Daniel Chatroux
email: daniel.chatroux@cea.fr

Performances of batteries technologies in vehicle applications

The main four batteries technologies for electric vehicles are lead acid, NiCd, NimH and Lithium-ion batteries. Mainly, these technologies are compared with the supplier specifications. In our laboratory, monitoring of electric vehicle indicates significant differences between the supplier specifications and real use performances, because of the specification conditions. The goal of this study is to focus on the main relevant parameters to select battery technologies for electric vehicle applications and to give some figures on the technologies.

1.

Batteries for vehicles applications For lithium-ion batteries, the whole chain is involved: materials for Lithium ion accumulators, accumulator conception, production, battery packs design, battery management systems, electric vehicle integration and monitoring of vehicles.

In different projects, we replace existing Lead acid, NiCd or NimH battery packs by Lithium ion batteries. We observe a big difference between the stored energy according to the supplier specifications for different battery technologies and the ranges of the vehicles.

Another result is the difference between laboratory test results and number of cycles on vehicles in real operating conditions. This point impacts directly on kilometer costs. This presentation focuses on some CEA results to give a first level of overview on this subject.

Differences between supplier specifications and effective vehicle ranges

The first case of great difference measured between supplier specification and vehicle range was presented at PCIM in 2009 [START_REF] Béranger | Sébastien Fiette CEA-LITEN Experience feedback on electric vehicles of the French car fleet -battery impact PCIM[END_REF] for NiCd technology. The monitoring of a small electric car with NiCd battery indicates that the stored energy in the batteries in real use is 8.7 kWh instead of 12 kWh as specified. This small car is an AX from Citroën. The same result has been published by the Idaho National Laboratory [START_REF]Hybrid Electric and Plug-in Hybrid Electric Vehicle Testing Activities" from the Idaho National Laboratory[END_REF] for the monitoring of a Kangoo from Renault using the same NiCd technology.

For the AX battery pack, the gravimetric energy decreases from 46 Wh/kg (12 kWh specified for 250 kg) to 33 Wh/kg, (8.7 kWh in real use for 250 kg).

The difference between the real performance and the specification is the test conditions. This NiCd technology is tested for a three hours discharge. In real use, with a speed average of 90 km/h the battery is discharged in three quarters of hour. With this NiCd technology, the range is only 60-70 km. Because of the too fast discharge for this NiCd battery, a part of the energy is dissipated in the too high internal resistance and a part of the battery capacitance is not used in discharge because of diffusion phenomena. After the end of discharge and some times of storage, a part of this capacitance is available and it is possible to restart the vehicle for some additional kilometers.

Lead acid battery is well used for small electric vehicles. This technology has a low initial investment cost and seems to be an interesting challenge to provide a competitive kilometric cost. For three years, the laboratory has monitored Lead acid electric vehicles. In parallel we modify the vehicles to implant a lithium-ion one.

For a small two wheel vehicle (Trotty from Jean Claude Andruet) for the same level of energy, the range is presented in Figure 1 between the initial Lead Acid battery and our standard Lithium Iron phosphate (LiFePO4) module. With the same speed profile, the lead acid battery of 1.2 kWh provides a 32 km range and the 980 Wh LiFePO4 one provides a 45 km range. The LiFePO4 module has a lower internal resistance. The voltage drop is much lower than Lead acid one. As for NiCd batteries, a part of the capacitance of the lead acid battery is not used for fast discharges.

Synopsis of accumulator supplier specifications for different technologies

The document [START_REF] Schwarz | Le stockage électrochimique[END_REF] indicates the level of performance of different technologies of electrochemical accumulators.

Results on electric vehicles

Performances of packs on electric vehicles

In AX vehicle, for the initial NiCd battery pack, there are twenty battery modules 6V 100Ah in series. The gravimetric energy for module, based on supplier specification, is 54 Wh/kg. Each module weights 13.2 kg. So the energy per module should be 712 Wh. For the twenty modules, the energy should be 14 kWh at the beginning of the life to be at the level of 12 kWh at the end of the life (vehicle supplier specification). For main accumulators, the end of life is achieved for a loss of 20% of the initial capacity. Because of additional mass the weight of the whole pack is 290 kg,. According to the vehicle supplier specifications (12 kWh), the gravimetric energy for the pack should be defined at the end of the life. For the Vectrix, an electric scooter with NimH battery manufactured by Gold Peak (GP) the gravimetric energy for accumulator based on supplier specifications is 47 Wh/kg [START_REF] Chatroux | Power electronic and battery analysis of Vectrix Scooter[END_REF]. This value is low compared to literature values (60 -110 Wh/kg). This accumulator is designed for power to provide the accelerations to the scooter, that is why this value is low for NimH technology. Because of this choice, in real use, the energy supplied by the battery pack is in the same order of magnitude 40 Wh/kg. Figure 2 describes the gravimetric energy for three battery packs, one with Lithium ion with iron phosphate (LiFePO4) with an energy optimization design (red curve), one with LiFePO4 with power optimization (brown), and two for lead acid (yellow and black). For Lead-acid battery, Trotty is an example of two or three hours discharge (long discharge). For a car, the discharge time is around one hour only. The delivered energy decreases. The gravimetric energy may be less than 20 Wh/kg. The loss of performance between accumulator gravimetric energy and battery pack due to additional mass, electronic, and safety devices. The loss of gravimetric energy is around 20% because the integration of Lithium ion accumulators requires more connections, mechanicals parts and electronic than Lead acid, NiCd or NimH batteries. Another point is the ageing of the battery packs. The worst case is for lead acid in a small car, the distance is only 10 000 km for the pack deterioration (Fig. 2). This point has very important impact on kilometer cost. Lead acid seems to be a cost effective solution for electric vehicles but this technology is not adapted for the discharge in the range of one hour. The first impact is the gravimetric energy divided by two. The second impact measured is low number of cycles. With this technology, the initial cost is low but the kilometric cost may be high due to poor reliability. Lead acid batteries are more adapted for discharge in some hours, for vehicles with very low speed. Energy LiFePO4 accumulators provide more energy at room temperature, but this energy decreases at low temperature [START_REF] Dauchy | LITEN France Comparison between energy and power one for electrical vehicle applications PCIM 20010[END_REF] and the ageing is faster. This technology is adapted only for long range vehicles and temperature conditioning is necessary. Power LiFePO4 accumulators provide good performances with the possibility of fast charge and without temperature conditioning. We measure a difference between winter and summer around 10%.

Some new results of a small electric car with iron phosphate battery technology

Now with our small car equipped with power LiFePO4 battery (brown curve), two runs of 10000 km have been performed within a month. Each day the car runs 500 to 600 km with six runs with five fast charge and one slow night charge. The charge duration is limited by the maximum current of the existing charging station to 45 minutes. The first run of more than 10 000 km in one month was from 15th of July to 15th of August.

In 10 000 km, less than 1% of capacitance decrease has been measured. The second run from the 1st of October to the 31st of October confirms the first result in distance and capacity losses.

Energy density versus vehicle distance

In these tests, the capacity loss is only 1.7 % for 30 000 km with high temperature due to summer temperature and fast charge temperature rise. With standard linear decrease hypothesis, the battery could provide energy for 300 000 km in the same intensive utilization.

Conclusion

In our monitoring activity we measure great differences between batteries supplier's specifications and real pack measurement in vehicles applications.

In vehicle application the discharge rate of the battery pack, may be higher than supplier specification. A part of the energy is dissipated in series resistances. A part of the capacitance is not used.

Lead acid

NiCd NimH ZEBRA LiFePO4 According to supplier specifications, lead acid seems to be a low cost solution for small electric vehicles. In a car, because of the fast discharge (around one hour) the real gravimetric energy is very low, with a great impact on vehicle mass. This solution provides poor vehicle performances. The initial cost seems promising but the fast ageing has a very important impact on kilometric cost. Power LiFePO4 is a good candidate for electric vehicle because of the safety of the chemistry, the low cost of the material, and the great number of cycles in real applications. The gravimetric energy of the accumulators specified by the suppliers is lower than other Lithiumion chemistry but we see that the good criteria is the real measurement and the comparison of the gravimetric energy of the global pack The replacement of lead acid or NiCd or NimH provides lower mass, extended range from 50 to 100% and the promises of competitive kilometric costs in some years.

Figure 1 :

 1 Figure 1: Voltages of Lead acid and LiFePO4 Lithium ion batteries for the same discharge profile, X-axis time in second, Y-axis voltage in Volts

Figure 2 :

 2 Figure 2: Gravimetric energy (Wh/kg) for energy LiFePO4, power LiFePO4 and Lead-acid batteries

  In real use, it's possible to charge at negative temperatures but with lower current. The low temperature charge has a great impact on the number of cycles.

			Lead	NiCd	NimH		ZEBRA	LiFePO4	Li ion (1) Specific tests on Li Polymer
	-	acid this point are necessary in practical, (8) Lanthanum, a rare earth, is used in the Hydrid,	Iron phos-phate Li-ion
	Gravimetric energy -(9) Since 2005, the costs have decreased. 30-50 45-80 60-110	120	120-140	150-190	150-190
		(Wh/kg)
		Energy	75-120	80-150	220-330		180	190-220	220-330	220-330
		density
		(Wh/l)
	Power den-	up to 700		up to 900		200	up to 800 (3)	up to 1500	up to 250
		sity (2)
		(W/kg)
	Number of	400-600(1)	2000	1500		800	>2000	500-1000	200-300
		cycles	1200(2)
		Self dis-	5%	20%	30% (4)		12 %	5% (6)	10%	10%
	charge per					per day (5)
		month
		Nominal	2V	1,2V	1,2V		2,6 V	3,2V	3,6V	3,7V
		voltage
		Tempera-	-20°C +60°C	-40°C +60°C	-20°C +60°C	-40°C +50°C	0°C +45°C	-20°C +60°C	0°C +60°C
		ture range						(charge)	(7)
								-20°C +60°C
								(discharge)
	Advantages	Cost	Reliability,	Good energy	Good energy	Good energy	Excellent	Thin batter-
				low tempera-	density		density	density	energy and	ies are pos-
				ture	(Wh/l)		(Wh/l),	(Wh/l),	power	sible
							number of	safetyy, cost,
							cycles	number of
								cycles
		Disad-	Poor energy	Low energy,	Raw material	Power limita-	Low temper-	Security for	Low temper-
		vantages	instantane-	toxicity	cost (8),		tion,	ature charge	big accumu-	ature per-
			ous death		temperature	energy loss-	(7)	lator cost (7)	formance
					behavior		es		cost (7)
		Cost esti-	200 to 250	600	1500 to 2000		800 to 900	1000 to 1800	2000	1500 to 2000
		mation (9)	(a)
		(€/kWh)	200 (b)
	(a) sealed (b) tubular
	In this table, there are a lot of information.
	The table can be completed with new results or precisions:
	-	(1) Li ion has several main families with different behaviors,
	-	(2) For all accumulators, there are energy versions optimized for energy but with lower
		power, and power versions with lower energy density,
	-	(3) the A123Systems company designed LiFePO4 for electric tools at 2 600 W/kg (for
		10 s discharge) and VL10VFe of Saft -Jonhson Control have 5 kW/kg continuous dis-
		charge,
	-	(4) For high quality NimH, the initial self-discharge is high (30% for example) but de-
		crease a lot in a second phase. In a Vectrix scooter, we measure than a NimH battery
		from GP is only half discharge after more than one year of storage,
	-	(5) ZEBRA is a high temperature accumulator with high temperature conditioning. Re-
		sistors are used to maintain the temperature in storage. Per year, the electricity to
		warm the accumulators in storage increases significantly the electricity cost,
	-	(6) For A123Systems in format 26650, the accumulators have low discharge after
		more than two years of storage,

-(7) As for LiFePO4, most Lithium ion technologies are adapted for discharge at negative temperatures, but for charge the minimum temperature is around 0°C. It's necessary to know the problem and to ask the question to the supplier on this point.

kg (real use)

  The gravimetric energy based on real energy supplied in electric car running at 90 km/h is 30 Wh/kg. The supplier specification for gravimetric energy (54 Wh/kg) is in the range of the literature value (45-80 Wh/kg). The battery pack in real use provides only 30Wh/kg for only 10% additional mass between accumulators and pack.

	Accumulator	Energy kWh	Weight	Gravimetric energy
	6V 100Ah beginning	710 Wh	13,2 kg	54 Wh/kg
	of life			(specified)
	20 accumulators be-ginning of life	14.2 kWh	264 kg	54 Wh/kg
	20 accumulators end of life	12 kWh	264 kg	45 Wh/kg
	Pack beginning of life	14.2 kWh	290 kg	49 Wh/kg
	Pack end of life	12 kWh	290 kg	41 Wh/kg
	Pack measured	8.7 kWh	290 kg	30 Wh/

4.

Literature


Standard electric buses use battery energy storage for all the day long and night charges. The high energy imposes high weight, volume and cost with impact on bus performances and a long return of investment. Another solution is buses with ultracapacitors with a fast charge station at each bus stop, but with a huge infrastructure cost. The ElLiSup project is based on a small battery, to be light and to have a limited cost, and a fast charge but only at the ends of the buses lines, or at nodal points. The goal of the presentation is to detail the concept, the prototypes of buses (an plug-in hybrid one with fast charge and the electric one with four minutes recharges at the ends of the line) and the charge station.

1.

ElLiSup project presentation

Electric buses technologies

A standard electric bus uses night charges and a high volume and weight of battery to provide a 100km range for all the day long. For example, the battery weight is around 2 Tons.

Because the number of cycle per day is only one, the investment return of the battery is in the range of seven years. There is a difficulty for the bus supplier to prove the reliability of the battery for such a long time.

A second solution for electric buses is to use a small electric storage with ultracapacitors and a high power charge station at each bus stop. The advantage of this solution is the cost and high life time of ultracapacitors. For example, the electric storage is designed for twice the distance between two bus stops to be tolerant at one charge station failure. Ultracapacitors life time and number of cycle are well known, which is an advantage to calculate investment return. The disadvantage of the solution is the huge investment infrastructure for charge. For example, a 200 kW fast charge station with a 20kV connection is necessary at each bus stop. The great number of stations with the material cost and the one of square meters in town implies a huge investment. This point limits the development of this interesting technical solution.

The third solution is to use a small battery storage designed for the bus line distance and to charge at the ends of the line. We will detail this solution.

Battery buses with ends of the line charges

Lithium batteries are in constant progress. Now, some commercial power lithium batteries are adapted for fast charge. For example, for electric tools, small iron phosphate Lithium ion batteries are specified for a total fast charge in ten minutes. The impact on battery lifetime of such a fast recharge is limited.

In France, correlated to the size of the towns, around 30% of bus lines are less than eight kilometers long, and 65% of bus lines are longer but have nodal connection points and the segments are less than 8 kilometers long.

For the concept, fast charge stations are located at the ends of the line and at nodal points. Classically, the driver has a five minutes stops at this nodal and ends points. To provide the energy transfer in four to five minutes, a 200 kW station is necessary. Because of the small number of stations, the infrastructure cost is low. At the ends of lines the square meter cost may be less expensive than in the town center. The challenge of this solution is to have a battery with a competitive cost of use compared to diesel solutions. Because of electric vehicle markets the battery performances are in constant progress, and costs decrease.

In this application the number of cycles for the battery is around ten to fifteen per day. So the life time of the battery to be competitive is only about two or three years depending of the battery size. There is an optimization to do about battery technology and size with impacts on battery costs, losses, cooling requirements and number of cycles in real use.

ElLiSup project

ElLiSup is one of the projects of "Grenelle de l'environnement" in France to promote electric vehicle development. Two prototype buses are in development and a 350 kW charge station is build.. The first bus is the transformation of hybrid bus in a fast charge plug-in one.

The second bus is a full electric one with ends of the lines fast recharge.

The two buses and the high power charge station will be presented in this paper.

Bus prototypes

Fast charge plug-in hybrid bus

IRISBUS commercialize a CITELIS bus with a diesel electric series hybridization. The diesel engine is downsized. It's a six liter Tector with 300 horse power. BAE provides the hybridization chain [1]. The electric generator is a magnet synchronous one with a 140 kW power. The motor is an asynchronous one, with a nominal electric power of 120 kW. Its maximum power is 175 kW. The battery is an iron phosphate one (LiFePO4) with 2,3Ah A123Systems cells. The design of the battery is optimized for efficient air cooling and crash acceleration tolerance. The battery is an 11 kWh one.

In real use, diesel consumption is 29 to 39 % lower due to stop-start and energy recovery at each bus stop. In the ElLiSup project one of the objectives is the evolution of one hybrid bus to be charged with the high power charge station. Because of the small size of the existing battery the charge is limited to 50 kW in the first step.

A pantograph is designed and installed on the bus roof.

The objective of the plug-in modification is to provide a part of full electric range in the applications, for example for historic centers or pedestrian zones or to cross a tunnel without pollution. The full electric range is 3-4 kilometers. There is a compromise between battery lifetime and full electric range to adjust for the application.

Higher electric range may be provided by high energy storage in the battery. The 11 kWh existing battery is a very small one.

Electric bus with fast charges at the ends of the line

This electric bus uses batteries and ultracapacitors. Ultracapacitors store the recovery energy at each bus stop and this energy is used for the bus restart. The mean energy is stored in the battery to provide the height to ten kilometers range. At the ends of the line or at nodal points the energy is supplied in four or five minutes by the charge station.

For height to ten kilometers the energy to store is ten times less important than a standard electric bus with a 100 km range. To provide lifetime, the size of the battery is only tree time less important than a standard one. So the charge is a partial one with around 30% of deep of discharge. Ultracapacitors are well adapted for millions of cycles but for low energy storage, for example some hundreds of watt.hour.

In the ElLISup electric bus concept ultracapacitors and batteries are associated. Ultracapacitors are used for braking energy recovery because there is one or two millions of braking cycles in a bus life and ultracapacitors are adapted for this number of cycles. Batteries are used for the main storage. In the project, the impact of braking energy recovery on the lifetime of the battery is studied, to know if ultracapacitors use has a real impact on batteries lifetime, or if the fast charge batteries may bear the braking recovery cycles.

There are four 18 kWh battery packs and an ultracapacitors one. A bidirectional 80 kW converter is used in series with each battery pack and ultracapacitors one. So a total energy management is possible with or without ultracapacitors, with full battery or a partial one.

Michelin Active wheel

The bus uses Michelin active wheels. Michelin Active Wheel is known a standard wheel that houses a pair of electric motors. One of the motors spins the wheel and transmits power to the ground, while the other acts as an active suspension system to improve comfort, handling and stability. The technology is such that a vehicle equipped with it will no longer need any gearbox, clutch, transmission shaft, universal joint or anti-roll bar. Active Wheel's compact drive motor and integrated suspension system has also enabled designers to fit a standard brake disc between the motors, which means the braking, drive and suspension components are all fitted within the single wheel.

For ElLiSup bus a specific active wheel is designed with adapted power for heavy vehicle traction motor and a mechanical brake but without active suspension. The system also allows torque from the motors to be electronically controlled for each individual wheel independently. The results are similar to the effects of an active differential.

Due to Michelin electric wheel and specific tires the design of the bus is optimized to provide more space.

Lithium ion batteries

On the bus, four lithium ion battery packs are used. The nominal energy storage of each pack is 18 kWh. The packs are on the roof in parallel with an air cooling system. Each pack is associated with a bidirectional converter to manage energy transfer in charge and discharge. Each battery pack has a Battery Management System. In each module an electronic card monitors each cell voltage and module temperature. The data are transmitted with CAN network. The current is measured by shunt to provide high precision from some Amps to 200 Amps. In central microcontroller the algorithms calculate the battery states: state of charge, state of energy, state of health, state of security.

Recharge station

The recharge station in CEA is designed for 350 kW to supply the two buses in parallel. The recharge station is supplied with 15kV (or 20 kV) voltage to have a negligible impact on the electrical network. A standard transformer provides an isolated 400 V main.

An inverter absorbs sinusoidal current on the three phases to generate a 700 V intermediary bus.

Bidirectional buck converters are used to provide a constant voltage output to supply the bus.

To charge only one bus simultaneously the station may be a current source.

To charge buses in parallel, the station is a voltage source and each bus has converters to absorb the level of current on the station. This choice has been done for the project.

Because of the 350 kW power, nine 40 kW buck converters are used in parallel.

In the station in CEA, converters are connected in parallel for the four minutes buses charges and a used independently for battery cycling tests when no bus are present. So the station has to be bidirectional. High voltage vacuum relays are used for the connections and disconnections to the batteries in test or to the bus charge connexion.

The charge station is designed to minimize the pollution impacts: power factor, EMC, flicker and noise.

Conclusion

Now the plug-in hybrid bus is in test with CEA with the 350 kW charge station to optimize the different modes.

The electric bus with four minutes recharges at the ends of the line is totally defined and is in construction. Michelin electric motor was tested. Lithium-ion battery modules are qualified. Four battery packs for the bus and two additional ones for lifetime test on the station are now in construction.

Literature

[1] BAE/Orion Hybrid Electric Buses at New York City Transit, A Generational Comparison, R. Barnitt, Technical Report NREL/TP-540-42217, National Renewable Energy Laboratory

Figure 1 :

 1 Figure 1: plug-in hybrid bus

  Figure 2: bus electrical architecture

Figure 3 :

 3 Figure 3: Michelin active wheel

Figure

  Figure 4: space improvement

Figure 5 :

 5 Figure 5: Battery packs

Figure 6 :

 6 Figure 6: Battery Management System

Figure 7 :

 7 Figure 7: BMS central microcontroller

Figure 8 :

 8 Figure 8: CEA 350kW charge station


Laurence Noirez

Eni Kume

Patrick Baroni

From Hidden Shear-Elasticity to Opto-and Thermo-Elasticity in Confined Polymer Melts and Viscous Liquids

From the newly identified low frequency liquid elasticity, it turns out the possibility to reveal novel dynamic properties of liquids at mesoscopic scale. We present two examples illustrating the impact of the mesoscopic liquid elasticity in optics and thermodynamics. We show strain induced optical effects and thermo-induced effects that occur in the liquid state and reveal the need to take into consideration the elastic character of fluids at small scale. The fluids are submitted to a low frequency (~ Hz) mechanical shear strain (external stress). We probe the dynamic response by two methods, either accessing instant optical response of the liquid phase of liquid crystals and or probing the thermal response upon mechanical deformation. It is shown that the isotropic phase of liquid crystals can exhibit a dynamic birefringence in the isotropic phase while ordinary liquids exhibit cold and cold thermal waves upon low frequency excitation.

Introduction:

On the basis of a Maxwell gas model, it has long been suspected that liquids and polymer melts exhibit (shear) elasticity at high solicitation frequencies (MHz or GHz) only [START_REF] Frenkel | Kinetic theory of liquids[END_REF][START_REF] Scarponi | Brillouin light scattering from transverse and longitudinal acoustic waves in glycerol[END_REF]. Recent experimental improvements seem to indicate that liquids present also shear elasticity at very low frequency (0.1 to 10Hz); i.e. a nearly "static" shear elasticity. We developed an experimental protocol to reveal a "static" mesoscopic shear elasticity of polymer melts and liquids up to submillimeter scale (Fig. 1) [START_REF] Mendil | Solid-like rheological response of non-entangled polymers in the molten state[END_REF][START_REF] Noirez | Revealing the solid-like nature of glycerol at ambient temperature[END_REF][START_REF] Noirez | The missing parameter in rheology: hidden solid-like correlations in viscous liquids, polymer melts and glass formers[END_REF][START_REF] Noirez | Identification of a low-frequency elastic behaviour in liquid water[END_REF][START_REF] Noirez | Importance of Interfacial Interactions to Access Shear Elasticity of Liquids and Understand Flow Induced Birefringence from Liquid Crystals to Worm-Like Micellar Solutions[END_REF]. A shear elasticity of about few Pascal up to thousand Pascal was identified both on simple liquids and complex fluids (entangled and unentangled polymer melts, molecular glass formers, Van der Waals liquids, ionic liquids, H-bond liquids including liquid water) pointing out a probable generic character and extending the pioneering micron-scale works of Derjaguin et al [START_REF] Derjaguin | The complex shear modulus of polymeric and small-molecule liquids[END_REF][START_REF] Derjaguin | Shear elasticity of low-viscosity liquids at low frequencies[END_REF] and later of Reiter et al [START_REF] Reiter | Stick to slip transition and adhesion of lubricated surfaces in moving contact[END_REF] up to the sub-millimeter scale. Finite low frequency shear elasticity means that liquids are not viscous at all scales but that below an elastic threshold, they exhibit a solid-like behavior. Mesoscopic shear elasticity has profound implications on flow, surface instabilities, thermodynamics, fluidic transport mechanisms and make possible the identification of new liquid properties [START_REF] Lv | Viscosity of water and hydrocarbon changes with microcrevice Thickness[END_REF] such as the low-frequency birefringence or spectacular strain-driven thermoelastic effects that we illustrate in this paper and that were known in solids only. Low frequency shear elasticity challenges also molecular theories since long range elastic correlations imply collective dynamics that are not taken into account in conventional molecular models.

We show here on two representative examples the importance of considering long range elastic correlations in mesoscopic liquids. The first example comes from a revisited study of the dynamic behavior of the liquid (isotropic) phase of liquid crystals. In LC-displays the tunable optical properties are exclusively studied and expected in the mesophases, typically below the first order isotropic-nematic phase transition. The mechanism is based on the reorientation of the liquid crystalline (nematic) domains. This reorientation is usually achieved by applying external fields as electric, magnetic or mechanical fields [START_REF]The Physics of Liquid Crystals, PG de Gennes[END_REF]. No tunable properties are expected at temperatures above the liquid crystal phases; i.e. in the isotropic phase. This phase is usually considered as an ordinary liquid [START_REF]The Physics of Liquid Crystals, PG de Gennes[END_REF][START_REF]Rheology and Processing of Liquid Crystal Polymers[END_REF][START_REF] Turzi | Viscoelastic nematodynamics[END_REF]. But, the recent identification of "static" shear elasticity in polymer melts and viscous liquids at the sub-millimeter scale [START_REF] Mendil | Solid-like rheological response of non-entangled polymers in the molten state[END_REF][START_REF] Noirez | Revealing the solid-like nature of glycerol at ambient temperature[END_REF][START_REF] Noirez | The missing parameter in rheology: hidden solid-like correlations in viscous liquids, polymer melts and glass formers[END_REF][START_REF] Noirez | Identification of a low-frequency elastic behaviour in liquid water[END_REF][START_REF] Noirez | Importance of Interfacial Interactions to Access Shear Elasticity of Liquids and Understand Flow Induced Birefringence from Liquid Crystals to Worm-Like Micellar Solutions[END_REF][START_REF] Lv | Viscosity of water and hydrocarbon changes with microcrevice Thickness[END_REF] allows us to revisit the mesoscopic dynamic properties of the isotropic phase on new assumptions. By assuming a coupling between orientational pretransitional fluctuations existing in the isotropic phase and long range elastic interactions recently identified, we revealed that a strong and reversible dynamic optical response is generated in the isotropic phase upon a low frequency low strain mechanical excitation (Fig. 2) [START_REF] Kahl | Hidden solidlike properties in the isotropic phase of the 8CB liquid crystal[END_REF][START_REF] Kahl | From flow birefringence in the isotropic phase to the identification of shear elasticity in liquids[END_REF][START_REF] Noirez | Exploring "Dormant" Opto-Mechanical Properties of the Isotropic Phase of Liquid Crystals and Revealing Hidden Elasticity of (Ordinary) Liquids[END_REF]. A low-frequency optical birefringence, synchronous with the excitation, has been measured up to 15 °C into the isotropic phase with the advantage of producing a true black phase between crossed polarizers in the unexcited state. The isotropic liquid works as an optically active material when it is excited a low frequency mechanical shear strain. Such spectacular observation demonstrates the dynamic coupling that exists between the pretransitional fluctuations (clusters of molecules with identical directions coexisting within the isotropic phase) and necessary elastic properties of the liquid. The liquid phase of liquid crystals can thus behave as a low frequency optical oscillator in response to a low frequency mechanical excitation. The birefringence is produced by exciting the isotropic phase at low frequency; this means that the mechanical energy brought by the shear strain is not used for increasing the entropy as it should be the case for an entropic system (and as a liquid is supposed to be) but it lowers its entropy (the birefringence highlights an ordering), implying internal correlations. The low frequency elastic birefringence is identified in the isotropic phase of several liquid crystalline fluids, and including the isotropic phase of LC-polymers, indicating a likely generic property and is probably connected to spectacular optical properties very early reported by different authors for the isotropic phase [START_REF] Reys | Short-Range-Order Eff'ects in the Isotropic Phase of a Side-Chain Polymeric Liquid Crystal[END_REF][START_REF] Bailey | Large Flow Birefringence of Nematogenic Bent-Core Liquid Crystals[END_REF]. The second approach uses the thermal radiation, thus probing larger scale effect with respect to the optical birefringence, of the order of the infrared wavelength ( 10 µm against 500 nm). Few physical studies exploit the fundamental property that a thermal radiation is naturally emitted by the condensed matter above zero degree Kelvin. The thermal emissivity is also a tool to access the thermodynamic state. We illustrate microthermal results obtained on applying a low frequency shear deformation to polypropylene glycol (PPG-4000) confined at sub-millimeter scale (Fig. 3).

Fig. 3: By applying a low frequency mechanical stimulus (~ Hz), the liquid (here polypropylene glycol -PPG-4000) emits a modulated thermal signal synchronous with the stimulus at 1rad/s. a) Real-time mapping of the temperature confined in a 340µm gap (gap view at room temperature measurements at γ = 4000% gathering about ~800 frames, alumina substrate). b) Temperature variations of the bottom, middle and upper bands displayed in red, blue and green. The average bulk temperature is in orange. The continuous line is an eye guide for the applied shear strain. The zero ΔΤ is defined relative to the equilibrium temperature. c) Strain dependence of the maximum of the temperature variation (absolute value: T(K)). Thermal waves recorded for the bottom (), middle (), upper () bands and total liquid volume (♦) respectively at γ = 2500%,  = 1 rad/s and e = 340μm. (d) Amplitude of the thermal variation of the hot (bottom) wave versus shear strain at 1 rad/s and 340µm.

Fig. 3 reveals that a low frequency mechanical shear strain impacts the liquid thermal equilibrium in such a way that a thermal wave propagates through the liquid at low frequency; i.e. at time scales much below every known relaxation times (molecular relaxation times are of the order of 10 -6 s [START_REF] Yahsi | Relaxation time of polypropylene glycol and polypropylene glycol dimethylether-like polymers in terms of fluid-phase temperature and pressure dependent hole fraction[END_REF]). The thermal waves are multiple, synchronous with the excitation, harmonic at low strain amplitude (Fig. 3b). The study of the amplitude of the thermal wave indicates a linear behavior with respect to the strain rate (Fig. 3c). The hot and cold thermal bands of about 20µm width relax simultaneously when the excitation is stopped (Fig. 2b). It confirms the adiabatic nature of the thermal effect indicating that the temperature changes are only local and works without exchanges with the environment. These enounced non-dissipative properties are characteristics of an elastic response.

The identification of thermal bands in ordinary viscous liquids at the sub-millimeter scale under the action of a low frequency mechanical shear is a recent observation of utmost importance [START_REF] Kume | Strain-induced violation of temperature uniformity in mesoscale liquids[END_REF]. It reveals a collective behaviour of the liquid that is not described by conventional theoretical approaches. The adiabatic mechanism rules out convection or conduction effects. The existence of cold and hot bands thus indicates that the alternation of "stretched" states of the liquid coexisting with compressed ones. These two states compensate dynamically: i.e. the system does not exchange energy with the environment; in agreement with an adiabatic-like process. No heat is added (dQ = 0). (Strain) work offered to the fluid is transformed into internal energy by organizing liquid in temperature bands.

An analogy is the thermoelastic stress analysis in solids. The basic equation of the thermoelasticity in solids can be written as [START_REF] Biot | Thermoelasticity and irreversible thermo-dynamics[END_REF][START_REF] Pitarresi | A review of the general theory of thermoelastic efect[END_REF]:

T = -T0. /(.Cp).
where α is the linear expansion coefficient, ρ the density of the material, cp the specific heat at constant pressure, T0 is a reference temperature and  the stress. We have seen that the thermal change ΔΤ is linear in respect with the applied shear strain  which is here the only external field. Therefore the slope of T versus strain could give access to a shear elastic component supposing  = G'.. The equivalent shear elasticity is about 70 kPa for a band of about 20µm, which is of the order of values reported for microfluidic systems or of the order of the macroscopic shear elasticity determined using the wetting protocol by stress measurements (Fig. 1a) [START_REF] Mendil | Solid-like rheological response of non-entangled polymers in the molten state[END_REF][START_REF] Noirez | The missing parameter in rheology: hidden solid-like correlations in viscous liquids, polymer melts and glass formers[END_REF]. This equation supposes that we know the local shear strain which is an important assumption, that the density change is negligible, which is also an assumption in the case of a mechanism involving compressive and extensive waves. Finally, positive or negative T means that the liquid undergoes opposite stresses and that the strain parameter should be understood similarly as in the Hooke law with a restoring force opposite to that of the displacement (Fig. 4).

Fig. 4: The Hooke law illustrated by a balance between restoring energy and kinetic energy for a suspended spring evolving following a sin wave of its compressive state (extended or compressed).

Furukawa and Tanaka [START_REF] Furukawa | Violation of the incompressibility of liquid by simple shear flow[END_REF] suggested that liquids can be compressible by shear flow in order to explain the cavitation creation problem. They base their study on the existence of density fluctuations inside the liquid. The theory "does not involve any viscoelastic or structural relaxation". However, the expected shear rate is about 10 6 s -1 , much higher than the inverse of the timescales corresponding to the frequency range explored in the present study. At high shear rates or high shear strain, other effects are expected due to slippage and viscous friction in particular near the walls [START_REF] Giacomin | Viscous heating in large-amplitude oscillatory shear flow[END_REF].

Fig. 3 shows that the liquid thermal waves behave like harmonic oscillators; their response is synchronic with the applied strain. It cannot be coupled with relaxation times (1/ω >> τrelaxation), implying that the shear deformation is solely responsible for the creation of the bands. Consequently, the liquid integrates and converts (at least partly) the shear wave energy to organize in local temperature bands. Frenkel [START_REF] Frenkel | Kinetic theory of liquids[END_REF] was one of the first to propose the existence of transverse modes in liquids above ω > ωF. At high frequency, liquids are supposed to propagate both longitudinal and transverse waves. However, the applied mechanical excitations are from far away to the Frenkel frequency. Several authors using different theoretical approaches suggest that long range intermolecular interactions play a key-role in the dynamic liquid and melt behaviour [START_REF] Stillinger | Hidden structure in liquids[END_REF][START_REF] Guenza | Intermolecular Effects in the Center-of-Mass Dynamics of Unentangled Polymer Fluids[END_REF][START_REF] Volino | Théorie visco-élastique non-extensive[END_REF][START_REF] Schoen | Rheological properties of confined thin films[END_REF] and thus that collective responses are possible at smaller frequency. In particular, Trachenko et al. revisited the Frenkel theory [START_REF] Trachenko | Quantum dissipation in a scalar field theory with gapped momentum states[END_REF][START_REF] Trachenko | Lagrangian formulation and symmetrical description of liquid dynamics[END_REF][START_REF] Yang | Emergence and Evolution of the k-Gap in Spectra of Liquid and Supercritical States[END_REF][START_REF] Trachenko | Collective modes and thermodynamics of the liquid state[END_REF][START_REF] Zaccone | Explaining the low-frequency shear elasticity of confined liquids[END_REF][START_REF] Zaccone | Universal G′ ∼ L-3 Law for the Low-Frequency Shear Modulus of Confined Liquids[END_REF]. They use a generalized viscosity that contains short-term elasticity and predict a new frequency , where propagation modes can exist and show the existence of k-gap. The k-gap suggests that the liquid can be considered as collection of dynamical regions of characteristic size cτ corresponding to the maximum propagation length of the shear waves. This implies a scale-dependence of the propagation of the shear elastic waves that is now taken into account with new developments [START_REF] Zaccone | Explaining the low-frequency shear elasticity of confined liquids[END_REF][START_REF] Zaccone | Universal G′ ∼ L-3 Law for the Low-Frequency Shear Modulus of Confined Liquids[END_REF][START_REF] Baggioli | Gapped momentum states[END_REF][START_REF] Baggioli | Deformations, relaxation and broken symmetries in liquids, solids and glasses: a unified topological field theory[END_REF] in agreement with the present thermal observations that are restricted to low scale measurements.

Conclusions:

We have shown on two examples of fluids probed away from phase transitions, that the viscous property describes only a part of the liquid properties. Liquids are not deprived of shear elasticity, but this later is not easily accessible by conventional shear stress measurements and only access an apparent viscous behaviour. New approaches such as dynamic optical measurements or the very recent thermal approach have to be employed to reveal the elastic response of liquids, and describing as completely as possible hidden scaledependent liquid properties that dominate the dynamic behavior at mesoscopic scale. Thanks to elastic correlations, we are confident that many new features and related applications remain to be discovered together with the development of new theoretical models [START_REF] Zaccone | Explaining the low-frequency shear elasticity of confined liquids[END_REF][START_REF] Zaccone | Universal G′ ∼ L-3 Law for the Low-Frequency Shear Modulus of Confined Liquids[END_REF][START_REF] Baggioli | Gapped momentum states[END_REF][START_REF] Baggioli | Deformations, relaxation and broken symmetries in liquids, solids and glasses: a unified topological field theory[END_REF] in link with the experimental evidences.

Fig. 1 :

 1 Fig.1: Improving the wetting between the surface and the polymer melt, it has been shown that viscoelastic moduli of ordinary liquids do not vanish at low frequency but exhibit a solid-like response observable at the submillimeter scale (room temperature response of PBuA melt, Mw = 47500, T = T + 90°C -L. Noirez et al, Polymer Int. 58 962 (2009)).

Fig. 2 Fig. 2 :

 22 Fig.2illustrates the emergence of optical birefringence in the isotropic phase of a low molecular weight liquid crystal polymer (LCP95) in use for LC-displays in its liquid crystal phase. This molecule exhibits a particularly strong optical signal in the isotropic phase.

Scientific Report 2020, 10, 13340.

Acknowledgments:

This work has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 766007 and LabeX Palm (ANR-11-Idex-0003-02).


Daniel Chatroux
email: daniel.chatroux@cea.fr

Bruno Beranger

Sébastien Fiette

Marion Perrin

Eric Fernandez

Performance improvement of a small car with LiFePO4 batteries

In 2009, a presentation was done in PCIM about the experience feedback on electric vehicles of the French car fleet and the cost of battery use in this application. The initial expensive price of NiCd batteries, combined with poor cycles endurance, induce a kilometric cost two to three times higher than the one of traditional gasoline vehicles.

The replacement of the initial NiCd battery of an AX vehicle by an iron phosphate Lithium battery with 1600 small cells was presented in conclusion [1].

This presentation is focalized on the improvements of the vehicle performance due to this power iron phosphate Lithium ion battery.

I. Introduction

Nearly 10.000 electric vehicles have emerged from the French car fleet during these 15 last years. The initial expensive price of the NiCd batteries, combined with poor cycles endurance, induce a kilometric cost two to three times higher than the one of traditional gasoline vehicles.

To be competitive with thermal vehicle the cost of stored/restored energy must be lower than 0.5 €/kWh.

In CEA-LITEN, an AX from Citroën is used for vehicle monitoring and NiCd battery replacement by Lithium ion ones.

This car was chosen because of its low weight. Its one of the lightest car produced. With 280 kg of the battery, the global mass is less than a ton (950 kg).

For electric vehicle, a small weight is one of the key points to minimise the battery size and cost.

The CEA-LITEN has been working for 9 years on Iron Phosphate Lithium battery technology.

The LifePO4 technology provides a lower energy density than others Lithium ion batteries, but a high level of intrinsic safety. This chemistry has a very good level of intrinsic safety because of phosphate thermal stability compared to regular Lithium battery for laptops. These batteries use Cobalt Oxide Lithium ion battery. This chemistry proves low thermal stability. Sufficient security level has to be provided by additional components and electronics.

Iron phosphate Lithium ion technology has a good security level and its price is going down. The performances are very good for urban electric vehicles due to the low internal resistance and high number of cycles.

The cost of wear (the cost of stored/restored energy) may become as low as 0.3 €/kWh for high volume productions and good cycling performances.

In January 2009, new battery packs are designed and installed in the AX small car. The battery packs used 1600 small power cells from A123Systems. These cells are iron phosphate Lithium ion accumulators designed for electric power tools. This marked provides now first high volume production with good quality.

The cells are installed in the same place as initial NiCd batteries, in the same plastic boxes.

II. LiFePO4 Power battery

The A123Systems accumulators are designed to provide a very high level of power corresponding to power electric tool specifications. The cells are specified 2.3 Ah. The internal resistance is very low.

Discharge currents are specified up to 120 A for 10s.

The results of some discharge curves from 10A to 60A are presented upper.

With connections, the parasitic resistance is 10 milliohms for the 2.3 Ah cells. This low value provides low losses in charge and discharge.

With this high power LiFePO4 cells the losses are very low, so the battery packs are design without any forced air or water cooling.

LiFePO4 discharge curve is nearly constant around 3.2 Volts for standard discharge. This voltage provides a very important safety margin compared to electrolyte maximum voltage. This is a great difference compared to oxide Cobalt Lithium ion battery where the final charge voltage (4.2 Volts) is near the electrolyte maximum voltage.

III. LiFePO4 batteries

The original vehicle AX includes 3 batteries packs:

• A rear pack with 11 modules (154 kg).

• A lower front pack with 6 modules (84 kg).

• An upper front pack above the electrical motor with 3 modules (42 kg).

The total weight of the initial NiCd batteries ready to use with water cooling and electrolyte is 280 kg.

We replace this three NiCd packs by two LiFePO4 ones. We delete the lower front pack above the electrical motor.

The water cooling circuit for NiCd batteries is deleted. The whole cooling circuit is now dedicated to power electronic system.

The energy stored is 10.5 kWh in only two packs instead of three:

-A rear pack (72 kg).

-A lower front pack (66 kg).

Figure 2: front pack

The new energy density is: 76 Wh/kg, including all the battery system and the packaging.

The total weight of the new battery system is less than half of original one (138 kg).

The AX mass is now 810 kg, about the same value as for a thermal engine version.

The car gives now good accelerations.

IV. Batteries energy comparison

The specified energy of the initial NiCd battery is 12 kWh, but in use on road or motorway the real energy is only 7.4 kWh for our car. This result is due to the discharge rate of the battery in real use (C) which is more important than the discharge rate of the specification (C/3). The real energy depends on the driving conditions and on the battery (from 7.4 kWh to 9.8 kWh for different vehicles). The specified capacitance of the initial NiCd battery is 100 Ah (at C/3). In real use on road and motorway the measured capacitance is only 67 Ah (at C).

For the LiFePO4 battery, the capacitance is 81 Ah.

Because of the low voltage variation of the LiFePO4 battery, we design a battery for higher voltage than NiCd one to minimise the current losses in wiring and in power electronics.

V. Kilometric ranges and electric consumption

On road or motorway the range improvement is due to the more important energy stored.

With the initial NiCd battery, the kilometric range is only 75 km. With the LiFePO4 battery, the kilometric range is 110 km. On road or motorway, the electric consumption for the vehicle with LiFePO4 battery is 9.4 kWh/100 km. It's around the same value for NiCd batteries (9.6k Wh/100 km). The weight improvement has a low impact on energy consumption at constant speed.

In the city, with LiFePO4 battery the consumption is 10.9 kWh/100 km compared to 12.6 kWh/100 km. In the city, the impact of the weight improvement is important.

VI. Battery charge / discharge efficiency

The charge / discharge efficiency is 90 % for LiFePO4 battery compared to 80 % for NiCd ones.

Losses are divided by two. In fact, in real use, the improvement is more important. With NiCd battery there is an important overcharge for charge balancing after the charges. In real use, there is a lot of partial charge of the battery where the charge efficiency is less than 80 % due to overcharge for charge balancing.

VII. Battery cooling

The LiFePO4 battery has a very low internal resistance. The low level of losses, in charge and in discharge, allows having no additional cooling circuit.

Without any cooling circuit, the temperature rise is only 2 °C for a motorway drive in 50 min, and 3 °C in a city drive for the same time.

VIII. Fast charge

A lot of Lithium ion accumulators are specified as power ones. The gravimetric and volumetric power densities are specified. But these specifications are for discharge only. And the fast charge isn't really specified.

For a lot of Lithium ion technologies, fast charges are forbidden because of security aspects. Compared to maximum voltage the safety margin is not sufficient for fast charging, or the too high internal resistance provides a too high level of losses compared to maximum temperature rise.

With high power LiFePO4 Lithium ion battery fast charging is allowed because of the great voltage safety margin and of the intrinsic security.

A123Systems specified a 10 Amps charge current for 2.3 Ah cells (around 4-5 C).

We do a first fast charge test for the rear AX pack. The temperature rise is only 14°C because of the low internal parasitic resistance.

In fast charge the thermal behaviour is mainly adiabatic. Thermal losses are stored in the battery thermal capacitance. Thermal losses are dissipated after. The thermal constant is around some tens of hours.

Because of the high production quality, the charge dispersion between the accumulators in series is only 0.3%. This is a very good result. Cells charge balancing circuit losses are very low because of this very low dispersion.

IX. Conclusion

The LifePO4 technology provides a lower energy density than others Lithium ion batteries, but a high level of intrinsic safety.

We design a battery with 1600 high power small cells in two existing packaging of a small light car.

Because of the low internal resistance, the totality of the energy is accessible. Energy recovery during braking and downhill is efficient. The 140 kg weight improvement is important for acceleration performance and city electric consumption.

With the initial NiCd battery, the kilometric range on road is only 75 km. With the LiFePO4 battery, the kilometric range is 110 km.

In the city the kilometric range improvement is 38 km (95 km compared to 57 km).

The standard charge / discharge efficiency is 90 % for LiFePO4 battery compared to 80 % for NiCd one. Losses are divided by two.

The low level of losses, in charge and in discharge, allows having no additional cooling circuit. The temperature rise is only 2 °C for a motorway drive in 50 min, and 3 °C in a city drive for the same time.

In fast charge, with 250 Amps at 80% charge is done in 20 mn. In this case, the temperature rise is only 14°C.

The initial NiCd battery suffers from memory effect. In case of partial discharge, memory effect limits the effective range. So it's necessary to discharge totally the battery before recharging. This limitation is very difficult to manage for a collective fleet with different drivers. For Lithium ion battery, there isn't memory effect. So it's possible to recharge at the plug after each drive. This is a very important advantage for a collective fleet, and the total range is so available.

For electric vehicle, a small weight is one of the key points to minimise the battery size and cost.

Iron phosphate Lithium is a good candidate to provide a high performance, fast charge, low losses battery with a good level of intrinsic security.

In case of high volume production the cost of stored/restored energy should be lower than 0.5 €/kWh and so be competitive compared to thermal vehicles

X. References

[1] Bruno Béranger, Daniel Chatroux, Eric Fernandez, Sébastien Fiette: Experience feedback on electric vehicles of the French car fleet -battery impact PCIM2009 Nüremberg

  Figure1

  battery the real energy is 10.3 kWh.Because of the very low internal resistance, this energy is constant with a very low dependence on driving conditions.

Figure 3 :

 3 Figure 3: rear pack voltages (on road)The upper figure present the two mid rear pack voltages for a road driving.

  Figure 4: 250 Amps fast charge A C/2 discharge is done to measure the charge stored in the battery.


Toyota PRIUS: battery in microcycle mode: the cost of use is divided by five

Daniel Chatroux

Introduction

In CEA-LITEN, the LPAC laboratory has two activities. First, it develops fuel cell systems for automotive applications, off-road ones and stationary gensets. The second activity of the laboratory is electric vehicles, with monitoring actions of existing electric or hybrid vehicles and development of battery stacks or power electronics for electric vehicles. The goal of the monitoring actions is to know the real behavior and dimensioning of existing vehicles to have technical feedback and to provide expertise for new vehicles. The Toyota PRIUS is the first large diffusion hybrid vehicle. A lot of information is available on the developed technology [START_REF]Evaluation of 2004 Toyota Prius Hybrid Electric Drive System[END_REF]. Because of the complexity of the technology and the different aspects involved, Toyota PRIUS technology is difficult to summarize. The presentation is dedicated to Toyota PRIUS concept and to focus on the technical and cost aspects of the battery. Because of two battery laboratories on CEA, and one of these laboratories dedicated to battery use for photovoltaic application, there is some knowhow in CEA on real cost of battery utilization in different applications. In the Toyota PRIUS, the battery is used in microcycle mode, in this paper an estimation of the real cost of the battery used in microcycle mode is done. The main information used for this synthesis is extracted from Oak Ridge National Laboratory studies [START_REF]Evaluation of 2004 Toyota Prius Hybrid Electric Drive System[END_REF] ordered by US Department of Energy.

Continuous Variable Transmission with high efficiency

It is often said than the key point of the Toyota PRIUS concept is due to the battery. For us, the first key point of the Toyota PRIUS concept is not the battery, but the use of a high efficiency electrical continuous variable transmission. The all transmission is presented in figure 1.

Figure 1

The different parts of this transmission are the power split device, the generator, the two inverters (generator and motor ones), and the electrical motor. The figure 2 details the different mechanical parts.

Figure 2

The engine is connected to planetary carrier of the split device. The generator is connected to the sun gear. The ring gear is the mechanical output axle. The electrical motor is directly connected to this mechanical output axle. It's able to provide additional torque, up to four times the one of the engine.

The figure 3 presents the system configuration involved. In fact there are two inverters, one for the generator and one for the motor in the same box. In this part of the presentation, we present the nominal mode, and the battery is not used. In nominal mode, there is no energy exchange with the battery.

Figure 3

The power split device separates the engine mechanical power in two parts, a first one is used as a mechanical power; the second one is transformed in electricity by the generator. This electrical power is converted by the generator inverter in a high DC voltage (high voltage DC bus). The second inverter uses this DC voltage to supply the motor. The figure 4 presents the mechanical transmission and the electric transmission:

Mechanical transmission

Electic transmission

Gazoline thermal engine

Figure 4

In the Toyota Prius, the gasoline engine provides a maximum torque of 115 Nm. The electrical motor has a maximum torque of 400 Nm at low rotation speed. So this motor can provide the adequate level of torque to start and accelerate the vehicle, as a mechanical gear box on the first transmission ratio. Because of electric motor property, this component can provide enough torque up to the maximum speed of the vehicle. The figure 5 presents the power and torque of the electric motor for the first Toyota Prius (THS) and the 2004 version (THS II). The differences between the two versions will be explained later.

Figure 5

The generator is a high speed (10 000 r/mn) and low torque electric machine. The motor is a high torque and medium speed machine (5000 r/mn). The magnetic surfaces are very different, because surfaces are proportional to maximum torque value.

Figure 6: generator and motor magnetic surface

The all system (power split device, generator, generator inverter, motor inverter, and high torque motor) associated with adequate controller provides a continuous variable transmission with a very good efficiency and speed variation. On the Toyota PRIUS, there is no additional manual gear box or automatic one. There is only a mechanical reduction with constant ratio with a metallic chain and gears with fixed reduction ratio. The electrical motor is directly connected to the wheels with only constant reduction gear. So, it will be possible to provide additional power from the battery for transient phase. For example, for quick acceleration, it's not necessary to wait for gasoline engine acceleration to provide more power (as it's the case when there is only mechanical continuous variable transmission). Additional power could be provided instantaneously by battery to the motor to provide fast acceleration. This point will be detailed later in the presentation.

Engine is used only in the high efficiency zones

With a continuous variable transmission, it's possible to use the engine in a high torque zone where the specific consumption is low. This point is described now. Figure 7 present the hybrid drive system associated to the gasoline engine.

Figure 7

Compared to diesel engine, the gasoline engine has a lower efficiency due to Carnot law because of its lower compression ratio and lower combustion temperature. But this theoretical efficiency difference is lower than the real fuel consumption difference (in liters). One additional point is the difference of energy between one liter of diesel (10 kWh) and one liter of gasoline (9 kWh). The last and important point is that the gasoline engine has a lower efficiency, far from theoretical one, because of air aspiration losses at low charge (low torque). At low charge, the throttle is only partially open, to have the adequate quantity of air to be stochiometric. In a diesel engine, there is no throttle and no additional losses. The diesel engine works with air excess. Without a continuous variable transmission (and with additional power for transient acceleration) the driver uses the gasoline engine in low charge (low torque) for long times, to have adequate level of safety margin for the accelerations. The figure 8 presents the Toyota PRIUS gasoline engine cartography [START_REF]Development of new-Generation Hybrid System THS II -Drastic Improvement of Power Per-formance and Fuel Economy[END_REF]. This engine is a specific optimized one, but the rules between efficiency and torque are the same than standard ones.

Figure 8

With a standard manual or automatic gear box, the engine is used mainly in low torque zone to have safety margin to provide acceleration if necessary. For example, 9 kW is adequate to provide 80 km/h speed, but there is no gear box with 70 km/h for 1000 rotation per minute. Standard manual gear boxes are 35 km/h / 1000 rpm for gasoline engine and 45 km/h / 1000 rpm for diesel engine. With such a too long ratio it would be very difficult to drive because it's necessary to change the gear box ratio for each power demand variation. The hybrid drive system of the Toyota Prius is firstly a high efficiency electrical Continuous Variable Transmission and secondly it provides the safety margin for acceleration instantaneously by the battery. So the engine can be used only in high charge zone with high efficiency. The consumption is lower. In this case, the global efficiencies of diesel and gasoline engine (associated with hybrid drive system) are the same. The energy consumptions are the same. The consumptions in liters are different because of the difference of density between the two fuels: -For diesel, the energy is 10 kWh/l, CO2 emission is 2.6 kg.

-For gasoline, the energy is 9 kWh/l, CO2 emission is 2.4 kg. If the price was related to energy or CO2 emission, the price for diesel fuel should be 10% more than gasoline one. The Toyota PRIUS has an Atkinson/Miller cycle. This is an optimization of the gasoline engine with controlled valve with angle control. This is not a key point. This kind of technology (controlled valve with angle control) is now developed in new Japan or European gasoline engine to minimize air aspiration losses and for global higher efficiency.

Electric motor is always connected to the wheels, gasoline engine is decoupled

As said previously, the electric motor is always connected to the wheels, so it's possible to:

-provide additional acceleration with battery power, -provide continuous torque without interruption (as an electrically control gear boxes with different transmission ratio), -drive in pure electric mode. The engine is not linked directly to the wheels. It's linked by the split device. If there is no torque demand on the generator, there is no torque on the engine axle. With the control command and the generator inverter, it's possible to decouple totally the engine of the wheels.

Compared to some other hybrid electric cars, the Toyota Prius engine can be totally decoupled. It's an advantage for energy saving in deceleration and electric mode (no engine losses).

Gazoline consumption and pollution emissions at low speed in the city are reduced

As it was said before, at low speed the consumption is reduced because of:

-the engine is used in high torque zone with high efficiency, -the sequence of electric mode for short distance on the battery, and recharge by restarting the engine for some tens of seconds (in high torque zone), and engine stop again for pure electric mode, and so on. This sequence is used at low speed (< 60 km/h) in the cities or traffic jams.

In the city, the consumption is limited:

-for standard gasoline vehicle, the city consumption is 50 to 60 % more important than the mean consumption, -for standard diesel vehicle, the city consumption is 30 % more important than the mean consumption, -for Toyota Prius, the city consumption is 20 % more important than the mean consumption,

Power electronic evolution between first generation and 2004 version

The first Toyota Prius has limited driving performances compared to the 2004 version. This high improvement is due to an electric architecture modification [START_REF]Development of Hybrid Electric Drive System Using a Boost Converter[END_REF].

In the first Toyota Hybrid System (THS) the battery is directly linked to the high voltage DC network between the generator inverter and the motor inverter. With this electrical architecture, the motor is triangle coupling because of the low level of voltage, and torque is limited by the maximum current of the inverter. The current of each phase of the inverter is separated between two windings. The figure 9 presents the two electrical architectures.

Figure 9

The architecture of the 2004 version is THS II. An additional converter is used between the high voltage battery and the DC bus between the generator inverter and the motor inverter. In this case, the voltage can goes up to 500 volts. With this voltage, the same motor can be connected in star coupling. The same inverter, with the same maximum current, supplies only one motor winding per phase. The maximum torque of the motor is improved from 300 Nm up to 400 Nm. The maximum power is improved from 33 kW up to 50 kW [START_REF][END_REF].

Figure 10

The voltage of the DC bus is variable between 200 Volts and 500 Volts. The voltage is proportional to the vehicle speed in stabilized mode and goes to 500 Volts in transient mode.

A 1.2 kWh battery in microcycle mode

In the Toyota PRIUS, the battery is used only in some very specific conditions. This strategy is due to cost and reliability aspects. In this part, we will describe these points. The Toyota Prius battery is from NimH technology. The energy storage is around 1.2 kWh. The elements are small ones with a capacity of 6.5 Ah and the individual voltage is 7.2 Volts. Each element has 6 cells in series. In the battery, 28 elements are connected in series, so the nominal voltage is 201 Volts. In the first version of Toyota Prius, the elements are tubular ones. In the 2004 version the elements are prismatic ones.

Figure 11

This 2004 version is very well optimized for thermal dissipation and temperature homogeneity. Air conditioning from the vehicle is used and spreads in parallel all the elements. Between the elements, there are only small spaces for air with corrugated surfaces. All this points are adapted for very good air exchange, with high speed of air and a lot of turbulences. The electrical technical points are: -the NimH battery stores a maximum energy of 1.2 kWh, -to provide a long life, the mean state of charge of the battery is 60%, far from the extremes, -a state of charge windows of 20% is allowed around the mean value of 60%. This windows amplitude is 240 Wh.

-In most cases, the real cycle amplitude is only 5% in the windows of 20%. So, most cycles have amplitude of only 60 Wh. The specified maximum power for the battery is 21 kW. For this level of power, the 5% cycle correspond to 15 s of range.

Microcycles: Battery use cost is divided by five

The use of a battery has a cost which is the investment cost divided by the number of cycles and the cycle amplitude. In most application, this cost is very high compared to the cost of the electricity. For example, in photovoltaic application isolated from the main, for 30 years, the cost of the lead acid battery replacement is as important as the cost of photovoltaic modules. For 30 years, the cost of electricity is 0.5 € per kWh. The cost of the storage is about another 0.5 € per kWh.

For lead acid battery the cost of the battery use is around 0.5 €/KWh. This cost is approximately the same for high quality tubular lead acid battery for telecom or photovoltaic applications and for lower cost but lower number of cycle lead acid battery for starting vehicle. In this application the cycle amplitude may be maximum, around 80% and more, depending from the solar resource.

For the NimH Toyota Prius, with high cycle amplitudes battery, the cost of use should be around 1.5 €/kWh (1500 €/KWh and 1000 cycles). This cost is prohibitive compared to energy provided by gasoline and efficiency of engine.

A good order of magnitude for mechanical or electrical energy provided by an engine supplied by a fuel is 0.5€/kWh (For example, gasoline to 1.4 €/liter, 9kWh/liter and a 30% efficiency for the engine). This value is continuously changing because of oil price variation.

For the NimH Toyota Prius battery with high cycle amplitudes the cost of use should be three times the cost of gasoline use. This cost is prohibitive. With this level of cost a plug-in Prius is not competitive.

With a cost of use of 0.5 €/kWh, lead acid batteries are competitive. This technology is often used in small electric vehicle. The battery strategy in the Toyota PRIUS is to be around 60% of state of charge (far from the extremes to provide long life time) in a window of 20 % of variation, and a great number of real cycles around 5%. It's important to know, the real cost of use with this strategy. The battery supplier's specifications are 100,000 cycles for 5% of state of charge variation. With such cycles the cost of battery use is now: 1500 / 100,000 x 0.5 = 0.3 €/kWh. In microcycle mode, the cost of storage use is divided by five. This cost is competitive compared to the energy provided by gasoline and efficiency of the engine.

The global battery energy is 1.2 KWh. A cycle of 5% is only 60 Wh, so the battery uses are limited to:

-acceleration assist (with high torque and high electrical power), -energy saving in decelerations and braking, -electric mode for short distances, -sequence of electric mode for short distances on the battery, and recharge by restarting the engine for some tens of seconds, and again engine stop for pure electric mode, and so on. This sequence is used at low speed (< 60 km/h) in the cities or traffic jams. But the energy is too limited so there are:

-almost no energy saving in downhill, only 5% or 2 or 3 cycles of 5% maximum, -no assistance for long uphill. To provide the power, the engine rotation rate is high, and the efficiency decreases. There is a lot of noise compared with the standard driving conditions.

-no plug-in.

Because of the very low level of energy, plugin has no sense.

In case of additional battery for plug-in in modified vehicles, the battery cycles are high and the battery use cost is five time higher. With the indicated costs, the plug-in vehicle is not competitive. But:

-Because of Toyota Prius influence, the high energy batteries are produced in high volume. One million of vehicles were produced and this only battery is one half of the all NimH world market.

-New technologies of batteries are emerging, with lower material costs than NimH. In NimH batteries, Lanthanum is often used in hydrides and is expensive. For example, the components of Iron phosphate lithium ion batteries are less expensive.

The cost of battery use is not competitive now, but the boundary is near.

Conclusion

The first key point of the Toyota PRIUS concept is a high efficiency controlled variable transmission.

The second one is that this CVT allows using the gasoline engine only in the high efficiency zone at high charge (torque).

The third key point is the possibly to provide acceleration without delay by using the electrical motor and additional power provided by the battery. This possibility is due to the electrical motor directly linked to the wheels and a high power battery used for additional power for a short time. The fourth one is the microcycle mode. The cost of storage use is divided by five (0.3 € compared to 1.5 €) and is competitive compared to energy provided by gasoline and engine use.

In microcycle mode, the level of energy is very low, around 60 Wh. It's only compatible with acceleration assist, energy saving in decelerations and braking, electric mode for short distances, and sequence of electric mode for short distances on the battery, and recharge by restarting the engine. This sequence is used at low speed (< 60 km/h) in the cities or traffic jams. But the energy is too limited so there is almost no energy saving in downhill, no assistance for long uphill, and no plug-in.

With the Toyota PRIUS concept, city overconsumption is limited compared with diesel and gasoline engine, with a very low level of pollution. For plug-in vehicles, the cost of battery use is not competitive now, but the boundary is near. Quantity production, competition between manufacturers and higher number of cycles in real use are the key factors to have a competitive cost of battery use. For us, Iron phosphate Lithium ion batteries technology can be a good candidate for cost and number of cycle points of view. This technology provides a high level of intrinsic security.

Literature


Bruno Béranger

Daniel Chatroux

Eric Fernandez

Sébastien Fiette

Experience feedback on electric vehicles of the French car fleetbattery impact

Nearly 10.000 electric vehicles have emerged from the French car fleet during these 15 last years. The presentation details the accumulated experience on these vehicles, to highlight the technical points that limited the performances and to analyze the technological accessible progress. The reliability weakness of the batteries, and of course its cost effect, is the main problem that has been met with this kind of vehicles. The initial expensive price of batteries, combined with poor cycles endurance, induce a kilometric cost two to three times higher than the one of traditional gasoline vehicles. Some problems on the electronics and on the traction motor were also noticed. In CEA-LITEN the native Ni-Cd batteries of a Citroën AX was replaced with high security Lithium Iron Phosphate batteries.

Introduction

In CEA-LITEN, the LPAC laboratory has two activities. Firstly, it develops fuel cell systems for automotive applications, off-road ones and stationary gensets. The second activity of the laboratory concerns electric vehicles, with monitoring actions of existing electric or hybrid vehicles and development of battery stacks or power electronics for electric vehicles [START_REF]Experience feedback on electric vehicles of the French car fleet[END_REF]. In France, the 10.000 electric vehicles were produced. Their conception results from prototypes born fifteen to twenty years ago. Now, electric car production from all French manufacturers has stopped, and only a few thousands of them remain in circulation. Many electric cars have been destroyed and others could be found outside France, especially in Scandinavians countries. The aim of this paper is to detail the accumulated experience feedback on these vehicles, to highlight the technical points that limited the performances of these vehicles and to analyze the technological changes that make it possible to minimize these limitations.

Main vehicles and their characteristics

The two most famous French manufacturers have produced several series of electric vehicles since 1990: Express, Clio, Kangoo, AX, Saxo, Berlingo, 106, Partner. These vehicles are all based on Ni-Cd batteries technology coming from the same supplier (SAFT). Some are air-cooled, others are watercooled.

The motors are all D.C. current type with separated excitement, except for the Kangoo, which is driven by a wound rotor synchronous motor (separate excitation).

Electronic control units for PSA vehicles

All electronics devices of the vehicle are gathered in an aluminium box cooled by water (cold plate on the lower face).

Characteristics of the 12 V converter:

-Input voltage 120 V or 162 V.

-Output voltage 14.1 V.

-Output current: 70 A maximum.

The load of the battery 12 V occurs in the following cases: ignition key in position APC or battery of traction in load or voltage of battery < 11 V. Technology: Mosfets transistors; galvanic insulation.

Characteristics of the traction battery charger:

-Input voltage: mains 230 V 50 Hz -Maximum input current: 10, 13 or 14 A RMS configurable with the diagnostic tools ("Elit" at Citroen).

-Correction of the power-factor.

-Output current controlled: 0, 5, 10 or 15 A approximately according to the programmed loads.

The technology is traditional for a PFC charger of this power: an elevator "boost" including the function of correction of the power-factor (PFC), fol-lowed by a DC/DC conversion with galvanic insulation realized with an "H bridge". The semiconductors selected are Mosfets (figure 2). The computer generates a PWM signal to control the charge of the battery according to the programmed loads.

Characteristics of the motor converter:

The driving information of the vehicle is exchanged between the calculator and the electronic card of the converter. Measurements and the control of the main currents and of the excitement are done on this card. A medium power converter drives the excitement of the motor. This converter with two 20 A rated relays allows choosing the rotational speed of the motor and its direction.

A high power converter controls the current in the rotor of the motor for the acceleration and braking phases.

The control strategy of the currents in the rotor and the inductor depends on the rotational speed of the motor: -In acceleration phase from 0 to 1 600 rpm, the excitement current is fixed at 11 A. The armature current is cut out so that its mean value can vary between 0 and 200 A according to the position of the accelerator.

-In phase of acceleration from 1 600 to 5 500 rpm, the full voltage of the battery is applied to the armature while the excitement current is cut out so that its mean value varies from 11 to 1.2 A according to desired speed.

Deceleration is managed in the following way:

-From 5 500 rpm to 1 600 rpm the excitement current grows from 1.2 to 11 A and the battery is charged by currents varying respectively from 165 A at 5 500 rpm to 75 A at 1 600 rpm (accelerator pedal completely slackened).

-From 1.600 rpm to 0 the braking converter raises the voltage in order to keep on recharging the battery with 75 A at 1 600 rpm towards 0 at null speed.

-The action on the brake functions differently: the battery is charged at 75 A from 1 600 rpm down to 300 rpm.

Technology of the converter:

The switching function for the current of traction and braking is entrusted to an IGBT module (Insulated Gate Bipolar Transistor) Toshiba 400 A/600 V. This component integrates two IGBT (arm of bridge), each one with a diode in anti parallel, noted T, F, DF and DT on figure 6. One of the components provides the function of buck converter (acceleration at low speed), the other makes it possible to carry out the boost converter function (deceleration at low speed).

The switching function of the excitement current is done by an IBGT of smaller power (case TO 220).

Fig. 3. : synoptic diagram of the converter

A contactor opening on two poles ensures galvanic insulation between the battery of traction and the electronics of motor control (figure 3).

Reliability problems

Power electronics problems

Generally, the thermal aspects of power electronics were not sufficiently taken into account in the design of these power electronics. For example on PSA vehicles, the weak point is the thermal management of the battery charger which suffers particularly.

Electric motor problems

Information collected tends to show that there could have had manufacturing problems concerning the commutator of the traction motor. By warming up, the commutator would become deformed, causing sparks between the brushes and the commutator segments, involving the fast degradation of this one and a premature wear of the brushes. Thanks to the monitoring of electric vehicles, we also note that the motor works regularly beyond its range of nominal output power while the car is going up a slope. The armature of the engine is cooled by air flow. However, ventilation does not include any filter. That causes pollution of the commutator and brushes. Some defects of insulation may occur because of the graphite dust generated by the wear of the brushes and transported by the air flow of the cooling.

Batteries of traction problems

The batteries of traction used (open Ni-Cd technology) encounter various problems with a lot of different root causes. The comprehension of the causes of failure is difficult and dubious.

The experiment shows that some profiles of use make recurring breakdowns:

-The systematic recharge of the battery as soon as the vehicle is being parked, without taking into account energy gauge or the long periods of nonutilization, battery plugged for loading, causes the "metallization of the plates" followed by an internal short-circuit (figure 5).

-The high depth discharges of the batteries contributes to keeping a good range of the vehicle, but can lead to the destruction of the weakest elements. The monitored voltage of the whole battery (with a lighting indicator in the instrument panel when the recharge is necessary) does not make it possible to detect a block of 6 V which would be weak. Therefore its voltage may be reversed.

-The calculator (according to the number of overloaded energy) orders the frequency of water The filling of water every 3 000 to 4 000 km rather than 6 000 km is desirable. Some well used batteries of 2001 (regularly charged and discharged, used in rather fresh temperatures, frequent filling of water) allow, now, Berlingo to travel more than 200 000 kms.

Operating costs

Operating costs of the Electric Citroen Saxo

Energy cost

Taking into account the following input data: -Energy consumption: 21 kWh/100 km -Distance: 15 000 km/year (200x75km) -Electricity price in France, o "off-peak hours": 0.062€/kWh o "full hours": 0.103€/kWh -4 water filling a year: 250 € leads to an energy operating cost of:

-0.0297 €/km "off-peak hours" charges -0.0383 €/km "full hours" charges.

Saxo maintenance cost

Despite some common ideas, the maintenance of the electric Saxo is not cheaper than the maintenance of a similar gasoline vehicle, but this is due to some unsuitable technology choices:

-cooling pump has to be changed every 20 000 km -motor brushes have to be changed every 50 000 km The maintenance cost then reaches 1 400€/100 000km or 0.014€/km.

Battery lifetime cost without leasing agreement

An electric Saxo includes 20 Ni-Cd 6 V/100 Ah modules. The battery price reaches 7 000 € and the battery is specified for 120 000 km (1 500 cycles of 80 km). But, battery lifetime is in fact highly linked to how it is used and charged. A lot of batteries haven't even run more than 30 000 kms, whereas some others have run up to 200 000 kms. The overall costs of use are obviously radically different between these two extreme cases.

Assuming:

-7 000€ battery price -500 cycles of 70km (35 000km) lifetime due to the misused of the battery (long storage period battery at full charge, high operating temperature). leads to a lifetime cost of 0.2€/km and a lifetime of 28 months. The corresponding cost is: 1.4 €/kWh.

Assuming:

-7 000€ battery price -1 500 cycles of 70km (105 000km) lifetime thanks to optimal operating conditions (water filling frequency twice than the manufacturer specification, low temperature of use, normal use without too deep discharge, no storage at full charge). leads to a lifetime cost of 0.067 €/km and a lifetime of 7 years. The corresponding cost is: 0.47 €/kWh.

Battery lifetime cost with leasing agreement

The PSA maintenance leasing agreement of the batteries cost 1 980 €/year (including 4 water filling a year) Assuming 15 000 km/year and excluding the water filling annual cost lead to a battery lifetime cost of 0.115 €/km.

Total costs:

The total cost can be estimated with or without the PSA leasing agreement cost.

With the PSA leasing agreement the total cost is comprised between 0.159€/km and 0.168€/km. Without it, the total cost, including battery lifetime cost, is comprised between 0.111€/km and 0.252€/km.

Operating costs of traditional Diesel car

Energy cost An economic diesel car in urban cycle uses 5.5 L/100 km. Assuming a gas oil price of 1.4€/L, the energy cost reaches 0.077 €/km.

Car maintenance cost

For a Clio 1.5 DCI 80 the maintenance cost is roughly 1 200€ per 100 000km or 0.012 €/km.

Total cost

The total cost is 0.089€/km.

Cost conclusion

Energy cost of electric car is roughly half the energy cost of similar diesel car. Maintenance costs are close, but electrical car design can be easily improved to be very reliable. Battery lifetime cost is high and the range is very large depending of the battery use. Mainly, the total cost is two or three time more important than a diesel one. Only in the best case, the Saxo total cost and the diesel car cost are close. Battery lifetime improvements must be achieved in order to provide a more attractive total cost. (Notice that these results are related to energy, electric and petroleum, costs)

Results of monitoring from an original electric Citroën AX

The energy stored in the batteries of our AX in real use is 8.7 kWh instead of 12 kWh. The same ratio was found on the Kangoo, measured by the Idaho National Laboratory [START_REF]Hybrid Electric and Plug-in Hybrid Electric Vehicle Testing Activities[END_REF]. The efficiency of charging/discharging the batteries is good, up to 85 %.

The efficiency of the charger is only 80%. This value is low compared to current commercial products available.

The original vehicle AX includes 3 batteries packs:

• A rear pack with 11 modules (154 kg).

•

A lower front pack with 6 modules (84 kg).

•

An upper front pack with 3 modules (42 kg). The total mass of the batteries ready to use with water cooling and electrolyte is: 280 kg instead of 260 kg (the packaging is included).

The real energy/mass is: 31 Wh/kg. This value is very low but it includes all the battery system and the packaging.

CEA batteries developments with LiFePO4 technology

The CEA has been working for 8 years on Lithium Iron Phosphate battery technology. This technology has a good security level and its price is going down. The performances are very good for urban electric vehicles due to the low internal resistance and high number of cycles. The cost of wear may become as low as 0.3 €/kWh, which will be cheaper than the lead acid batteries technology.

In CEA Grenoble, the replacement of the original Ni-Cd batteries by lithium iron phosphate batteries has been successfully done on an electric vehicle demonstrator (Citroën AX which is one of the lightest vehicles).

The total weight of the new battery system is less than half of original one (138 kg). The AX mass is now 810 kg, about the same value as for a thermal engine version. The car gives now very good accelerations.

The energy stored is 10.5 kWh in only 2 packs instead of 3: -A rear pack (72 kg).

-A lower front pack (66 kg). The new energy/mass is: 76 Wh/kg, including all the battery system and the packaging.

The measurements of the new kilometric range of the car are in progress.

Conclusion

A new departure for the development of electric vehicles is update today.

The power electronics can be designed to ensure a high level of reliability.

The batteries remain the most important point and should be studied firstly to ensure a cost of stored/restored energy lower than 0.5 €/KWh, (to be competitive compared to the thermal vehicles) and a good security level. The purchase cost must be controlled; the number of cycle in real use (temperature range) must reach 1 000 to 2 000 cycles. The lithium iron phosphate technology battery which offers an intrinsic high level of safety and which exists industrially today is a good candidate for this need. In CEA, the replacement of the original Ni-Cd batteries by lithium phosphate batteries is done. The weight is now 810 kg compared to 950 kg initially. Weak range is not a blocking point for urban vehicles. Maximum speed is also sufficient (approximately 90 to 100 km/h). Therefore, a power of approximately 15 to 20 kW on a light vehicle is suitable.

Fig. 2 .

 2 Fig. 2. synoptic diagram of the battery charger of traction

Fig. 4 .

 4 Fig. 4. IGBT and polypropylene capacitor having overheated

  filling of the batteries. An open Ni-Cd Battery supports the overload quite well, but this causes the electrolysis of the water contained in the electrolyte. The requests for maintenance of the batteries (put out of water) by the calculator are too much spaced, and overheating of the emerged plates can then occur.

Fig. 5 .

 5 Fig. 5. : Degradation of the separators, follow-up of fusion.

current with separated excitation, reducing and differential by integrated epicyclical gear). The motors of the Express and Clio are also with D.C. current and separate excitation but the dif- ferential reducer is not integrated.

	2.	POWER TRAIN	Characteristics of the motors:
	2.1. Batteries
	Contrary to the other vehicles presented, Kangoo
	is driven by a three-phase wound rotor synchro-
	nous motor.
	The motor with D.C. current with brushes is sim-
	ple to control: the armature requires a simple re-
	versible DC/DC converter.
	An inverter drives the Kangoo synchronous mo-
	tors.
	For these two types of motors, a DC/DC con-
	verter regulates the excitement.

Batteries are common to PSA and Renault vehicles: open Ni-Cd technology requiring of the regular water handing-over (ideally all 3.000 to 4.000 km rather than 6.000 to 8.000 km such from the calculator of the vehicle). The express uses 18 modules 6 V/140 Ah cooled by air. Kangoo embarks 22 modules 6 V/100 Ah cooled by air. This air cooling appears insufficient and badly distributed, taking into account the level of the thermal losses. Clio is supplied with 19 modules 6 V/100 Ah, cooled by water with antifreeze agent. The Saxo/106 uses 20 modules 6 V/100 Ah, cooled by water with glycol. On Berlingo and Partner, the 27 batteries are cooled with water with glycol. Fig. 1. Ni-Cd battery module 2.2. Electrical traction motors PSA vehicles use motors of the same technology (D.C.

Vehicles / motors Type Power (kW) Nominal/ Maximum Torque (Nm)

	AX / Saxo / 106	DC	11/20	127
	Partner / Berlingo	DC	15/28	180
	Clio / Express	DC	16/21	125
	Kangoo	AC	15/30	165
	Vehicles / motors	Voltage (V)	Current (A) Nominal / Maximum	Speed (rpm)
	AX / Saxo / 106	120	110/200	6 700
	Partner / Berlingo	162	110/200	6 700
	Clio / Express	114	150/190	-
	Kangoo	132	110/220	9 500