EVS26Los Angeles, California, May 6 - 9, 2012

Development of a modular lithium-ion battery for asub-compact electric vehicle

Peter Burda1, Peter Keil2, Markus Lienkamp1, Andreas Jossen2

1Institute for Automotive Technology (TU Munchen), Boltzmannstr. 15, 85748 Garching, Germany, burda@tum.de2Institute for Electric Energy Storage Technology (TU Munchen), Arcisstr. 21, 80333 Munich, Germany, keil@tum.de

Abstract

The Technical University of Munich is currently developing the electric vehicle MUTE. MUTE is alight-weight sub-compact car, which has been presented at the Frankfurt Motor Show (IAA) 2011 as afully functional prototype. The concept of this electric vehicle is optimized to the demands of usage inurban areas or as a secondary car. This allows a reduction to a driving distance of 100 km with a 15kW traction system. Cost reduction as one key success criterion leads to an air-cooled battery with adownsized energy content of 10 kWh.Within our paper we want to give a detailed overview of MUTE’s design goals and the developmentprocess of MUTE’s modular lithium-ion battery system.After gathering relevant requirements for the battery system, the ideal lithium-ion cells were determined.Simultaneously to the from-scratch development of the vehicle, the battery system was constructed.Important design aspects were a durable and safe mechanical construction which is also suitable forseries production. A FMEA helped to identify weak spots early in the development process. An advancedbattery management and an optimized cooling/heating system for improved lifetime complete the energystorage system.The MUTE project presents an affordable and economical vehicle concept for a contemporary marketintroduction scenario.

Keywords: Battery, BEV, Thermal Management

1 Introduction

One of the biggest drawbacks of current electricvehicles is their very high purchase price. Beingtens of thousands Euros/Dollars higher than forcomparable conventional cars their market shareremains marginal. Most of the currently avail-able battery electric vehicles are highly expen-sive cars, e.g. Tesla Roadster, Mitsubishi iMievor Nissan Leaf, which are only sold in very smallnumbers.Surveys show, that customers have an in generalpositive attitude towards electric cars and are in-terested in drive one, but are not willing to paymore than 1000 EUR surcharge to an compara-ble gasoline powered car. [1, 3, 6]

Therefore it is obvious why current electric ve-hicle concepts are not able to reach high marketpenetration.Consequently, developing affordable vehicleconcepts is the key strategy to increase marketshares.

With the energy storage system being the mostexpensive part of an electric vehicle, small andlightweight cars are the most promising way toachieve electric mobility at affordable prices.Reducing weight helps to reduce the requiredengine power and to develop smaller propulsionand especially energy storage systems.

The Technical University of Munich is currentlydeveloping the electric vehicle MUTE (fig. 1)

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Figure 1: The MUTE BEV

which is a sub-compact car specially designedfor inner cities and suburban areas or as sec-ondary car for rural areas. Basic requirement forthe MUTE is to transport up to 2 persons and 2pieces of baggage at a range of 100 km and topspeed of 120 km/h.With a minimum travelling distance of 100 kmsuch a car still covers more than 90% of all tripsundertaken with a conventional car.The average daily range driven is below 40 km[7]. Fleet tests, e.g. with the Mini-E performedby BMW (fig. 2) or by the E-Flott project, per-formed at TUM, affirm this usage behaviour.

Figure 2: Results of the BMW Mini-E Fleet Test [2]

In Europe, there is the special vehicle admissioncategory L7E for compact lightweight vehicles.L7E has low demands on vehicle equipment andallows for high design freedom. Additionally,there are virtually no taxes and admission feescompared to the common vehicle category M1.However, L7E vehicles are limited to a tractionpower of 15 kW at the wheels and a maximumweight of 400 kg (batteries excluded).Calculations made for such a L7E vehicleshowed that 15 kW engine power is sufficient forthe demanded top speed and an acceptable ac-celeration of about 0.7 m/s, if the energy storagesystem does not add more than 100 kg of addi-tional weight. Further simulations resulted in ananticipated energy consumption of 6.6 kWh/100km for the New European Driving Cycle. Con-sidering a maximum depth of discharge of 80%and a remaining capacity of 80% at the end oflife of the battery pack, an initial energy contentof 10 kWh is required.Although L7E demands for no special safety re-quirements, except proper brake and illuminationsystems, MUTE is aimed to a safety level com-pared to a M1 car. For a vehicle that has the lookand feel of a fully equipped M1 car, customerswill absolutely not tolerate low levels of safety.[6]

According to the above mentioned requirements,a safe and cost-efficient lightweight battery sys-tem for the MUTE with an energy content of atleast 10 kWh had to be developed.

2 Cell SelectionThe development process of the battery sys-tem started with the selection of appropriatecells. In principal, lithium-ion cells of manydifferent shapes and chemistries are available.However, not every cell is suitable for usage inelectric vehicles. Many requirements like weightgoal, provided construction space and requiredamount of electric energy to reach the desireddistance restrict the set of usable cells.

Finally, more than 800 different cells wereevaluated regarding energy density, safety,availability, production quality and costs. Asvery limited information was available for mostcells, less than 100 cells remained available for awell-grounded decision.

Electric vehicles require battery cells with highenergy but low power demand. Such cells arealso used in consumer devices. They can befound as small cylindrical 18650 or small pouch-bag and prismatic cells. As the latter two areusually specifically designed for certain con-sumer products they are not available on freemarket. Large automotive cells are usually fo-cused towards higher power demands than theMUTE with its very small 15 kW electric en-gine. The good availability of 18650 cells fromvarious suppliers is a result of them being a well-established form factor for usage in laptops. Forthe MUTE project, the set of cells was soon re-duced to 18650 cells because of several reasons:Well-known and experienced suppliers with reli-able production are available for the 18650 celltype. A change of cell supplier can be performedquickly as the form factor of the cells remainsidentical. Moreover, future improvements in bat-tery chemistry can find their way into the bat-tery pack easily, as it is highly probable that thenew cells will also be available in 18650 stan-dard. Hence, current enhancements in lithium-ion technology may directly lead to further im-provements in power and energy density until apotential market entry of a MUTE-like vehicleas the battery system and vehicle concepts wouldnot need major changes.Usually, 18650 cells have disadvantages in max-imum energy density due to the cylindrical formwhich causes void volume when arranging mul-tiple cells to battery packs. This, however, couldbe neglected for the MUTE BEV, as an air-cooled battery pack is intended. Therefore, dis-tances between neighboring cell rows are neces-sary for air ducts which lead the cooling air flowthrough the cell array. Another major advantageof using small cells is that the available construc-tion space can be filled up very efficiently (fig.3). Often regarded as pricey because of the highamount of cells needed to be integrated, recent

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calculations e.g. by VW showed them as verycost effective solution [4, 5].

Figure 3: Cell Package-Alignment

Furthermore, 18650 cells show beneficial behav-ior in case of statistical failure or premature ca-pacity loss. As single cells only have a small ca-pacity the total loss of energy in a parallel con-nection is limited. Most of the energy content ofthe entire battery pack is still usable. In contrastto that, battery packs with large cells suffer muchmore from the failure of a single cell. In the ex-treme case, when there is only a connection ofcells in series and no connection in parallel, thefailure of one cell results in a complete failure ofthe battery pack. (fig. 4).

Figure 4: Impact of cell failure when 10% of the cellsare defective

The selection process led to three differentsuppliers of high energy cells of the cylindricaltype 18650. The final decision was made fora cell from a Japanese manufacturer with agravimetric energy density of 208 Wh/kg.

Cell safety tests, such as overcharge, nail pen-etration, short-circuit, drop test, performed withthis cell showed only minor spill of electrolyte.This led to a classification as Hazard Level 2which is convenient for the use in a prototype ve-hicle.

3 ConstructionPrimary focus of the development was a batterysystem which is well-adapted to the requirementsof a sub-compact car without compromises atsafety and reliability. The placement of lithium-ion batteries in a car is critical for crash safety.Moreover, the battery has to be protected againstenvironmental influences such as water and dirt.Regarding these requirements, the space behindthe passenger seats has turned out to be the bestlocation.Due to the small size and capacity of 18650cells, a large quantity of 1232 cells has to be in-terconnected. Therefore, a modular constructionwas desired, in which the battery is split intoseveral identical modules. Each module shouldhave a maximum voltage below 60 V, whichenables safe handling and improves servicingand exchangeability. This led to a set of elevenmodules connected in series.

The cells of each module are embedded in asystem of two supporting frames and spacingelements between the cells. The well-definedseparation minimizes thermal interaction in caseof failure of single cells and allows for the cool-ing air to flow around the cells. The orientationof the modules in the car is chosen in a waythat vibrational loads onto the welding spotsduring driving are minimized. The parts and theassembly process of the modules are optimizedfor a simple construction as a prototype as wellas cost effective large-scale manufacturing.For the prototype, all parts can be producedby simple milling and assembled with manystandard parts. For series production the partscould be produced by injection die molding andoptimized for a less extensive assembly.

Figure 5: Module Assembly

A design FMEA on module and system levelallowed for identification and correction ofweak spots early in the design process. Weak-nesses of the current flow through the terminalcontacting of the module were identified inexpert interviews. The improved contacting forthe battery module is shown in figure 6. Anelectric connector is clamped onto the hiluminconnectors along the complete module width.It is also integrated into the module’s cover.

EVS26 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 3

Adequate current distribution and low transferresistances are expected of this solution.

Figure 6: Electric Contact

The finished modules can be simply inserted intothe outer casing, as figure 7 shows exemplary forthe first of the eleven modules.

Figure 7: Pack Assembly

There is no additional fixation needed, as the cov-ering lid of the casing and spacers keep hold ofthe modules. The air distribution channels (fig.8) inside the lid, which are able to resist high tor-sion, guarantee for the required firmness.Using air distribution channels as an integral partof the battery casing to retain movements of themodules in the z-direction of the vehicle, weightfor additional module fixation can be saved.

Figure 8: Sectional View

In a last step the air-cooling system is attachedto the battery pack. The air ducts lead cooling

or heating air to the eleven modules. The fullyassembled system is shown in figure 9.

Figure 9: The complete MUTE Battery

Positioning all high voltage components, such asbattery, engine, power electronics and charger inthe rear leads not only to a perfect weight dis-tribution with 60% of the overall weight on thepowered axle, but also good EMC is expectedas all major 12V systems and controllers are lo-cated in maximum distance at the front space ofthe car.

4 Battery Management

To guarantee safe operation of the lithium-ionbatteries a battery management system (BMS)is essential. Its task is to continuously monitorvoltages, currents and temperatures inside the ebattery. In case of overvoltage, undervoltage aswell as too high currents or temperature values,the BMS requests the load to be reduced or shutsdown the battery pack in case of emergency.The BMS (fig. 10) is also required for state ofcharge determination in order to display the re-maining energy and control cell balancing mech-anisms during the charging process.

Figure 10: The Battery Management System

For MUTE, a BMS in master-slave topology withone slave for every battery module has been de-veloped.By using powerful processing units on eachslave, cell-oriented state-estimation based on ad-vanced mathematical methods (e.g. KalmanFilters) can be processed decentralized on theslaves. In comparison to conventional systems,

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where all calculations have to be performed cen-trally on the BMS master this allows an func-tional separation between state and parameter es-timation algorithms on the slaves and safety rel-evant functions performed on the master board.As lithium-ion batteries are strongly dependenton temperature, a thermal management is alsoneccesary. At very low temperature performancedecreases significantly, at high temperatures ag-ing of the cells is increased, leading to losses incapacity. To ensure uniform aging of all cellsa homogeneous temperature distribution insidethe battery is important. Therefore, a countercurrent cooling architecture (fig. 11) is used toreduce temperature gradients inside the batterymodules. The cooling system is also used for pre-heating the battery in winter through air warmedby a bioethanol heating system. This enables forhigher performance and greater distances even atlow temperatures.

Figure 11: Counter Current Cooling

Simulations (FEM, CFD) (fig. 12) and measure-ments show the expected improved temperaturedistribution in the battery modules.

Figure 12: Thermal Simulation

5 ConclusionWithin the MUTE project a modular batterysystem has been developed which is speciallyadapted to the requirements of a lightweight sub-compact car with reduced energy consumption.Although cost and weight reduction played animportant role in the development process, no

compromises on safety and durability were ac-cepted. This can be observed throughout thewhole development process of the battery sys-tem: Reliable lithium-ion cells combined with asafe mechanical integration into the vehicle andan optimized heating/cooling system are the ba-sis for a comfortable, robust and long-lasting op-eration of the entire battery system.Through using specific lithium ion cells best fit-ting to the characteristic demands of a small citycar on the one hand and a concurrent develop-ment of vehicle and battery pack on the otherhand, a small and cost efficient battery pack wasdeveloped and built-up.This allows for a gravimetric energy density ofmore than 130 Wh/kg (fig. 13) on system leveleven for the prototype battery pack which pro-vides an energy amount of 12.3 Wh at a weightof 94 kg.

Figure 13: Battery Weight

This battery system demonstrates that affordableelectric mobility is already possible today.The MUTE BEV was successfully presented atthe 2011 International Automotive Roadshow(IAA) in Frankfurt.

More information on the MUTE is available athttp://www.mute-automobile.de/

Acknowledgments

Acknowledgements go to all students adding tothe success of the MUTE project, especially toTobias Pletzer, Josef Eggersdorfer, Tobias Al-brecht, Matthias Kerler, Michael Huber, Mar-tin Rodriguez, Daniel Mogele and Georg Waldercontributing to the development of it’s energystorage system.

References

[1] GfK Group, Germany - High propensity to buyelectric cars, Press Release: 2010-09-21.

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[2] Klimaentlastung durch den Einsatz erneuer-barer Energien im Zusammenwirken mit emis-sionsfreien Elektrofahrzeugen, BMW AG finalreport, http://tinyurl.com/834mjzo,p.39, 2011

[3] Barkawi Management Consultants, http://tinyurl.com/6qa5kur, accessed on2012-01-23.

[4] Financial Times Deutschland, http://tinyurl.com/FTD-18650, accessedon 2011-11-20.

[5] SAE International, http://ev.sae.org/article/8645, accessed on 2012-02-08.

[6] Kooperative Konfrontation: E-Mobility ausMarkt- und Entwicklersicht, ATZ, 2011.

[7] Mobility in Germany, infas, DLR, sum-mary report p.4, http://tinyurl.com/88m3l6u, 2008.

Authors

Dipl.-Ing. Peter Burdais a research associate at the in-stitute for automotive technologyat TU Munchen since 2010 andwas responsible for the powertrainand developer of the energy stor-age system of the MUTE electricvehicle. Currently he works as re-search associate in the field of en-ergy storage engineering for TUMCREATE in Singapore.

Dipl.-Ing. Peter Keilis a research associate at the in-stitute for electrical energy storagetechnology at TU Munchen since2010. Main activities are electricand thermal simulation of lithium-ion batteries. He is member of theenergy storage system team of theMUTE electric vehicle.

Prof. Dr.-Ing. MarkusLienkampis head of the institute for automo-tive technology since 2009. Afterhis study and PhD at the Universityof Darmstadt he joined VW, wherehe hold different positions from ve-hicle dynamics to department man-ager of electronics and vehicle re-search in the end.He initiated the MUTE BEVproject, successfully developed ina cooperation of 21 of TUM’s in-stitutes and displayed at the Frank-furt Motor Show (IAA) 2011.

Prof. Dr.-Ing. Andreas Jossenearned his doctorate, dealing with”Management of photo-voltaicplants using energy storage sys-tems” at University of Stuttgart.From 1994 he was group leaderfor different battery related topicswith ZSW, Ulm. Since 2010 heis full professor at the Institutefor Electrical Energy StorageTechnology, TUM.

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