HV Traction Battery: From Layout to Realization As especially public transport is a promising sector for electric mobility, technical questions such ... simulation is 77 kWh, ... underﬂoor

EVS26Los Angeles, California, May 6 - 9, 2012

HV Traction Battery: From Layout to Realization

S. Rothgang1, B. Lunz1, I. Laresgoiti1, G. Geulen2, J. Homann2, F. Topler2, S. Gehrmann3, L.Eckstein2, and D. U. Sauer1

1Chair for Electrochemical Energy Conversion and Storage Systems, Institute for Power Electronics andElectrical Drives (ISEA), RWTH Aachen University; Jaegerstrasse 17-19, D-52066 Aachen, Germany;

[email protected] for Automotive Engineering, RWTH Aachen University

3Air Energy GmbH und Co. KG

Abstract

From system layout to the vehicle integration a lot of design steps have to be taken into account regardinghigh-voltage (HV) batteries for electric vehicles. Electrical performance has to be traded off againstmechanical and thermal issues several times during development. Besides the physical setup also themanagement of the battery system has to be considered and adapted to the chosen system layout, whichleads among others to an optimized amount of sensors that are needed to ensure the safety of the system.As especially public transport is a promising sector for electric mobility, technical questions such as thethermal management and cell contacting shall be considered and shown by the example of a battery fora full-electric bus in the pilot-project “Smart Wheels”, supported by the German Ministry of Economicsand Technology (BMWi). Within the project the battery system as well as a fast charge concept for thewhole day operation of the vehicle were developed and taken into operation. This work aims to give anoverview of the full development cycle in the design of a new traction battery including test results ofthe prototype and solutions for technical questions such as the thermal management.

Keywords: battery, BEV, fast charge, public transport, thermal management

1 Introduction

Currently the industrial nations are responsiblefor around 60% of the worldwide CO2 emis-sions. The overall CO2 emissions in 2010 were30.6·109 tons. The combined share of electricity,heat generation and transport represents nearlytwothirds of the yearly global emissions withtransport having a share of 23% in 2009 [1]. Withthe ongoing industrialisation of the developingcountries and the related urban megacities andincreased energy needs, the impact of these sec-tors and especially the transportation will evenincrease. Hence, it is especially important to re-duce the emissions within these growing living

areas which can be achieved by locally emission-free individual transport. Benefits like using thebus lanes or paying less city toll are already pro-vided by several cities like London to encouragepeople to drive electric cars and electric busesare mostly allowed to enter traffic-calmed innercities. In addition to it, a global emission freetransport could be realized using renewable ener-gies only.Nevertheless, only reducing emissions won’t lastas only justification for electric mobility as theecological advantage over diesel becomes lessimportant with more severe emission standardsas EURO VI being introduced by 2013. Thesestrict standards lead on the other hand to high ef-forts and costs for the car manufacturers to match

EVS26 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 1

World Electric Vehicle Journal Vol. 5 - ISSN 2032-6653 - © 2012 WEVA Page 0350

the regulations for diesel propulsion that mightclose the financial gap to electric vehicles. Thisfinancial aspect is reinforced by the rising oilprice. With 107 $ per barrel as an annual aver-age in 2011 the oil price reached the highest levelever [2].As the driving profiles, ranges and route charac-teristics of a public bus are known exactly anda high mileage is ensured, electrification is espe-cially promising and might offer chances to intro-duce electric mobility in the market. Moreover,it is possible to charge only at defined placeswithout a need for a highly comprehensive publiccharging infrastructure. Hence, a roll out is pos-sible relatively independent from infrastructuralcircumstances. To enable cost effectiveness it iscrucial to guarantee a high number of operationhours and to optimize the battery in terms of op-eration and lifetime as it is the cost-dominatingcomponent in electric vehicles.Based on the project “Smart Wheels” this paperdiscusses the full development cycle of a batteryfor a public bus regarding the reliability in op-eration and an optimal battery size in terms oflifetime, vehicle costs and operability. Moreover,a concept for the thermal management as crucialpoint for the aging of the cells and the operabilityof the battery is addressed.

2 Design Steps

The design of a large scale battery can be dividedin three fundamental development steps, the de-mand analysis, the system design as well as thesystem production. During the first stage the di-mensioning of the system as well as the batterytechnology and the cell types are addressed. Inthe following the system layout focuses on theoverall design of the battery. Important steps asshown later in this work are the thermal manage-ment of the battery, the contacting of the cellsas well as the mechanical stability and the crashsafety. Moreover, the precharge of the interme-diate circuit has to be considered and a parame-terized battery management is needed. In the laststage the system is build-up and tested. Duringall steps safety and conformity to the regulationshave to be assured.

2.1 Demand Analysis

Lithium ion battery cells are the most promis-ing technology for electric mobility as they com-bine a high energy density with a high efficiencywithin certain limits. Therefore, cells with ahigh energy density are chosen for battery elec-tric vehicles (BEV) and plug-in hybrid vehicles(PHEV) whereas high power cells are chosen forhybrid electric vehicles (HEV) or – dependingon the needed C rate – for fast charge applica-tions. Moreover, with around 5,000 full cycleslithium ion cells have a cycle stability superiorto other battery technologies such as lead acidor nickel metal hydride. Nowadays, lithium ioncells are one of the most expensive battery tech-nology and are responsible for 50 - 65% of the

total battery pack costs, but due to mass produc-tion large economy of scale effects are expected.Therefore, it is estimated that the costs for nickeloxide (NCA) cells will decline by 60% to 270 -330 $/kWh [3].Regarding the typical driving range of a publicbus, an approximate range of 95 miles is needed.With 93 miles on an Aachen inner city line thisis also true for the project Smart Wheels. There-fore, the energy consumption according to simu-lations would be around 120 kWh which wouldlead to a battery size of at least 850 l and a weightof 1,100 kg. As the specific vehicle in the projectis a small inner city bus with an admissible totalweight of only 5650 kg the battery itself wouldreduce the payload tremendously. Hence, re-energizing during operation is necessary by allmeans. To reduce weight and costs of the systemit was decided to limit the energy content of thebattery to 45 kWh and to fast charge in betweenthe rounds with a 60 kW charger. Two differentcharging strategies can be considered:

• discharging the whole battery and recharg-ing via fast charge in a longer break or

• partly discharging the system and recharg-ing every other round.

Deciding on the strategy, aging has to be takeninto account. It can be divided between calen-dric aging and aging due to cycling whereas bothlead to an increasing internal resistance and a de-creasing available capacity. Calendric aging ef-fects are parasitic side reactions in the cell thatare strongly dependent on cell voltage and tem-perature. Cycling has an impact on lifetime dueto voltage displacement, temperature rise andmechanical stress inside the cell [4]. Mechan-ical stress is caused by volume changes of theactive material due to lithium (de-)intercalationand therefore dependent on the cycle amplitudes.Deeper cycles lead to a stronger aging. Hence,a reduction of the used capacity range by charg-ing more often can limit aging albeit the over-all charging time per day remaining the same[5]. Therefore, it was decided to charge the bat-tery every other round for maximum 15 minuteswhich leads to a stepwise declining SOC duringthe day as shown exemplarily by the simulationin Fig. 1. Even though the overall energy con-sumption in the simulation is 77 kWh, the sin-gle cycles are only 15.4 kWh and the maximumoverall discharge amplitude is limited.

SO

C[%

]

2 4 6 8 10 12

20

40

60

80

100

0t [h]

Figure 1: Simulation of a full day driving cycle witha fast charge every other round



Due to safety reasons, it is essential to ensure afailure-free operation of the battery in mobile ap-plications. A shutdown of the battery system isthe ultimate possibility that should only occur aslast choice, as it might happen in a crucial driv-ing situation. Examples for that are turning left,crossing railroad tracks or when the active mo-ment distribution is activated. In these situationsa shutdown of the battery might lead to a highersafety risk as the fault in the battery itself. There-fore, it was decided to divide the battery in twopacks that are directly in parallel to minimize therisk of complete failure as an unmanageable faultwould have to occur in both packs at the sametime.Regarding the axle weight distribution and theavailable constructed space it was decided to takepouch bag cells as they have the best energy tovolume ratio. Due to the vehicle requirementsa working voltage between 200 - 450 V wasneeded. That leads to a series connection of 98cells with a nominal voltage of 3.7 V. To achievethe desired energy content a capacity of at least60 Ah per battery pack was needed. Due to therelatively high load during the quick charge, ahigh power cell was favored. The agglomera-tion of the cumulative decision criteria broughtforth the choice of the SLPB110255255H, a60 Ah high energy cell of Kokam [6].

2.2 System Design

After having chosen a cell type and decided onthe general system layout in the demand analysis,the system design addresses the specific layoutof the battery packs including the cell contactingand the overall mechanical setup as well as thebattery management system.For the housing of the battery it is essential totake the mounting position into account. On theone hand cells have to be protected against me-chanical impact during a crash by all means, onthe other hand it has to be guaranteed that thebattery packs don’t cause further damage in thepassenger cabin. Moreover, the weight distribu-tion of the vehicle before the conversion to elec-tric propulsion has to be considered as it shouldonly be changed within certain limits. This leadsto the conclusion that the packs can either be in-stalled on the underfloor or on top of the vehicle,with the latter affording the bigger changes in thevehicle structure. In the approached city sprinteran underfloor-installation outside the crashzonesof the vehicle behind the two axes was cho-sen. This limited the available battery space to870 x 580 x 360 mm each and required strongvehicle-mounted ground plates to protect the bat-tery against damage due to e.g. speed bumps.The housing itself consists of 4 mm thick alu-minum U-profiles and a ground plate that is alsoused as cooling plate as shown later in this work.For waterproofness the housing is welded and thelid sealed with a gasket and it is made sure thataccording to IP 4X no contact to active parts ispossible to protect people against electric shock[7]. Therefore a specified HV-plug for the auto-motive sector was needed.

This completely sealed housing would also pre-vent the battery from emitting gas. Cell failuresas a thermal runaway normally go along with anoverpressure within the cell. Due to that over-pressure pouch bag cells vent as the vacuumedfoil opens next to the cell contacts. An 18650cell of only around 2.6 Ah emits approximately2.5 l of gas [8]. Therefore, it is essential to adda pressure vent in the housing of the battery aswell. The pressure vent consists of a membranethat is jammed in between two aluminum plateswhich are screwed to the housing, as shown inFig. 2. Test results show that the membrane isblown out by a pressure of 1.0-1.2 bar.

Figure 2: Front of the housing with the IP compatibleHV-plug and the vent at the right hand side

Having the highest energy density, pouch bagcells on the other hand go along with the dis-advantage of the lowest mechanical stability. Toprevent the cells from damage, they have to bestabilized in the designated position. Therefore,the pack was divided into 8 equal sections by alu-minum partition walls. The 98 cells were dividedin 7 modules of 14 cells each, the remaining com-partment houses the switch box and the electron-ics.In each module the cells are interconnected alter-nating by two copper plates that fixate the con-tacts with screws in between. The copper platesare constricted with a rubber strip that fixates thescrews and ensures insulation. The modules werewrapped in rigid foams to pad the cells and to en-able a slight expanding of the cells. It is essen-tial to use only self-extinguishing materials in-side the pack to prevent a single cell fault to in-flame further cells. The final layer of the modulesshown in Fig. 3 is Teflon foil as insulation. Thesections itself are also lined with Teflon. This in-sulation is important, as the constant vibration ina vehicle might lead to a fraying of the cell foiland consequently to an insulation fault.The bottom area of the sections is coated with asoft silicone foil. The material has a high flex-ibility to enhance the connection to the coolingsystem and ensure at the same time an outstand-ing electrical insulation. At this juncture a tradeoff has to be found as good electrical insulationnormally does not go along with a good ther-mal conductivity. However, the insulation is ofparticular importance in that case, as there are– as explained hereafter in section 4.2 – alu-minum plates installed in between the cells dueto cooling. A direct contact of these plates to the



ground plate would basically equal a huge capac-itor which has to be prevented in the vehicle dueto EMC issues.The battery is monitored by the battery manage-ment system of Air Energy, that is divided in amaster-slave architecture. Each module is moni-tored by a slave that interacts with the joint mas-ter. A joint master is necessary as the two packsare directly parallel-connected and thus the in-terconnecting of the packs has to be prevented ifthey have a different voltage level. Different volt-age levels can occur due to e.g. self dischargeafter a longer downtime and would lead to inad-missible high compensating currents. The mea-sured voltages and temperatures of each cell areA/D converted at the slaves and sent to the mas-ter. The master controls the precharge and inter-connecting of the packs and sets, as the commu-nication interface to the vehicle, the maximumload levels for the charge or discharge procedure.The master controls the adherence of the thresh-old values and can either derate the load or shut-down one of the packs. As a fall back level for amalfunctioning master, the slaves can shutdownthe pack directly via a safety circuit if a sec-ond threshold is exceeded. As already the A/Dconversion for the safety circuit is done indepen-dently a complete redundancy is given.

Figure 3: Module of 14 cells with the contactsscrewed together and a Teflon insulation (constructedby Air Energy)

The battery system can be disconnected from therest of the drive train by relays at both poles.The relays are normally open which is the safestate of the system. During driving operationthey have to be held shut actively. Addition-ally to the switches the battery pack is protectedagainst overcurrent by two DC-fuses and thepack current is measured. Those melting fuseswith a nominal value of 200 A ensure a protec-tion against external short-circuits.

While the front battery pack contains the masteras shown in Fig. 4, the rear pack houses the in-sulation monitoring device that can monitor thecomplete electric drive train. To enable drivingoperation with a shutdown rear pack, the insu-lation monitoring device has to be placed at thedrive train side of the relays. This requirement ismotivated by the high risk that goes along with acomplete system shutdown due to the particulardriving situation.

+ -

98 series-connected cells

98 series-connected cells

98 temperature and voltage sensors

98 temperature and voltage sensors

7 slave BMS

7 slave BMS

Insulationmonitor

Pack 2

Switches, fuses, Switches, fuses,

Switches, fuses, Switches, fuses,

current sensor current sensor

current sensor current sensor

MasterBMS

Pack 1

Figure 4: Schematic of the system layout

Regarding the voltage measurement, it is indis-pensable to monitor every single cell voltage ina series connection. Overcharging a cell is oneof the highest safety risks as it might lead toa thermal runaway. A thermal runaway is anexothermal process that is initiated by a triggerevent like e. g. an overcharge or a deep dis-charge. The evaporated heat in turn acceleratesthe exothermal process due to the decompositionof the active material. Hence, the highest volt-age in the string limits the charging and the otherway round. Voltage inequalities can be causedby aging or different temperatures and due to thisdifferent internal resistances of the cells. There-fore, it is important to balance the cells, which isrealized by a passive balancing system.Additionally to the cell voltages the cell tem-peratures were measured by a temperature sen-sor in the area in between the positive and nega-tive contact. Measurements have shown, that thewarmest spot of the cell is in the upper cell areaon the middle axis. By installing a sensor at thatposition in the module and not directly on a cellsurface, each cell temperature is measured twicewith a slight time delay and therefore a redun-dancy against a sensor failure is given.As the pack was produced for a prototype bus andacademic research it was of great interest to mon-itor all cell temperatures in detail. However, in-stalling a sensor per cell would be too expensivefor series production. Therefore, it was analyzed



what aftermaths a single cell fault has and howmany sensors are really needed to detect it. Forthis purpose a module was simulated with Com-sol Multiphysics. In the simulations it was esti-mated that the sensor is in a thin block of air asit has no direct contact to the cell surface. Thevolume-related power losses of the cell

Qnom =Plosses

Vactive

=8.17 W

51.94 · 104 mm3≈ 15729.69

W

m3

(1)

could be calculated by the measured internal re-sistance of the cells and therefore the overalllosses of the cellPlosses. As the pack itself hasa cooling plate the ground plate was simulatedwith a constant temperature of 20C and it wasestimated that the module is thermally isolatedas it is packed in hard foam.To simulate single cell faults the losses of thecells were risen one by one while the rest of thecells in the module produced the standard ther-mal dissipation lossPlosses. The increased lossesPfault were set to 20,4 W, as this causes tempera-tures above 60C. In Fig. 5 it is shown exemplar-ily for the cells 7 and 14. Those are the extremecases due to their position in the module. It canbe seen that a fault in a middle cell is cooled bet-ter due to the connection to the neighboring cells.At the same time, it is shown that a fault defi-nitely will be detected by the directly adjoiningsensors.

65C

50C

35C

20C

cell 7 cell 14

Figure 5: Temperature distribution for a single cellfault in cell 7 on the left side and cell 14 on the rightside

Having shown that a cell fault can be detectedwith one sensor per cell, it is of interest to seewhether or not a safe system operation is stillpossible with faulty sensors. Therefore, a sin-gle cell fault at cell 6 was simulated. The adjointsensors 4, 5, 6 and 7 were assumed as out of workand therefore gave no or a misinformation aboutthe cell temperatures. In Fig. 6 is shown, that atemperature difference in the module can be de-tected by a comparison of the temperature valuesof the sensors 3 and 8 with the lowest measuredtemperature in the pack. Therefore, the distanceto a functioning sensor should not be more thantwo cells as it is otherwise not possible to get ameasurable significant temperature gradient of 2to 4C. Moreover, it is essential for a save detec-tion that the farthest sensor is working properlyto measure the temperature gradient. Accord-ing to this it might be more severe if the faulty

sensors are evenly distributed. A sensor is ex-cluded from the measurement if it is not react-ing or sending implausible values three times ina row as otherwise a technically fully functionalsensor might be excluded premature. A mea-sured temperature above 50C leads to a warningmessage to the vehicle and a reduced providedoutput power. If 55C are exceeded the mastershuts down the affected pack or if this does notwork the safety circuit will ultimately open therelays at 65C.

111 2 3 4 5 6 7 8 9 10 12 13 144041424344454647

sensorsT

[C

]Figure 6: Measured temperatures of the sensors in amodule with a single cell fault in cell 6 and defectsensors 4,5,6,7

Next to the supervising of the operation, the mas-ter also has to control the start up of the battery. Ifthe relays of the battery would be closed withouta start up procedure the input capacitors of theconverter would be damaged by the inrush cur-rent as the load capacitance is charged up to thebattery voltage. Moreover, the main fuse won’tcarry the inrush current and the relays would bedamaged as well. Last but not least the cells arealso not rated for the inrush current. Therefore,a precharge circuit is needed that consists of aprecharge resistorRprech to limit the inrush cur-rent in series with a precharge relay and the mainrelays at both poles that ensure the safe state ofthe battery during downtime as both poles aredisconnected. The precharge resistor is usuallyconnected to the positive pole of the battery andneeds to be dimensioned based on the capacity ofthe load and the desired precharge time. The cur-rent reaches1

eof its initial value after the period

T = Rprech · C (2)

and has dropped to a manageable value after 5·T .The energy

E = (C · U2bat)/2 (3)

that can be stored in the load’s input capacitorsC has to be dissipated by the precharge resis-tor. Hence, the power dissipated by the prechargeresistor during precharge is that energy over theprecharge time

P =E

5T. (4)

Though, the instantaneous power

Pimmediate =U2bat

Rprech

(5)



the precharge resistor has to dissipate is a lothigher and due to this a high power prechargeresistor is needed, even though it won’t need todissipate any significant power during overall op-eration. Yet, as most available resistors have adistinctly higher peak power dissipation, down-sizing is possible as admissible precharge peri-ods are below one second. Hence, a cooling ofthe resistor is also not needed. As many com-ponents are susceptible to the inrush current ifthe precharge fails, the proper performance of theprecharge circuit should be checked before themain relays are closed. This can either be doneby auxiliary contacts at the relays or measuringthe voltage difference of the battery and the drivetrain.

3 System Production

Several regulations like e.g. the ECE R100.01 orthe ISO 26262 have to be fulfilled during the sys-tem layout and also the system production. Thegoals are to protect people against electric shockand to enable a safe and reliable operation of thevehicle later on.The protection against electric shock has to beassured on the one hand at the battery box buton the other hand also inside the battery system.Therefore all enclosures and barriers, which,when removed expose live parts of high voltagehave to be marked with a symbol for HV. More-over, the electrical parts have to be engineeredand constructed in a way that insulation faults areprevented. Therefore, also the leakage distanceas well as the distances due to insulation materialhave to be fulfilled. In the event of a battery faultincluding flames it is essential that there is as lit-tle as possible flammable material in the pack.Especially outer insulation parts and insulationparts that keep active parts in their designated po-sition have to be sufficiently thermostable and re-sistant against ignition and fire spread to preventa knock-on effect. During the commissioning theinsulation resistance of the pack has to be mea-sured regarding the dielectric strength and elec-tric safety.Moreover, the hardware components and themaster-slave communication have to be checkedduring the system production. A representativeamount of sensors has to be tested on accurateoperation and it has to be ensured that they aremounted in the designated position. Moreover,the wiring of the boards and sensors has to becontrolled. The boards have to be fixed tightly inthe pack because of the high strain due to vibra-tion in the vehicle and have to be housed EMC-safe by all means. The proper functioning of theBMS has to be tested during the initial startup.This includes a test of the correct reaction to allthreshold values.

4 Thermal Management

In vehicle applications with on the one hand ahigh power requirement such as sport cars or ve-

hicles with a fast charge application and on theother hand a tight packaging space an active cool-ing of the battery is mandatory. It is important tokeep in mind that a cooling always has to be de-signed for the worst-case scenario and an agedcell. The worst case scenario is a hot summerday with the highest admissible power require-ment of the vehicle the battery was designed for.An aged cell has a higher internal resistance andtherefore higher thermal losses as a new one. Acooling can be done either with a cooling fluid orair and is either dependent or independent fromthe cooling system of the rest of vehicle.

4.1 Necessity of a Thermal Management

To analyze the necessity of a thermal manage-ment for the designed pack, a cell was cycled atthe test bench with limited heat dissipation by thesurface and without an acitve cooling. To ther-mally isolate the preheated cell, it was housed instyrofoam and four temperature sensors were at-tached to the cell surface at one side. One sensorwas positioned in the lower part of the cell nearat the middle axis, the sensors 2 to 4 in the upperarea at the two sides and the middle axis. Thecell was cycled with a 1C-rate.In Fig. 7 is shown that in the upper middle areain between the two contacts most of the heat isset free. After less than one complete discharge-charge cycle the cell has already reached the pre-defined threshold value of 50C and the experi-ment was interrupted. The relatively fast coolingrate indicates that the cell is by no means fullyisolated. Thus, in the real operation the housinghas a high thermal capacity and emits heat to theambiance so that it can be assumed that the testedcell is isolated more than in the real operation inthe module. Still it is unquestionable that a cool-ing is needed, as the experiment was run with anew cell and the load with 1C is less than in real-ity due to the fast charge. Moreover, a significanttemperature difference could be detected in thecell which should be diminished by the coolingas well.

0 1 2 330

50

35

40

45

T1

T2T3

T4

ChargeDischarge Cooling

T[

C]

t [h]

Figure 7: Cell surface temperatures of a thermally iso-lated cell during a 1C charge/discharge process



4.2 Thermal Connection of Cells

Due to the large surface and the thin housing,pouch cells are superior to other cell geometriesregarding the cooling. The upper area of the cellin between the contacts being the warmest areaof the cell, it is important to dissipate the heatfrom there as well. As a heat dissipation in thechosen module design is only possible via theground plate, an aluminum plate of 1 mm thick-ness covering the whole surface of the cell wasneeded in between the cells to ensure an evenlycooling within the cells. This effect is intensifiedby the close packaging, as the other side of thecell is pressed against the aluminum plate of thenext cell. The cells were connected to the alu-minum plate with a silicone to compensate sur-face irregularities, as it is essential to guaranteethat there is no air in between as this would im-pair the cooling considerably. In Fig. 8 a cellwith a detached aluminum plate is shown. Thestructure of the compensated cell surface irregu-larities can clearly be seen in the silicone layer.Still, small bubbles indicate air locks in betweencell and aluminum plate. That shows that eventhough the silicone was carefully applied a com-plete coverage was not achieved. Therefore, it isessential to really apply a well-defined amount ofsilicone and to groud the two parts with an evenpressure among the surface.

Figure 8: Cell and detached aluminum plate with thesilicone layer indicating cell surface inequalities andair locks

4.3 Homogeneity of the TemperatureDistribution

Due to aging it is important to get an as even aspossible cooling, as the temperature is one of thecrucial factors for calendric aging [9]. To designthe optimized cooling, several factors have to betaken into account:

• available delivery rate of the pump,

• minimum pressure drop in the cooling sys-tem,

• minimum temperature gradient within thecells and

• minimum temperature gradient within thebattery pack.

Within the vehicle only a relatively small pumpwas available that has a working curve below0.15 bar. Moreover, the cooling fluid itself wasonly recooled by the ambient air. Therefore, acooling system with the available pressure shouldbe designed for an ambient temperature of 30Cwith a average heat generation of 0.26 W/cm2 atthe surface of the cold plate. This leads to a totalheat influxQ to the cold plate of 1161 W. To findthe optimal cold plate by a Comsol simulation thevelocity

vw =w

A(6)

with the water flow

w =mw

ρ=

Q

cp∆Tρ(7)

and the cross-section A of the cooling channel isneeded. The velocity is dependent on the admis-sible temperature gradient∆T between inlet andoutlet and the specific heat capacitycp. Know-ing the velocity, the Reynolds number to choosethe right differential equation in Comsol can becalculated by

Re =ρvwDh

µ(8)

with the hydraulic diameterDh, the densityρ ofthe cooling fluid and the dynamic viscosityµ. Toenable an as even as possible cooling it was de-cided to focus on a bifilar cooling as shown inFig. 9. Those spiral windings are for examplealso used for underfloor-heating where an eventemperature distribution is a prerequisite as well.The plate was build out of a thick aluminum platewith the windings milled into it and a covering lidof 3 mm thickness. To add an additional air gapof 2 mm in between the windings, which have adiameter of 15 mm, was simulated but scrappedas the further decoupling of inlet and outlet onlyimproved the temperature distribution by 0.2Cat an associated reduction of the available adher-ent of the two half’s of the cold plate.

Figure 9: Bifilar cooling structure with the cool inlet(blue) and the hot outlet (red).



As channel depth 4 mm to 6 mm were simulatedwith an inlet velocity of 0.6 m/s to 0.8 m/s. Asshown in Fig.10 a channel depth of 6 mm andan inlet velocity of 0.7 m/s are the optimal cool-ing option and with a pressure drop of 0.09 barstill admissible. With the optimized cold platelayout a temperature gradient below 3.5C wasachieved and the temperature gradient within thecell could be reduced to 2.5C by the aluminumplates in between the cells.

-0.2 -0.1 0 0.1 0.231

32

33

34

35

36

37

38

39

T[

C]

y [m]

0.7 m/s; 5 mm0.8m/s; 4 mm0.65 m/s; 6 mm0.6 m/s; 6 mm0.7 m/s; 6 mm

Figure 10: Temperature profile for different channeldepths and velocities. The profile is a cross sectionalong the y axis in the middle of the cold plate on topof the aluminum cover.

5 Test Results

Finally a few results of the designed and buildbattery pack shall be shown. During the initialstart up first of all the insulation resistance of thesystem was measured to ensure electric safety.Hence, a test voltage of 1000VDC was appliedfor one minute according to ECE R100.01 be-tween each pole and the housing. The measuredinsulation resistance has to be above 1 MΩwhichwas clearly exceeded for the tested packs, as itcan be seen in Table 1. The following test on di-electric strength with 2.6 kV was also succeededwithout any disruptives.

Table 1: Insulation resistance [MΩ]

Pack 1 Pack 2positive pole 28.3 29.8negative pole 50.4 49.1

In the operation on the test bench the systemwas tested on the reaction to all threshold val-ues. For all threshold values the warning mes-sages and the correct opening of switches wereproven. Moreover, a pulse test with the maxi-mum admissible load was run and the procedure

to connect packs with a differing voltages wastested. Having different voltages a reconnectionis only possible during the charge process. Thepack with the lower voltage level is charged untilthe voltages differences declined below a thresh-old value. In this moment the second pack is en-gaged and only minor compensating currents oc-cur.Additionally to the behavior of the battery man-agement system, the cooling system was testedby simulating a full day operation. In Fig. 11the measured daily cycle is shown. The sys-tem was discharged with the calculated mean dis-charge power for one hour and afterwards al-ways fast charged for 15 minutes with 150 A.As the temperature rise was relatively low due tothe cold ambient temperature during the test andthe new cells with a low internal resistance thebattery system was heated purposefully with a150 A charge-discharge current after 4.5 hoursof operation. On reaching a temperature above35 C the cooling of the second pack with acooling fluid of 30C was started whereas thefirst pack remained uncooled. It can be seenthat the temperature of the second pack heats onfor a short time until it reaches a steady valuewhereas the first pack heats on until the systemshuts down due to overtemperature. The tem-perature gradient within the cooled pack is only2.5 C and therefore even better than the calcu-lated 3.5C maximum gradient.

1

1

2

2

3

3

4

4

5

5

6

6

7

7

-100

100

200

300

400

15

25

35

45

0

0

0

U1,2

I1I2

T1

T2

U[V

]&I[

A]

t [h]

T[

C]

Figure 11: Daily cycle with a verification of the cool-ing concept by heating the system purposefully with acharge and discharge current of 150 A after 4.5 hoursof operation and cooling only one of the packs in thefollowing.

Having succeeded in all test bench experiments,the system was taken into operation in the vehicleand tested in real conditions. Moreover, the fastcharge was successfully tested.Finally an overview of the weight ratio of thecomponents shall be given as the potential to



reduce weight is of great importance for seriesproduction. Each of the packs weights approx-imately 215 kg with a cell weight of 149 kg.Therefore, 66 kg are added to the weight neces-sary for providing the propulsion power. As itcan bee seen in Fig. 12 the switches and mon-itoring system are negligible as there are threepredominant weight adding factors. The cool-ing system with the ground plate and the 98 alu-minum plates in between the cells weighs 23 kgwhereby 80% are the aluminum plates. There-fore, a direct cooling of the contacts could reducethe weight significantly. An even bigger poten-tial can be seen in the weight of the housing. Asthe modified vehicle was a conversion design ve-hicle, the battery had to be underfloor-installedand is therefore designed solid with an overallweight portion of 10% due to the crash safety. Ina purpose design vehicle the system could be in-tegrated in the floor directly to reduce weight. Fi-nally the cell contacts could be welded instead ofscrewed together. This would reduce the weightby approximately 4 - 5% even if still a structureto keep the contacts in place would be needed.

Switch Box

Contacting

CellsCooling

Housing

BMS

Rest

1 %

1 %6 %

2 %

10 %11 %

69 %

Figure 12: Weight ratio of the components

6 Discussion

It could be shown that the costs and size of a bat-tery for public buses can be significantly reducedby using a smaller battery and fast charging ev-ery other round. The consequently smaller max-imum driving range due to the smaller energycapacity does not limit the operation ability ofthe vehicle as the charging breaks are relativelyshort and can be done at the final stop of the ve-hicle. By recharging every round the chargingtime could even be reduced to less than ten min-utes. Moreover, the charging time can be usedat least as a small buffer time in operation forslight delays. With this approach electric mo-bility might in the future not only be a nice tohave option for public transportation due to itslocal emission-free operation but competitive todiesel fuel driven buses that are nowadays stateof the art. Contrary to private electric mobilitythe cycle life of the battery can completely beexploited and therefore a higher cost efficiencyis achievable. The cells for a mobile battery sys-tem in general have to be chosen based on the

estimated driving range, the needed load factorsand the actually usable cycles. All of these de-termining factors are known in detail for publicbuses. Therefore, it is possible to choose the op-timal cell for the respective application and to de-cide on a fundamental system structure. In thesystem structure especially the reliability of thesystem should be taken into account due to safetyreasons.The design steps in the system design can be di-vided in mechanical and electrical ones. Me-chanical issues are the stabilization of the cellsas especially pouch cells are quite flexible andthe crash safety. It was shown exemplarily in theproject Smart Wheels how this can be realized ina conversion design vehicle. Especially the hous-ing could be realized lighter for a purpose designvehicle and also the cell alignment in modulescould be modified for a more flexible construc-tion space. Electrical issues are the insulation forelectric safety and the management system thatis usually divided in a master-slave architecture,which can include a fall back level. Regardingthe measurements it can be stated that the volt-age measurement of every single cell in a seriesconnection is inevitable, whereas it is possible –depending on the architecture – to monitor notall temperatures in a module. For the temper-ature monitoring in series production it is alsoimaginable to combine the actual measurementof temperatures, that is needed for the load man-agement, with bimetall switches that shut downthe pack independently by a safety circuit in caseof an overtemperature. This would lead to onlya few temperature sensors per pack and there-fore reduced costs. The positioning and neededamount of sensors can be simulated beforehand.For the thermal management it could be shownthat there are driving applications such as fastcharge where cooling is mandatory. Therefore,a liquid cooling system in the ground plate wasdesigned and taken into operation. A possibil-ity to connect the cells to the cooling system wasgiven by the aluminum plates. With the designedcooling system it was possible to reduce the tem-perature gradient within the pack and the cellsbelow 2.5C which is of great importance for aneven aging of the pack. Still, further cooling con-cepts such as a direct cooling of the contacts canbe considered to reduce the overall weight of thesystem.

Acknowledgments

This work has been kindly supported by theGerman Ministry of Economics and Technology(BMWi) within the project Smart Wheels.

References

[1] IEA StatisticsCO2 Emissions From Fuel Com-bustion 2011 – Highlights, Paris, IEA Publica-tions, 2011.



[2] Tescon, http://www.tecson.de/, accessed on2012-01-15.

[3] A. Dinger et.al.,Batteries for Electric Cars –Challenges, Opportunities and the Outlook to2020, Boston Consulting Group, 2010.

[4] J. Vetter et al.,Aging mechanisms in lithium-ionbatteries, Journal of Power Sources, 146(2005),269-281.

[5] P. Sinhuber et.al.,Conceptional Considerationsfor Electrification of Public City Buses – EnergyStorage System and Charging Stations, Emobil-ity - Electrical Power Train, 2010, 1-5.

[6] Kokam, http://www.kokam.com/, accessed on2012-01-17.

[7] DIN EN 60529, http://www.vde-verlag.de/, ac-cessed on 2012-01-17.

[8] Ch. J. Orendorff Mitigating Catas-trophic Failure in Lithium-Ion Batteries,978-1-59430-217-6, Battery Safety – Advancesin System Design, Integration & Testing forSafety & Reliability , 2010, 88-109.

[9] M BrousselyAging mechanism in Li ion cellsand calendar life predictions, Journal of PowerSources, 97-98 (2001), 13-21.

AuthorsSusanne Rothgang received herdiploma in Electrical Engineeringfrom Friedrich-Alexander Univer-sity Erlangen-Nuremberg in 2010.In June 2010 she joined ISEA asa research associate. Her areas ofinterest are HV batteries and therelated vehicle integration with aspecial focus on safety aspects.

Benedikt Lunz received hisdiploma in Electrical Engineeringfrom Friedrich-Alexander Univer-sity Erlangen-Nuremberg in 2008.In September 2008 he joined ISEAas a research associate. His areasof interest are charging strategiesfor electric vehicle batteries andthe grid integration of stationarystorage systems.

Dirk Uwe Sauer received hisdiploma in Physics from Univer-sity of Darmstadt in 1994 and thePh.D. degree from Ulm University,Ulm, Germany, in 2003. From1992 to 2003, he was a Scien-tist at Fraunhofer Institute for So-lar Energy Systems, Freiburg, Ger-many. In October 2003, he wasa Junior Professor, and since 2009he has been a Professor at RWTHAachen University, Aachen, Ger-many, for Electrochemical EnergyConversion and Storage Systems.



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