improved performance of serially connected li-ion batteries with active cell balancing in electric...

2448 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 60, NO. 6, JULY 2011

Improved Performance of Serially ConnectedLi-Ion Batteries With Active Cell Balancing

in Electric VehiclesMarkus Einhorn, Student Member, IEEE, Werner Roessler, and Juergen Fleig

AbstractThis paper presents an active cell balancing methodfor lithium-ion battery stacks using a flyback dc/dc convertertopology. The method is described in detail, and a simulation isperformed to estimate the energy gain for ten serially connectedcells during one discharging cycle. The simulation is validatedwith measurements on a balancing prototype with ten cells. It isthen shown how the active balancing method with respect to thecell voltages can be improved using the capacity and the stateof charge rather than the voltage as the balancing criterion. Forboth charging and discharging, an improvement in performance isgained when having the state of charge and the capacity of the cellsas information. A battery stack with three single cells is modeled,and a realistic driving cycle is applied to compare the differencebetween both methods in terms of usable energy. Simulations arealso validated with measurements.

Index TermsBatteries, battery management systems, dc-dcpower converters, electric vehicles, energy storage.

I. INTRODUCTION

S INGLE battery cells are usually connected in parallel andin series to achieve higher capacity and voltage. The par-allel connection is simple to handle since the cells appear as abig single cell, and no management is necessary (similar to theparallel connection of capacitors). The current splits accordingto the internal impedance of the single cell, and the terminalvoltage of each cell is equal.

Overcharging, as well as overdischarging, of lithium-ion(Li-ion) cells causes irreversible damage and is also a majorsafety issue [1][3]. Therefore, reliable monitoring of each cellvoltage is necessary. The range between the charging voltagelimit (CV L) and the discharge voltage limit (DV L), wherein

Manuscript received December 14, 2010; revised March 23, 2011; acceptedMay 4, 2011. Date of publication May 12, 2011; date of current versionJuly 18, 2011. This work was supported by the Austrian Research PromotionAgency (Oesterreichische Forschungsfoerderungsgesellschaft mbH, FFG) un-der research project 8219115: Active Balancing fuer Lithium-Ionen-Batterienin Automobilanwendungen (BALI). The review of this paper was coordinatedby Dr. A. Davoudi.

M. Einhorn is with the Mobility Department, Electric Drive Technolo-gies, AIT Austrian Institute of Technology, 1210 Vienna, Austria (e-mail:[email protected]).

W. Roessler is with the System Engineering Automotive, Infineon Technolo-gies, 85579 Neubiberg, Germany.

J. Fleig is with the Institute of Chemical Technologies and Analytics, ViennaUniversity of Technology, 1060 Vienna, Austria.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TVT.2011.2153886

Fig. 1. Typical cell voltage of a Li-ion battery during charging/discharging [4].

Fig. 2. Cell voltages of three serially connected cells with a capacity of 35,40, and 45 Ah when applying a 40-A constant discharging current.

a Li-ion cell can be utilized, is shown in Fig. 1, and cell voltagemust not get in the shaded area.

In a serially connected battery stack, the discharging, as wellas the charging, process has to be stopped immediately as soonas one of the terminal cell voltages fall below DV L or exceedsCV L. The current through serially connected cells is the same.Therefore, when the cells initially have the same state of charge(SOC), the cell with the lowest capacity is the first one thatreaches DV L and CV L when being charged and discharged,respectively. Fig. 2 shows the simulated cell voltages of three

0018-9545/$26.00 2011 IEEE

EINHORN et al.: PERFORMANCE OF SERIALLY CONNECTED LI-ION BATTERIES WITH ACTIVE CELL BALANCING 2449

serially connected cells with a capacity of 35, 40, and 45 Ahwhen applying a 40-A constant discharging current. The cellvoltage of the cell with the lowest capacity (35 Ah) is the firstthat reaches DV L.

The capacity of the whole battery stack is thus limited bythe weakest cell in the stack. Charging of the battery stackcannot be continued when one cell (usually the cell with thelowest capacity) is completely charged although some cellsare not. Discharging of the battery stack must stop when onecell (usually again the cell with the lowest capacity) is empty,although the others still have some charge left [5].

There are many reasons why the capacities of the cells ina battery stack are not identical. One of them is the variationwithin the manufacturing process due to technical and eco-nomical limitations. Hence, the cell capacities are initially notequal, and moreover, there is a different capacity drift over thelifetime.

When single cells are built together to a battery stack, eachcell has a different temperature even with a well-designedcooling system [6]. Therefore, the cells age unequally fast.This is a second reason why a cell capacity diversification ina battery stack can occur [7], [8].

The performance of a battery stack with different single-cellcapacities can significantly be increased when the charge fromthe cells is equalized with an electronic circuit [9], [10]. This iscalled cell balancing. It can basically be divided into two maingroups: passive cell balancing and active cell balancing [11][19].

Passive cell balancing uses a resistor to discharge the cellwith the highest cell voltage so that charging can be continuedtill all cells are fully charged. This method is only suitableduring the charging process and not efficient due to powerdissipation and energy waste. With active cell balancing, chargecan be transferred between the cells in a battery stack using ashort time storage element, which can be either a capacitor or aninductor [20], [21]. This paper focuses on a promising topologyusing a flyback converter as storage element and particularlyaddresses the performance gain [22][25].

The balancing circuit is described in detail, and the per-formance gain with ten serially connected cells with a largecapacity diversification is simulated and measured. Moreover,it is shown how the balancing strategy can be improved whenhaving each cell capacity and each SOC instead of the cellvoltage as the balancing criterion. This method is simulated andvalidated with three cells.

II. BALANCING CIRCUIT

The balancing circuit principle is based on a flyback con-verter. The key component is a transformer with a winding foreach cell and a winding for the whole battery stack [26]. Fig. 3shows the balancing circuit for ns serially connected cells. Thebidirectional use of the multiple winding transformer allowstwo different balancing strategies. Energy from one single cellcan be transferred to the whole stack (top balancing), andenergy from the whole stack can be transferred to one single cell(bottom balancing), as shown in Fig. 4. A detailed descriptionof the balancing circuit can be found in [24], [25], and [27].

Fig. 3. Balancing circuit for ns serially connected cells with the multiplewinding transformer T as key component of the flyback converter structure.The microcontroller C operates the switches S and S1 . . . Sns (typically low-voltage MOSFETs) according to the balancing strategy.

Fig. 4. Charge transfer from one cell to the whole stack (top balancing) andfrom the whole stack to one cell (bottom balancing).

Top balancing is typically applied during charging to avoidovercharging of a cell. When the voltage of a cell is closeto CV L, charge can be transferred to the other cells, and thecharging process can be continued. With this method, each cellcould be completely charged, although it would take much time,depending on the charging current, the balancing current, theSOC, and the capacity of each cell.

Bottom balancing is typically applied during discharge modeto increase the usable energy. When the voltage of a cell isclose to DV L, charge can be transferred to this cell, and thedischarging process can be continued. With this method, thebattery stack can be discharged until all cells are completelydischarged (depending on the discharge current I , the balancingcurrent, the SOC, and the capacity of each cell).

The influence of the balancing current on the capacity of awhole battery stack can be approximated. Without balancing,


TABLE ICELL CAPACITIES C AND REMAINING CAPACITY RELATED TO THE

INITIAL CN = 2.3 Ah FOR THE BALANCING SCENARIO [28]

the capacity of the cell with the lowest capacity Cx in a batterystack with ns serially connected cells defines the capacity of thewhole battery stack

C = Cx = min{C1, C2, . . . Cns}. (1)If just cell x is supported during discharging, the usable

capacity of the battery stack is increased to

C = Cx + Ixbal t (1 1

ns

)(2)

with the total balancing time t and an ideal converter. A batterystack with 11 40-Ah and one 35-Ah cells, a balancing currentof Ibal = 5 A, and a balancing time of t = 1 h would have acapacity of

C = 35 Ah + 5 A 1 h (1 1

12

)= 39.6 Ah (3)

which is a higher value than that without balancing.If there is more than one weak cell, the balancing time ratio of

each cell to the total balancing time ti/t needs to be considered.The capacity of the battery stack is then increased to

C = min{

Ci + Ibal t (

tit 1

ns

)}i=1...ns

. (4)

III. SIMULATION AND EXPERIMENTAL VALIDATIONOF THE VOLTAGE BALANCING METHOD

Ten Li-ion cells have diversely been cycled to decrease thecapacity and are then serially connected to a battery stack. Theremaining capacities are shown in Table I.

The theoretical stored energy in the battery stack fromTable I is

W =nsi=1

Wi =

10

OCV dSOC 10

i=1

Ci = 57.2 Wh (5)

with the SOC versus open circuit voltage (OCV ) curve ex-tracted from the cell datasheet [28]. Fig. 5 shows the SOCversus OCV curve exemplarily for a Li-ion battery cell witha Li[NiCoMn]O2-based cathode and a graphite-based anode.

The balancing circuit from Fig. 3 is modeled in Modelica/Dymola [29], [30], using the electrical energy storage library

Fig. 5. Linear interpolation of the measured OCV at different SOC valuesfor the Li-ion polymer cell.

Fig. 6. Simulation arrangement.

[31]. Together with the validated battery model from [32], theincrease in energy by using an active balancing circuit withthe ten serially connected cells from Table I is simulated. Anextraction of the simulation arrangement is shown in Fig. 6.A constant discharging current (Load) is applied to the batterystack (Batterypack), and the balancer (Balancer) equalizes thecell voltages with a balancing current (single-cell side of thebalancing circuit) of Ixbal = 4 A. This operation mode is calledvoltage balancing. The balancer is in operation mode only whenthe difference voltage between the cell with the lowest and thatwith the highest voltage is greater than 20 mV. Fig. 7 shows theschematic operation mode during voltage balancing for threecells with different cell capacities. The efficiency and powerloss in control are taken into consideration with an assumedefficiency of the dc/dc converter in the balancer of = 90%.The energy and the charge from the battery stack are estimated(Energy).

This scenario has also been measured using the cells fromTable I and the active cell balancing prototype shown in Fig. 8.The measured cell voltages during discharging without balanc-ing and with a balancing current of Ixbal = 4 A are shownin Fig. 9. Without balancing (top chart), cell 1 is the one


Fig. 7. Schematic operation of voltage balancing. A positive cell currentdischarges the cell.

Fig. 8. Active cell balancing prototype with ten serially conected Li-ion cells.

that first reaches DV L because of its lowest capacity, andthe discharging process has to stop. The battery stack cannotfurther be discharged, although cells 210 are not completelydischarged. With balancing (bottom chart), all cells reach DV Ltogether because the weak cells are supported during the wholedischarging process. The cell voltages of the balanced cellchange during balancing according to its balancing currentand its internal impedance (oscillations). Fig. 10 shows thesimulated and measured gain in terms of energy [see Fig. 10(a)]and charge [see Fig. 10(b)] of this balancing experiment. Withthis scenario, the discharge energy is increased with activevoltage balancing by 15%.

Compared with the rated capacities of the used cells (CN =2.3 Ah) and the constant discharge current (I = 1.8 A), thebalancing current of Ixbal = 4 A is very large. For cells with ahigher capacity and a larger discharging current, the balancingcurrent might be too small to equalize the cell voltages com-pletely in real time. It can also happen that the wrong cells arebalanced if just the cell voltages are considered, as discussed inthe next section.

IV. CAPACITY BALANCING

Up to now, charge is transferred from the battery stack tothe cell with the lowest voltage during discharging. Duringcharging, charge is transferred from the cell with the highestvoltage to the battery stack. In this so-called voltage balancingstrategy, only the cell voltages are considered. However, it isshown in the following that these criteria do not always lead toan optimal decision that cells have to be balanced [33]. Whenthe cells are not balanced in an optimal manner, the usableenergy in the battery stack is lower than that with an optimalbalancing strategy.

The main aspects of the balancing procedure can alreadybe analyzed with a three-cell battery stack because just thecells with the highest and the lowest voltage are crucial. Threeserially connected cells are assumed. Cell 1 has the lowestcapacity (e.g., 35 Ah), cell 2 has an intermediate capacity (e.g.,40 Ah), and cell 3 has the largest capacity (e.g., 45 Ah).

Fig. 11(a) shows the schematic charge transfer with voltagebalancing during one typical charging and discharging period.The top diagram shows the cell voltage of each cell, the chartin the middle shows the SOC of each cell, and the bottomdiagram shows the energy in each cell. In phase I, the cells arecharged, starting from a different SOC, and charge is takenfrom cell 3 because of its highest voltage and transferred tocells 1 and 2. Indeed, the energy from cell 3 is transferred tothe whole battery stack, and since only three cells are present,the energy is split into three equal parts and spread to cells1, 2 and 3. The net charge transfer though is from cell 3 tocell 1 and to cell 2. Cell 3 also has the highest capacity, andso, it will take more time to fully charge it than the othercells when the current for each cell is equal. The slopes of thecurves in the middle and bottom diagrams indicate how fastthe cells are charged. When the cell voltage of cell 1 exceedsthe others at the beginning of phase II (it is assumed thatall cells have the same voltage and, therefore, have the sameSOC = SOC at this moment), charge is removed from cell 1and transferred to the other cells 2 and 3 because cell 1 now hasthe highest voltage. When the first cell is fully charged (cell1 because of its lowest capacity) ,the charging process muststop immediately to avoid overcharging this cell. Beyond thispoint, the charging process could be continued with a severelyreduced charging current and an active balancing system untilall cells are completely full. This would take much more timeand is not considered here. In phase III, charge is transferred tocell 3; although stored in cell 3 is the largest amount of energy,it has the lowest cell voltage. In phase IV, cell 1 has the lowestvoltage, limits the duration of the discharging process, and istherefore supported. Voltage balancing works inefficiently in


Fig. 9. Measured cell voltages of the serially connected cells from Table 1 during discharging (top) without balancing and (bottom) with voltage balancing. Thebalancing current in the bottom chart is 4 A using the active cell balancing prototype from Fig. 8.

Fig. 10. Simulated and measured (a) discharging energy and (b) discharging capacity without balancing and with active balancing.

phases I and III because the wrong cells are balanced, andthe transferred charge is partially retransferred in phases IIand IV.

By using the SOC and the capacity of each cell as the bal-ancing criterion, the drawback of voltage balancing in phases Iand III can be eliminated, as shown in the stack in Fig. 11(b).This is called capacity balancing.

During charging, energy from the cell with the lowest energyto full charge is taken. The energy to full charge of each cellis the difference between the total capacity of each cell (C1,

C2, and C3) and the corresponding curves in the bottom chartof Fig. 11. This is cell 1 during the whole charging process.Therefore, charge is transferred from cell 1 to cells 2 and 3(phases I and II), and the slopes of the curves in the middleand bottom diagrams do not change. During discharging, thecell with the lowest amount of energy is supported. This is forthe whole discharging process (phases III and IV) cell 1, andcharge is transferred to this cell. Hence, the slopes of the curvesin the middle and bottom diagrams do not change in phases IIIand IV.


Fig. 11. Schematic balancing (a) based on the cell voltage and (b) based on the cell capacity and SOC. The arrows indicate the charge transfer betweenthe cells.

TABLE IIRATED CELL CAPACITIES CN , MEASURED CAPACITIES C, AND INITIAL

SOC FOR THE CAPACITY BALANCING SCENARIO

With capacity balancing, the charge time can be decreasedbecause charge is not retransferred as with voltage balancing.The usable energy of the battery stack during discharging is in-creased because the weak cell is supported from the beginning.In addition, the cell with the lowest capacity is utilized with alower current for both charging and discharging. In the long run,this could have a positive effect on the aging since the capacitydecrease is related to the cell current.

V. SIMULATION AND EXPERIMENTAL VALIDATIONOF THE CAPACITY BALANCING METHOD

Three Li-ion polymer cells with different capacities andSOC, as shown in Table II, are serially connected to a battery

stack [34]. Cell 1 with CN1 = 20 Ah is a single cell, cell 2 withCN2 = 40 Ah is two single cells in parallel, and cell 3 withCN3 = 60 Ah is three single cells in parallel.

The battery stack with the configuration from Table II has atheoretical stored energy of

W =W1 + W2 + W3

=C1 1

0

OCV dSOC

+ C2 0.90

OCV dSOC + C3 0.80

OCV dSOC

=406.17 Wh. (6)The OCV is a function of SOC and can be extracted from

the cell datasheet. This battery stack is discharged until one cellreaches DV L (typically cell 1 with the lowest capacity). For thedischarging process, the current profile gained from the FTP72driving cycle, as shown in [35, Fig. 12], is continually appliedto the battery stack, as well as to the simulation.


Fig. 12. Definition of the FTP72 driving cycle, power consumption of atypical compact electrical vehicle, and current requirement from a batterystack with 100 serially connected single cells with a cell voltage of 3.6 V,respectively [35].

Fig. 13. Test circuit to validate the capacity balancing simulation.

Fig. 14. Experiment setup for the circuit from Fig. 13.

The simulation from Fig. 5 is used and extended by thecapacity balancing operation mode, and the battery stack isconfigured according to the stack from Table II. The active bal-ancing system is connected to the battery stack with a balancing

Fig. 15. Cell voltages during the FTP72 discharging cycle from Fig. 12 [topchart of (a)] without balancing, [middle chart of (a)] with voltage balancing, and[bottom chart of (a)] capacity balancing. With voltage balancing during t = 0and t 71 min, cell 1 is supported. After t 71 min, cell 3 is supported.(b) With capacity balancing, cell 3 is supported during the entire dischargingprocess.

current of 3 A (single cell side of the dc/dc converter). Theavailable charge and energy over the whole discharging processis calculated. The simulated capacity balancing scenario is thenvalidated with the experimental results related to the circuitfrom Fig. 13 since the capacity balancing strategy has yet to beimplemented in the prototype in Fig. 8. Instead of the flybackconverter with the prototype, a current source with a power(P ) coupled electronic load is used to perform the chargetransfer assuming an efficiency of 0.9. During the whole test,the cells are in a climate chamber to minimize temperatureeffects. Fig. 14 shows (from left to right) the battery test benchfor discharging the stack with the FTP72 cycle, the climatechamber with the battery stack in it, an electronic load coupledwith a current source, and a PC for the measurement.

In Fig. 15, the measured cell voltages during the FTP72 cur-rent profile are shown for the three different balancing scenarios(no balancing, voltage balancing, and capacity balancing). With


Fig. 16. Measured and simulated (a) discharging energy and (b) discharg-ing capacity without balancing, with voltage balancing and with capacitybalancing.

no balancing (top chart), cell 1 with the lowest capacity isthe first that is completely discharged, although it started withSOC = 1. With voltage balancing (middle chart), cell 1 is stillthe one that first reaches DV L, but the discharging processis significantly longer (71 min) than that without balancing.The first 50 min of the discharging process correlate withphase I in Fig. 11(a), where cell 3 with the highest amount ofstored energy is supported (not optimal). In the bottom chart ofFig. 15, the capacity balancing strategy is applied, and cell 1is supported during the whole discharging process. Cells 1 and2 reach DV L almost at the same time, just cell 3 has energystill left because of its much larger capacity. If the balancingcurrent were higher, all three cells would reach DV L simulta-neously. This would be optimal because the battery stack is thencompletely discharged, and all energy in the cells could be used.

The measured and simulated discharging energy for thedifferent balancing scenarios are shown in Fig. 16(a). Theavailable discharging capacity for different balancing scenariosis shown in Fig. 16(b). Since the measured discharging currentis applied to the simulation, there is no difference between themeasured and the simulated discharging charge. Without anybalancing, the capacity of the battery stack is as weak as the

smallest cell. In this case, the battery stack has a maximumcapacity of 21.9 Ah, which correlates with a usable energyof 240 Wh. Voltage balancing increases the capacity by 27%to 28.3 Ah and 306 Wh, respectively. The best performancefor this scenario is accomplished when balancing the availablecapacity. The capacity of the battery stack can be increasedby 32% to 29.1 Ah and 318 Wh, respectively. Even withcapacity balancing, the usable energy is just around 79% ofthe theoretical value from (6). For improving the result, thebalancing current must be increased as mentioned earlier.

When all cells are fully charged, there is no differencebetween voltage and capacity balancing during discharging (forcells with the same chemistry). The cell with the lowest voltagealso has the lowest amount of stored energy, and therefore, thebalanced cells for both voltage and capacity balancing are thesame. There is also no difference between voltage and capacitybalancing during charging if all cells are completely dischargedbefore starting the charging process (the cell with the highestvoltage is also the cell with the lowest energy to full charge,and therefore, the balanced cells for both voltage and capacitybalancing are the same). When the cells are not all completelycharged before discharging, capacity balancing increases theamount of usable energy of the battery stack. When the cellsare not all completely discharged before charging, the batterystack can be charged in a shorter time, and more energy can beloaded into the battery stack when using capacity balancing.

To use the active balancing system in capacity balancingmode, a method to estimate the cell capacities and the SOCduring operation is necessary. The SOC can, for example,be estimated by measuring the OCV and by using Fig. 6 or[36][39]. Since the capacity of a battery cell changes over itslifetime due to aging [8], [40], it is not enough to estimate thecapacity just once (e.g., after production). In general, the capac-ity of a single battery cell can be estimated by fully discharg-ing it and integrating the measured current (charge counting)[41], [42]. However, this approach is very difficult in a seriallyconnected battery stack because the cell with the lowest capac-ity is the first one that is completely discharged, and the batterystack cannot be further discharged. Hence, with charge countingonly, the capacity of the cell with the smallest capacity in aserially connected battery stack can be estimated. Methods toestimate the capacity of each battery cell in a serially connectedbattery stack are, for example, presented in [43] and [44].

If the capacities and the SOC of the cells are not well esti-mated during capacity balancing (e.g., due to a drift of the val-ues over time), it could happen that a cell is charged/dischargedover/below CV L/DV L. Therefore, it is recommended to ad-ditionally monitor each cell voltage to prevent the cells fromovercharging/overdischarging.

VI. CONCLUSION

If several Li-ion cells are serially connected to a batterystack, the worst cell defines the limit of the whole battery. Whenthe charging/discharging voltage limit of one cell is reached,charging/discharging has to be stopped, regardless of how muchenergy is left in the other cells. If the cell capacities in a batterystack are different (due to production diversification or aging),


active balancing remarkably improves the performance of abattery stack.

A flyback dc/dc converter topology has been presented tobalance the cell voltages of a battery stack with ten seriallyconnected cells. The cell capacities are considerably different,and with active balancing, the usable energy of the battery stackcan be improved by 15%.

Balancing the cell voltages is not always the most effectiveway to improve the usable energy in a battery stack withdifferent cell capacities. When the cells are not completelycharged/discharged before discharging/charging, balancing thevoltages would lead to a suboptimal result. The usable energyof a battery stack can significantly be improved when the storedamount of energy of each cell, and not the cell voltages, isconsidered (balancing the available capacity).

Further work will focus on implementing the capacity bal-ancing strategy to the prototype and estimating the cell capaci-ties and the SOC for cells in a battery stack during operation.

ACKNOWLEDGMENT

The authors would like to thank C. Kral and F. V. Contefrom the AIT Austrian Institute of Technology for reviewingthis paper.

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Markus Einhorn (S11) was born in Vienna,Austria, in 1984. He received the B.Sc. and Dipl.-Ing. degrees (with distinction) in electrical engineer-ing from the Vienna University of Technology in2008 and 2009, respectively, where he is currentlypursuing the Ph.D. degree.

He is currently with the Mobility Department,Electric Drive Technologies, AIT Austrian Instituteof Technology, Vienna, as a Research Associate. Hisrecent work has focused on the design and modelingof power electronics and battery systems.

Mr. Einhorn is a member of the Modelica Association and of the OVEAustrian Electrotechnical Association.

Werner Roessler was born in Bavaria in 1958. Hereceived the Dipl.-Ing. degree from the TechnicalUniversity Munich, Mnchen, Germany, in 1983,after studying communications engineering.

Since 1983, he has been with the SemiconductorDivision, Siemens, Neubiberg, Germany (in 1999,it became Infineon Technologies). Until 2000, hewas an Application Engineer for television engineer-ing with focus on power supply, microcontrollers,and teletext. He is a member of the InternationalStandardization Group for High Level Teletext. After

two years of Hardware development for a speech recognition chip, he moved tothe Automotive Division as Application Engineer for automotive sensors whichfocus on magnetic and pressure sensors. Since 2006, he has been a SystemEngineer for automotive hybrid applications with focus on battery managementsystems.

Juergen Fleig received the Diploma degreein physics from the University of Tuebingen,Tuebingen, Germany, in 1991 and the Ph.D. degreein chemistry from the Max-Planck-Institute ofSolid-State Research, Stuttgart, Germany, in 1995.

After working as a Researcher with the Max-Planck-Institute of Solid-State Research for severalyears, he accepted a position as Professor of electro-chemistry with the Vienna University of Technology,Vienna, Austria, in 2005. His main research subjectsare electroceramics and materials for electrochemi-

cal energy conversion devices, including basic investigations on the physicaland chemical processes determining cell efficiencies.

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