impact of single cell faults in parallel...
TRANSCRIPT
POSTER 2016, PRAGUE MAY 24 1
Impact of Single Cell Faults in Parallel Connections
Philipp DECHENT
1 Institute for Power Electronics and Electrical Drives, Electrochemical Energy Conversion and Storage Systems, RWTH Aachen University, Jaegerstrasse 17-19, 52066 Aachen, Germany
Abstract.
The focus of this work was on the impact of nondestructive
single cell faults in parallel connections in lithium-ion
batteries. The faults were expected to impact the ageing
rate as well as the homogeneity of the parallel connections.
Therefore single cell ageing tests as well as prototype
battery pack ageing and measurement tests were
performed.
Keywords
Lithium-ion battery, Ageing tests, Battery pack, Parallel connection, Differential Voltage Analysis
1. Introduction
Lithium-ion batteries play a prominent role as energy storage systems based on their exceptional energy and power densities as well as their life span. Increased experience and quantity of produced cells also compensate for the biggest disadvantages of lithium-ion batteries, i.e. cost and safety. Therefore they will play an important role in the foreseeable future.
Most lithium-ion battery cells are relatively small, so connections of several cells are necessary for applications demanding high power and energy levels. The most common form factor is the 18650 cylindrical cell containing up to about 12 Wh [1].Using this type of cell the Tesla Motors Model S for example contains over 7000 connected battery cells. This system includes massive parallel connections for redundancy. The aim of this work was the prediction of the impact of single cell failures in such connections. The main focus is hereby not on a catastrophic failure leading to a thermal runaway, but more on subtle defects and failures such as loss of connection of a single cell.
Fig. 1. Cell failure modes in parallel connections.
Fig. 1 shows different scenarios of single cell failures covered in this work and the intended way of examining their effects. The first considered possibility is an internal short circuit with a low resistance. This will lead to total failure and is therefore not part of further considerations in this work. Another possibility of a cell failure is an internal short circuit with a high resistance leading to an increased self-discharge. This possibility was considered for further studies in this work and therefore included in the design of the prototype packs.
The main focus of this work was the evaluation of the impact of loss of contact and increased internal resistance. Loss of contact was simulated with virtual packs and calculated increase of currents subjected to test cells (see 2.1). For the increased internal resistance prototype packs were designed and built and subjected to cycle ageing (see 2.2).
2. Material and Methods
The cell used for all tests is a commercial available cylindrical high power cell from Samsung SDI (Model NR18650-15L1) with a rated capacity of 1.5 Ah. All initial and ageing tests were performed at 35 °C either in Memmert UFE 500 ovens or Binder MK53, MK 240 dynamic climate chambers. Single Cell Tests were conducted with Digatron MCFT 20-5-60 ME Cell-Test-Systems and Pack tests with Digatron MCT 50-06-12 ME Systems.
2 P. DECHENT, IMPACT OF SINGLE CELL FAULTS IN PARALLEL CONNECTIONS
For determining the initial capacity and resistance, all test cells were subjected to an initial test program. This initial test program included a full discharge with 1C (1.5A) after a full charge with constant current of 1C up to 4.2 V followed by a constant voltage charge at 4.2 V until the current droped below 0.05C. The discharged capacity at 1C and 35 °C was used as the reference capacity. After the capacity measurement the cell was fully charged again, followed by a discharge with 375 mA until the cut off voltage of 2.5 V was reached. This step was performed as a Quasi-Open-Circuit-Voltage curve with a reasonable small current, while decreasing the effect of the ohmic voltage drop at higher currents. The last part of the check-up started again with a full charge, followed by pulse resistance measurements at 70 %, 50 % and 30 % state of charge, determined by coulomb counting. Each pulse test consisted of 5 charge and discharge pulses at current rates of 0.5C, 1C, 2C, 4C and 10C. The calculated 10sec pulse resistance at 2C was used for the most part of the following analysis.
A suitable voltage window was chosen after recording a Quasi-Open-Circuit-Voltage discharge curve at 375 mA (C/4). For further investigation of the effects taking place during the cycling, a differential voltage analysis was performed [2]. A smoothing spline was applied to the scattered raw data before differentiating the discharge curve [3]. To achieve fast ageing a range of 10 % SOC up to 90 % was chosen because fast ageing was aimed at and because cycling over phase changes is thought to be linked to fast degradation [4]. The corresponding QOCV charge window was determined to 3.3 V up to 4.1 V.
Fig. 2 Discharge curve at C/4 and dV/dSOC for evaluation of voltage window recorded at 35 °C.
The reference charge current was chosen to 1.5 A (1C) and the discharge current to 9 A (6C) to achieve fast cycling while keeping below the maximum charge temperature of 50 °C. For the cycle tests of the cell, 250 constant current charge and discharge cycles were performed between check-ups.
2.1 Single cell tests with different current
load
If a cell in a parallel connection loses contact to the rest of the parallel connection, the remaining capacity is decreased by the capacity of the cell loosing contact. In
addition the remaining cells are subjected to an increased current under the condition of the same system load currents. Part of this work was to evaluate the effect of this current increase and whether it has a significance compared to the immediate loss of capacity.
Tests were carried out with single cells with slightly different current rates derived from virtual battery packs. For example if one out of 10 equal cells in a parallel connection loses contact all other will be subjected to a 11% higher current. For all performed tests and the increase in charge and discharge current see Tab 1. Higher increase was not considered, since a loss of 20 % equals a remaining capacity of 80 % which is considered as the end of life in many applications.
Cell con-figuration
1S10P 1S8P 1S10P 1S15P 1S15P 1S20P
Cell faults 0/10 1/8 1/10 1/15 2/15 1/20
Charge current in C
1 + 14% + 11% + 7% + 15% + 5%
Discharge Current in C
6 + 14% + 11% + 7% + 15% + 5%
Tab 1. Test setup for single cell tests for evaluation of loss of contact. Three cell per setup and all tests at 35 °C.
2.2 Increased internal resistance
To assess the impact of an increased internal resistance, small battery packs with five cells connected in parallel were built. Cell connector PCBs were created (see Fig. 3) to allow connecting the cells in parallel through soldering woven cooper bands onto the cell terminals and the connector board.
Fig. 3. Cell connector PCB. On the left, bottom side of the PCB to connect to the cells, on the right top side with pads for serial resistance or jumper connection.
The cell connector PCV on the negative side of the battery connection was used for implementing different resistance configurations. In addition to the reference packs where all SMD resistor pads were bridged, two scenarios of increased resistance were emulated. To one of the cells in the parallel connection a resistor was added in series. The resistance values were chosen to 25 mOhm and 50 mOhm corresponding to an increase of 100 % and 200 % at an assumed internal cell resistance of 25 mOhm. This was derived from initial cell impedance measurements
POSTER 2016, PRAGUE MAY 24 3
performed at 1 kHz to eliminate outliers before further testing took place. This value is slightly different to the values derived by current pulse resistance tests performed at check-ups during cycling. Tab. 2 contains all test setups realized as battery packs with 5 cells each.
Cell configuration 1S5P
reference
1S5P
+25 mOhm
1S5P
+50 mOhm
Resistance increase reference +100% +200%
Charge current in C 1 C 1 C 1 C
Discharge current in C 6 C 6 C 6 C
Tab. 2. Test setup for increased cell resistance in a parallel connection. For each configuration 3 packs were built and subjected to cycle ageing at 35 °C
2.3 Current distribution
The increased resistance of one cell will lead to an uneven current distribution, therefore the PCB connector board was used as well with shunt resistors for current measurements. Previous to assembly a defined current was applied to the resistors with a HAMEG HM7042-5 to get a calibration measurement. The voltage drop over the resistor was measured with an Agilent 34401A multimeter and recorded. During testing a Gantner Q.Station 101 with Q.bloxx A104 was used to record the currents of 5 cells in parallel. As the shunt resistor, a TE Connectivity TLRS1050 with a low resistance of 0.5 mOhm and a low TCR of ±75 PPM/°C was chosen to reduce the influence of the resistor connected in series as part of the parallel connection of the batteries. Fig. 4 shows the 5 cell battery pack with the cell connector PCB connected to the battery test system as well as the individual cell voltage measurement.
Fig. 4. Battery pack with 5 cells connected in parallel on battery test system with individual current measurements.
3. Results and discussion
3.1 Chemical analysis of cells
To classify the cells used in this work, two cells were opened under a protective argon atmosphere in a glove box for further analysis.
Anode Cathode
Mass in g 13,5 14,9
Lenght in mm 677 624
Width in mm 58 57
Area activ material (calculated) in cm2 785 711
Charge density in mAh/cm2 1,9 2,1
Thickness current collector and two sided active material in µm
130 137
Thickness current collector in µm 22 27
Thickness active material (calculated) in µm
54 55
Tab. 3. Geometrical properties of disassembled cell
Tab. 3 includes the geometrical properties of parts of the disassembled cell and their masses. The weight of the cell before disassembly was measured to 42.7 g. The casing was weight at 9.9 g.
The composition of the cathode was identified with an Inductively-coupled-Plasma (ICP, Varian 725 ES) analysis and determined to a Nickel-Manganese-Cobalt ratio of 0.33:0.51:0.16 (Nickel:Manganese:Cobalt).
Fig. 5. Laserscanning microscope recording of the anode. New cell on the left, cell with 80 % of initial capacity on the right. Both cells were disassembled at 0 % state of charge and under protective argon atmosphere.
Fig. 5 shows a laser scanning microscope image of the anodes of two cells. One of the cells (left) was opened directly after an initial check up to assure the functionality, the other was opened after cycling until at only 80 % of initial capacity were left.
The most prominent difference is the increased roughness of the electrode and missing particle lumps on the right electrode. This might be an indicator for the ageing effect exfoliation, the loosening of active material during cycling. This was endorsed by the observation of anode active material tightly sticking on the separator during disassembly.
3.2 Differential Voltage Analysis
Fig. 6 shows discharge curves with different current rates at 35 °C and the corresponding Differential Voltage Analysis. At current rates of 0.1C and 0.25C there are noticeable maxima between 10 % and 20 % state of charge, whereas those maxima decrease or were not present at higher current rates. It is likely that the maxima in high energy are not visible at even lower current rates compared to the high power cells used in this work. This was not
4 P. DECHENT, IMPACT OF SINGLE CELL FAULTS IN PARALLEL CONNECTIONS
tested in this work, but should be kept in mind for determining operating voltage windows.
Fig. 6. Discharge curves at different current rates at 35 °C and their differential Voltage over SOC.
In addition to the changes in the Differential Voltage Analysis due to higher current rates, it is obvious by comparing the discharge curves at 5 °C in Fig. 7 to Fig. 6, some of the phase changes are not visible very well. At 0.1C the maxima between 10 % and 20 % SOC is barely visible and completely invisible at 0.25C.
Fig. 7. Discharge curves at different current rates at -5 °C and their differential Voltage over SOC.
Fig. 8 shows the 0.25 C discharge curves recorded at every check-up during cycling in order to detect changes with decreasing cell capacity. The blue curve was recorded in the initial measurement while the
Fig. 8. Discharge curves of reference cell during cycle ageing at different state of healths. Blue line represents new cell, color gradient to red represents time.
other lines are all following check-ups with the color gradient towards red representing the time that passed. Fig. 9 shows the Differential Voltage Analysis over time, where the blue line was recorded at the first check-up and red at the last one. The capacity loss over the ageing period shown was 24 % leaving a remaining capacity of 76 %. Interestingly the first Ah discharged of the battery remain similar in the Differential Voltage. This is an indicator that the ageing primarily occurs at processes taking place at the later stages of discharge.
Fig. 9. Differential Voltage Analysis of a reference cell during cycle ageing at different state of healths. Blue line represents new cell, color gradient to red represents time.
3.3 Single cell tests with different current
load
All test scenarios were carried out with 3 specimen therefore error bar plot were chosen for the plots. The remaining capacity was adjusted to the initial capacity for better comparability. In Fig. 10 it is clearly visible that all cells behaved similar as expected from the similar current profiles. Never the less there are differences in ageing and the end of life criterion of 80 % of initial capacity was reached earlier on cells with higher current rates.
Fig. 10. Error bar plot of remaining capacity of different test scenarios. All cycling was carried out at 35 °C.
Fig. 11 only contains the reference scenario and the two scenarios derived from the virtual parallel connection of 15 cells. The cells representing one failure show a little bit of accelerated ageing and the cells with two failures even more. The reference scenario reached the end of life at about 1850 equivalent full cycles with one failure at about 1760 and with two failures at about 1690 cycles. But the difference between the scenarios when the first cells
POSTER 2016, PRAGUE MAY 24 5
reached the end of life criterion was under 2 % and therefore quite low compared to the 15 % immediately lost when loosing contact to two of 15 cells.
Fig. 11. Error bar plot of remaining capacity of different test scenarios. All cycling was carried out at 35 °C.
The increase of pulse resistance over the ageing was much lower as seen in Fig. 12 when comparing to a common end of life criterion of doubling in resistance. Interestingly after the first cycling there was a significant drop in pulse resistance compared to the initial check-up. This was not experienced as dramatically in previous analysis and will therefore be a topic for further analysis.
Fig. 12. Error bar plot of 10 sec pulse resistance of different test scenarios. All cycling was carried out at 35 °C.
3.4 Increased internal resistance
The battery packs used in the cycle tests to determine ageing and the effect of an increased internal
Fig. 13. Remaining capacity of pack tests. All cycling was carried out at 35 °C.
resistance showed faster capacity degradation than the single cells used for the other tests. The packs also spread more within the same test scenario. Therefore each individual pack of the three scenarios is shown in Fig. 13. SA_PACK_001-003 are part of the reference scenario, SA_PACK_004-006 with one cell with a resistance increase of 100 % and SA_PACK_007-009 with an increase of 200 %. No clear trend was visible until the time of writing, some packs performed better than the reference some worse, therefore testing was continued, to see if a trend would develop.
4. Conclusion
The impact of acceleration of ageing can be neglected in simulations compared to the immediate loss of capacity due to loss of contact. The impact of increased internal resistance cannot be estimated right now, but further testing is necessary.
Acknowledgements
The author wants to thank Dr. Madeleine Ecker and Dipl.-Ing. Susanne Lehner for their support during his study. Further thank goes to Prof. Dr. Dirk Uwe Sauer, who made this master thesis possible and helped with his expertise.
References
[1] AVL List GmbH, “THE TESLA MODEL S BATTERY A Battery Pack Analysis Study.” Jun-2015.
[2] I. BLOOM, A. N. JANSEN, D. P. ABRAHAM, J. KNUTH, S. A. JONES, V. S. BATTAGLIA, AND G. L. HENRIKSEN, “Differential voltage analyses of high-power, lithium-ion cells 1. Technique and application,” Journal of Power Sources, vol. 139, no. 1–2, pp. 295–303, Jan. 2005.
[3] C. H. REINSCH, “Smoothing by spline functions,” Numer. Math., vol. 10, no. 3, pp. 177–183, Oct. 1967.
[4] J. SCHMALSTIEG, S. KÄBITZ, M. ECKER, AND D. U. SAUER, “A holistic aging model for Li(NiMnCo)O2 based 18650 lithium-ion batteries,” Journal of Power Sources, vol. 257, pp. 325–334, Jul. 2014.
About Authors...
Philipp DECHENT was born in Wiesbaden, Germany and has studied Electrical Engineering at the RWTH Aachen University, Germany. He has been working at the Institute for Power Electronics and Electrical Drives, Electrochemical Energy Conversion and Storage Systems as a student researcher since 2010. His bachelor thesis in 2012 dealt with the ageing effect of Lithium-Plating in Lithium-Ion Batteries. With his master thesis on the ‘Impact of Single Cell Faults in Parallel Connections’ he graduated in December 2015 in Systems Engineering and Automation.