Li-ion battery internal short circuits are a major safety issue for electric vehicles, and can lead to serious consequences such as battery thermal runaway. An internal short can be caused by mechanical abuse, high temperature, overcharging, and lithium plating. The low impedance or hard internal short circuit is the most dangerous kind. The high internal current flow can lead to battery temperature increase, thermal runaway, and even explosion in a few seconds. Algorithms that can quickly detect such serious events with a high confidence level and which are robust to sensor noise are needed to ensure passenger safety. False positives are also undesirable as many thermal runaway mitigation techniques, such as activating pyrotechnic safety switches, would disable the vehicle. Conventional methods of battery internal short detection, including voltage and surface temperature based algorithms, work well for a single cell. However, these methods are difficult to apply in large scale battery packs with many parallel cells. In this study, we propose a new internal short detection method by using cell swelling information during the early stages of a battery heating caused by an internal short circuit. By measuring cell expansion force, higher confidence level detection can be achieved for an internal short circuit in an electric vehicle scale battery pack.
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TEMPERATURE CONTROL TO REDUCE CAPACITY MISMATCH IN PARALLEL-CONNECTED LITHIUM ION CELLS
The temperature and capacity of individual cells affect the
current distribution in a battery pack. Non uniform current
distribution among parallel-connected cells can lead to capacity
imbalance and premature aging. This paper develops models
that calculate the current in parallel-connected cells and predict
their capacity fade. The model is validated experimentally
for a nonuniform battery pack at different temperatures. The
paper also proposes and validates the hypothesis that temperature
control can reduce capacity mismatch in parallel-connected cells.
Three Lithium Iron Phosphate cells, two cells at higher initial
capacity than the third cell, are connected in parallel. The
pack is cycled for 1500 Hybrid Electric Vehicles cycles with the
higher capacity cells regulated at 40C and the lower capacity
cell at 20C. As predicted by the model, the higher capacity
and temperature cells age faster, reducing the capacity mismatch
by 48% over the 1500 cycles. A case study shows that cooling
of low capacity cells can reduce capacity mismatch and extend
pack life.
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- Award ID(s):
- 1662055
- NSF-PAR ID:
- 10165902
- Date Published:
- Journal Name:
- DSCC2020
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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