Battery Safety – Charging/Discharging a Li-ion Battery Pack with Defective Cell

To protect cells in parallel, you put a fuse in series with each cell. This protects the pack from the possibility of one cell failing short circuit, and the other cells then driving a fire-starting current through it.

Now use a single BMS to avoid over/under voltage on the entire parallel assembly.

With a series assembly, the cells can have different voltages, so a BMS channel per cell is required. In a parallel assembly, they all have the same voltage, so all have the same protection voltage requirements.

The answer depends on the Li-ion cell chemistry, and, more importantly, on the State of Charge (SoC) when the cell is not in use.

Specifically, a cell that is kept a long time charged at 95 % SoC degrades faster than one that is kept a long time charged at 80 % SoC. Therefore, if the cell is kept unused for a long time, 80 % is better than 95 %.

On the other hand, a cell that is cycled constantly is better charged at 95 % than 80 % because the cell degrades faster when discharging at 5 % that id does at 20 %. Also, a cell that is charged to 95 % holds more energy than one charged at 80 %. Therefore, you only need to discharge it to, say 25 % (not 20 %) to get the same energy out of it compared to charging it at 80 % and discharging it to 5 %.

At 95 % SoC, Li-ion cell with LCO (cobalt) chemistry will degrade faster than a LFP (LiFePO4) cell, because the voltage is higher and the electrolyte will dissasociate faster. Therefore, there are fewer concerns at high SoC levels with LFP cells than with LCO cells. On the other hand, an LFP cell that is held at 100 % SoC without turning off the charger will experience an ever-increasing series resistance, which is bad. Finally, LTO (Titanate) cells are pretty much impervious to many of these issues.

So, it's complicated. The designers or traction batteries spend years of research to answer this question.