Causes of burnt or blown cell in hybrid vehicle battery pack
One might wonder what can possibly cause just a single cell to get blown amongst many other cells in a hybrid vehicle battery pack. Users who encounter this situation are left with questions rather than answers. Even technicians are sometimes at loss on the cause of this type of situation.
There are two major causes of blown hybrid pack cell.
I will explain them one after the other.
High terminal resistance: As the hybrid vehicle and the battery pack age, the terminals of the cells develop high resistance due to the connection of the cells. Normally, preventive maintenance cleaning should be done on these terminals to ensure good contact and uniform resistance of the terminals across the cells. Now, let us look into how the high resistance causes a cell to get blown. From basic ohms law, voltage is the product of current and resistance therefore, it is expected that when the resistance across the terminals of a cell increases, voltage across that terminal and of course voltage being delivered to the cell will also increase. It is this situation that leads to excessive voltage of one cell with a high resistance across its terminals and eventually causes the cell to get blown due to constant high voltage across its terminal.
Faulty cooling fan: The battery packs are designed to be cooled by the battery pack's cooling fan. When this fan becomes faulty, the battery pack does not get sufficiently cooled and the cells overheat. It has been observed that the cells in the center suffer this more as they are more likely to get hot than other cells by virtue of their location in the battery pack. Now, if the battery's fan stops working, the entire pack is not cooled but the cells in the middle are likely to reach the highest temperature first which can cause them to get blown.
There is another cause but it mostly leads to swollen cells rather than blown cells; the cells are meant to be compressed, that is, tightly packed together during charging. When they are loose and being charged, they easily get swollen.
Precautions to avoid a blown cell:
Do scheduled preventive maintenance cleaning of the battery pack's cell terminals (I suggest once per year or at least once in two years).
Ensure that the cooling fan of the battery pack is clean and operational. Also ensure that the cabin's Air Conditioner is working as cooled air from the cabin if drawn into the hybrid's battery pack for cooling.
The cells of the battery pack should be compressed during charging.
In the event of a single cell being blown, it should be replaced with only a cell that has similar capacity with other good cells in the battery pack. If this is not so, the batteries will not equalize and the performance of the whole battery pack can be negatively affected.
It is also worthy of note that the charging system of the hybrid car is a grid charging system so it should equalize the cells on its own (if the cells are good) without needing external charging.