Very odd lifepo4 behaviour

[mezzle992]

Hi i have been building this battery over the last few weeks. Its a 12s 36v 15ah lifepo4 and so far its been testing perfecly fine. I have been getting 560wh at a 10a load.

The bms is connected up properly now this is just an older photo. The over charge and balancing is working well.

I went to charge it last night and it wouldn't charge (through the bms), so i tested the voltage across the sets of 3 which should give me 9.6v each and they did apart from one. One set of three was only giving 6.4 which would indicate a cell at 0v.

By tapping the offending cell it would come back up to 3.2 volts again and the pack will charge and operate as normal and by tapping it or moving it, the voltage will gradually drop to 0v after a minute or so.
There is no sign of energy being used when this happens, no heat, sparks or any reaction of any sort.

The connections between the batteries are solid and i would expect to see open loop if something had been disconnected but my meter has not shown open loop in any case.

I know i will likely have to replace the cell, it would just be nice to hear other peiples thoughts on whats actually going on here.

Thanks

 

[amberwolf]

One set of three was only giving 6.4 which would indicate a cell at 0v.

By tapping the offending cell it would come back up to 3.2 volts again and the pack will charge and operate as normal and by tapping it or moving it, the voltage will gradually drop to 0v after a minute or so.
There is no sign of energy being used when this happens, no heat, sparks or any reaction of any sort.

The connections between the batteries are solid and i would expect to see open loop if something had been disconnected but my meter has not shown open loop in any case.

That sounds pretty strange. The only way the pack should still show a voltage when a cell is not in series anymore is if there is a short around the cell, but if there was, it would as you note, exhibit some sign of catastrophic failure.

If the cell was either internally or externally disconnected from the pack's series connections, then there should be no voltage reading across the pack.


But..if it is wired to the BMS all this time as well, perhaps there is a high-impedance path still allowing the series connection. Try disconnecting the BMS entirely from the pack, and see if you still get a voltage across the series set of cells. If you don't, then it *is* a broken series connection. It could be inside the cell, if it is not at the connection itself.