I guess it is the source of the spark when I opened it. But looking inside the case, there are corresponding scorch marks which suggest it has happened when it was in place, not when I took apart. I can't see how this has happened, unless it is from vibration over time or impact. I am now going to try and check if these cells are in the groups that are showing low voltage, but then not sure if it really matters, I guess it is no good without replacing these cells? Even then I would have to work out what happened and resolve it when rebuilt.