Two new cells have just arrived from BMSbattery, and the low voltage cell replaced. The defective cell was down to almost zero, so it looks like the cell may be beyond repair. The single cell battery charger is on its way, but won’t be here until the end of the week or next week.
Before the battery was charged, I checked the voltage of the cells, and they were all just 3.4 volts, including the new cell. This was handy, as it meant that the cells were balanced enough to put the 12 cell charger on.
While it was charging, I kept an eye on the cell voltages, and everything was looking good, so I went away and did the washing up. When I got back, the new cell was up to well over 4 volts, but the rest were still down at 3.4!
Hopefully I have not damaged the new cell in that brief time. What it does tell me is that the BMS is not doing its job, no cell should get above 3.65v while charging (or 3.9v if the BMS is not as good as it should be). So obviously I have some testing to do to see what is going on. It could also be the new cell is faulty from new, which would really annoy me! As I have no way of discharging the battery at the moment (rear wheel is not yet sorted), it may be while before I sort this out!