Yes, they’re LiFePo4 cells.
The main board cells are all 3.31V. The Daughter board cells were 3.41V.
Voltage across C2 = 5.01V
I checked R16/23 and R13/R22 and re-soldered them. Just next to them is a diode array (BAV99DW?) which had solder between pins 5 and 6 which I cleared off. Maybe this short was a problem?
I conected the daughter board briefly and the cells voltage quickly went to 3.85V so I removed power and disconnected the daughter board.
Unfortunately I ran out of time so I will have to look at it some more to see if the mother board is functioning normally – after removing the solder bridge and then powering on, the indicator LED was lit, dimmed and slowly brightening and dimming – perhaps this is the normal speed dimmer. It didn’t do this before when charging – it was just off.
It may be a couple of days before I can look further. I will check the daughter board and maybe carefully try new cells.