Power FLARM battery temperature

I am using NiMh rechargable cells in my PowerFLARM portable.
Yesterday, while charging, I noticed that the base of the case was hot
and I disconnected the unit immediately. When I opened the battery
compartment I found all cells were hot to the touch and that each was
about the same temperature.

I was more interested in preventing damage than recording the
temperature but estimate it to have being the region of 35-40 deg C.

When charging the same cells in a stand-alone charger they don't get
anything like this hot. Applied voltage was 13.8.

If you use rechargeables in your portable what peak cell temperatures
are you seeing during charging?

Andy