Testing the charging and discharging of a 2x7 cell NiMh pack in a 222 has revealed a problem with temperature. The charging current can runaway. What happens is the pack gets warm (unlike NiCads, NiMh are exothermic when charging) and the float voltage drops, which increases the current etc. Temperature feedback is through a plastic wall and 2 PCBs away so there is basically no temperature sensing. The charging regulator is capable of about 1A which is about C/3 at charging end. It's fine if it's charged in steps allowing a cooling off period.
To make things worse the batteries are rated at 1800 "chinese" mAh but in standard units more like 1100 making the situation worse. The batteries get really warm.
Solutions? Maybe limit max current? Or use NiCad cells? If the prebuilt Lithium battery kit was available that would have been a good solution.
Any more ideas?
