[adciv]

This is a NiMH battery pack, it shouldn't catch fire spontaneously again after the damage it has sustained.

Top end = connector end
Bottom end = end opposite connector

Well, the batteries went into thermal runaway. Cause TBD. Unfortunately this is one of the failure modes shared with Lithium Batteries (see 787 for more details). The cells at the bottom appear to have swelled and cracked. The cells on top look to have burst and were probably the source of the smoke.

Does the controller still work?
What happened to the insulation & wire on the cells at the bottom of the battery?
Was it dropped recently?
Any idea how old the battery is?

At this point, the cause seems most likely due to a cell to cell short internal to the battery pack. This would lead to thermal runaway in the cells causing the deformation and smoking. Placing it in the pack was probably the straw that broke the camels back in the internal insulation.