There is definitely being generated a lot of heat, but I do not really know if it is the holder or the battery.
The inner pin do not generate any heat, the current is virtually zero.
The battery gets hot and probably faster than my thermo sensor can follow.
The contact surface between battery and holder may also generate heat, just 1 milliohm would be 2.5 watt
The outer part may also heat at 50A, I do not now how much resistance it has.
The wire connection at the back may generate some heat.
The cable is getting warm, I uses 2.5mm2 (13 awg) (It is a bit thin for 50A).