I've had this happen a couple times now and am curious over y'all's thoughts. In my VW Karman Ghia sometimes it get into a mode where it's literally crawling. The battery voltage reads ~ 72 volts, and the amps read very low (say, 30A). Each time it's happened after a few miles of modestly hard driving.
First time it happened I thought, oh, the batteries are really crapped out. But it was very puzzling why would the voltage be reading at 72 volts? A completely discharged pack under load would be reading, say, less than 50 volts. In any case I pulled to the side of the road and waited awhile - the "growing amps" strategy that's known to let a battery pack recover a bit so you can limp home. I limped the car home and put it on the charger. The AH's used weren't very high. Hurm?
The second time it happened I thought to get out and feel the motor. It was very hot. But I didn't think to feel the controller. The last time it happened I felt the controller - and it was so hot my hand involuntarily leaped away. Any hotter and the controller would have given me a burn.
The controller is not mounted in the air flow, and doesn't have a heat sink. It's an Alltrax 72 volt 450A rated controller - this is the one with the longer case. It doesn't have much innate heat sink capability. It is bolted to the engine compartment firewall, but there's a sheet of rubber in-between.
It seems obvious the controller is overheating. I haven't checked whether Alltrax designed in to detect high temperature and go into shutdown, or whether it's simply being less efficient due to the heat and getting damaged. The controller obviously needs to be on a heatsink or else have a cooling fan.