A few months ago I was cruising along and my EV suddenly lost power. It was like someone had pushed in the clutch. I lifted the bonnet but the problem seemed to have gone and off I went again. Then it happened again last week. I suspected the throttle pot, as I know they can wear or get dirty over time. The speed controller can sense if this pot is open circuit and will shut down for safety reasons. So I pulled off the connectors to the throttle pot and sprayed some circuit cleaner on them. This seemed to fix the problem for a days.
Then just today it became really bad. Stop-start, stop-start. I could hear the contactor clunking so I knew the 12V circuit was OK. Other possibilities were a loose connection in the battery pack or a bug in the (rather expensive) speed controller. So I popped the bonnet and started poking around. Then I noticed something. By moving one wire I could hear a small relay clicking on and off rapidly. This relay (I think, it’s been a while since I built the EV) supplies a “go” signal to the speed controller. As I moved the relay it would click on and off, and the motor power would cut off.
Then I wiggled the solder connections to the relay and sure enough one of them was loose. Ah-ha! A few minutes later the problem was solved. A bad solder joint is not bad for 3.5 years electric driving I guess. Shows how easy Electric Cars are to fix as well.