Short when hooking neg to bat post

[geparx]

If the tow run switch is off it should not spark. If the tow run switch is faulty,it would.
It's just charging the capacitor. Will kill the controller usually. Better check the switch first.

[scooterman]

Originally Posted by scottyb

Use a 470 ohm resistor - hold it on the cable end and touch the main neg for 45 seconds....... then quickly go for the connection.

This sounds like the safest way to reconnect power, this softens the blow of that high inrush current, almost like using a soft start for large motors.