Tod if you need a jump start on your car, Turn the boost car off and start your's off his battery, What happen's and this is for all makes is their car is running charging your battery then you crank yours and it starts now you have two altenators charging your battery for a few seconds the controller just can't handle this voltage before it can shut down the voltage has spiked it. Think about the spark when you remove the cables that is what is going thru the controller. welding archs :!: :!: