no its no truth to that. you bat. is made to carry a certain load for a certain amount of time whether you have an alt hooked to it or not. if you left the light on all the time with the motor running and the alt. couldn't keep the bat up then your bat would go dead but that shouldn't burn the alt. up. you would be taking more amps. out of the bat. than the alt could put back, so you would need to go to a bigger alt. but if you're just use a light every now and then for short period of time then go for it. its not gonna burn up anything just because you pluged it in.and if its a short in the light then it should blow the fuse, if for some reason its to much load for the wiring then it should blow a fuse. if youer gonna do this for long periods of time then you will need a bigger bat and higher amp. alt.