why not just wire the relay to your brake lights so that when they get power, it throws the relay and kicks the alternator in. have it run to a manual switch on the dash that bypasses the brake light wire, so that if your cruising alot and arent doing much braking, you can always cut it on manually (or when stuck in traffic idling alot anyhow).
Or do like he said (or everyone said) having it come on at idle speeds (low duty cycle) AND run a wire to the relay from a brake light bulb wire so it also kicks in when stopping.
Use a cigarette lighter volt meter. Takes up no space, is digital. Buy deep cycle battery. Regular batteries, no matter how BIG or how many CRANKING AMPS do not stay running long. They are made to charge all the time, and work for about enough to either start your car, or to run radio/lights for an hour. LOL... deep cycle are made to run 12v power full blast for hours on end. Take longer to charge, but they LAST for longer cycles without recharging. Thats why he is talking about deep cycle batteries. Also, if your car once had a stereo amp, you can dual battery setup, second one in the trunk. Works great
