Your alternator is certainly the easiest and cheapest way to keep your “house” battery bank charged. It’s already under the hood, keeping that “starter” battery nice and charged. At it’s simplest, all you’d need would be the correct gauge wire, a fuse, and (probably) a manual disconnect switch to separate your “starter” battery from the rest of the electrical system when you were boondocking.
A simple setup like that will work, but because the disconnect is manual, it’s only a matter of time before you forget and accidentally drain your “starter” battery. If you don’t have an additional source of power like solar or a booster pack you could be marooned somewhere. That’s why many people choose a battery isolator system of some kind.
Relay vs Diode Battery Isolator Systems
Both function to isolate your “starter” battery from the rest of the electrical system when the engine is off. Diode based systems are more reliable, but relay systems aren’t known to be unreliable and are trivially bypassed in the field with a jumper cable. Diode systems are supposed to be a bit kinder to your batteries, but we didn’t look into that much. Most people just use the cheaper, smaller, full voltage relays so that’s what we did. We went with the 200A stinger, sized well above the 130A max output of our 2006 Tundra’s stock alternator.
We want to say a lot more here, but we won’t have this wired up until next month. Suffice to say, the lack of clear information about how to wire one of these up was one of the biggest inspirations for this site. We aim to do a big write up with photos and video by mid 2017, wish us luck! In the mean time please check out our guide on drawing a wiring diagram for your camper conversion’s electrical system, it will show how our isolator integrates with the solar system, batteries, outlets, inverter, etc.