The $40 battery isolator ate hours of my time, and to no avail – never found a wiring combination that worked.
I decided to use a relay instead, since it’s cheaper and (most importantly) I know how it works…it’s not a mysterious black box with an annoying voltage drop.
So the quest began; I went to Strauss Auto and Pep Boys asking for a 30-amp continuous duty, normally open, 12v relay. The answer at both places was “uhh, duh, what kind of car is it for?” They could not look up a part by what it is, they could only drill down by application (they type make, choose a list of models, choose year, etc). At Strauss, nobody even knew what a relay was(!).
That’s OK, I’ll try Radio Shack, it’s a pretty common, normal kind of part. Nope. Radio Shack is apparently no longer interested in customers with the intelligence to build or repair anything; they stock ZERO electronic components – no resistors, relays, anything. So WTF is the point of their existance? All they have now is the same crap I can get 25% cheaper at a big box store.
I wised up and went to Danken auto parts, where they actually understand what they’re selling. I told the guy what I needed, and he knew more about it that I did – the way it SHOULD be when you go to a specialty store! The part was $9, and an hour or two later I had it all hooked up and tested.
Now my second battery charges when the engine is running and all the camping accessories are isolated from the starter battery.