The relay setup works great until the auxiliary battery develops an internal short. The batteries attempt to equalize, but since the internal resistance of one battery is zero, the other battery can only get weaker. The alternator won't charge either battery since they are connected in parallel when the engine is running. Eventually, both batteries need to be replaced.

Why would the auxiliary battery short out? Since the refrigerator in the Wolfsburg Camper model runs on only 110VAC and 12VDC, some people (including me) tend to leave it plugged in while parked, totally dis-charging the auxiliary battery. The fridge draws 8 amps/hour and will discharge a new starting battery in 6-7 hours, the first time. Every time the battery is deep discharged after that, you loose about 25% of its capacity. My 1 year old battery can run the fridge for a little over an hour right now without the engine running. Eventually, the battery will short out internally and not hold a charge.

The obvious solution is to install a deep-cycle battery in the auxiliary battery position. My concern is having dissimilar battery types wired in parallel and the effect it will have on the batteries. I will be installing an isolator with the deep cycle battery(s) to keep this from happening.

Another solution is to rotate the batteries every couple of months.

Or install all deep cycle batteries.

Tim Hannink
Goldibox - 1987 Vanagon Camper, Wolfsburg Edition
Winter Park, Florida
http://home.earthlink.net/~tjhannink/

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