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Quote: Originally posted by doit on 23/7/2010

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I would either use a split charger or some sort of manual switchover switch so only one battery is connected to the van at any one time. Otherwise the batteries will discharge into each other so instead of getting a bigger capacity from the 2 batteries, you will end up with 2 flat batteries. Somebody may sell a specialist bit of kit to do this job for you, if not some heavy duty 12v switches will be needed. You only need to switch the 12v +ve and leave the -ve connected to a common earth. Same electrical rules mean a controller is required for a decent solar battery charger and if you do not bother with one your charger will cause your battery to go flat! Sounds crazy but thats what happens. As always a simple problem has a complicated answer..

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I don't think one battery discharging into the other will end up with two flat batteries. The power doesn't disappear, it just passes from one to the other. The rule is quite simple. The two batteries must have the same internal resistance and as Saxo says so should the cables to each. Therefore make sure the batteries are the same type, capacity and condition. Be aware that the car battry and van battery are in parallel when towing. They don't charge equally due to the larger volt drop between the alternator in the front of the car and the van battery against the much less volt drop between the alternator and the nearly adjacent car battery..

American but may help

http://www.marxrv.com/12volt/12volt.htm