In article , David
writes
ssnip

Just looking for a cheap and simple way to use the spare battery
without any major electrical surgery to the car or caravan.

Charge it when the car runs, not when it isn't. The latter gains you
nothing.

Yes, I am intending to charge it when the car is running; I don't want to
flatten the car battery.
Then jut connect up a voltage sensitive relay and off you go. That's all
the caravan battery has. The regulation system on the alternator will
prevent overcharging. When you switch off the engine the relay will
ensure the two batteries are not connected in parallel.
--
bert