or
http://cgi.ebay.co.uk/SPLIT-CHARGE-RELAY-12V-30A-SELF-SWITCHING-FORD-TRANSIT-/200600941491?pt=UK_Campervan_Caravan_Accessories&hash=item2eb4bf73b3Although it says it's for a Transit, they will work for any vehicle.
This is an intelligent relay so will sense the condition of the van battery before it switches to charge your leisure battery. Once you start your van it will take about 20 secs for this relay to click in (assuming van battery is in good state of charge) as it ensures that charging the van battery takes priority.
All that is required is one suitable cable from your van battery to the relay, and a second cable from the relay to your leisure battery which will carry at least 20 amps at 12v. An inline fuse holder with a 10/15amp fuse as close to each battery as possible and an earth cable to the van body from the relay. Instructions are included and there is a dedicated technical helpline provided by the company if you have any problems.
Initially we both had relays that were triggered by another wire from the alternator's charging warning circuit, but this relay from Ebay is much easier to fit.
We have one of them operating successfully on my sons van.
If you are doing low mileage and you have compact rounds, this split charge relay will not charge your van battery fully. You will still have to put a charger onto it every second or third evening when you get home.
Spruce