Split-charge relay - More than 50amps can flow between the engine battery and your leisure battery, so always get a relay that is rated at 50amps or more.   Preferably 75 or 100 amps.  Many relays sold on eBay are 30amps and designed for systems on cars, and not for connecting heavy duty batteries.

http://www.iem-services.co.uk/showdetails.asp?id=1169