Estimating current draw from alternator (14.4v?) to 12v DC
You need a split charge relay, relevant fuse and good quality wire for 80 amps or so.

This is the type of components that I used for a couple of caravans and the second battery I had on my Landrover that I used for jump-starting other vehicles.

And as for finding info, this link may help :http://www.12voltplanet.co.uk/split-charging.html