If you are just wiring up the hella or ciggie socket, then buy a 30 or 40 amp relay and wire it
terminal 30 to vehicle battery (include a 30 amp fuse)
terminal 85 to ignition live/alternator
terminal 86 to negative (body or chassis)
terminal 87 to positive on socket (include a 30 amp fuse)
negative of socket to body or chassis
-ve to chassis
+12v inputs to car battery via fuses. looking at the connector block about 15a.
one of the switched outputs to the battery.
to be hounest I think you could do away with one in as much the only reason for having it is to give prioity to charging the car battey. as it is only going to be use occasionaly I think you would be fine. just get the car started first before hooking it up in parallel with the 12v from the car. fused of course
This is what you need just google this, easy to fit one fused live from vehicle battery,along with a small earth wire, one live wire from relay to aux battery. obviously you will also need to earth the aux battery to vehicle chassis. and ensure your aux battery is adequetly ventilated as whilst charging it will give off explosive fumes,