2nd Battery 88 Westfalia

Hi All

Hope you had a good 4th of july. Canada day was a blast here.

I wanted to "properly" wire a second battery in my van and have been all over the archives and the net trying to find a wiring schematic for the already existing relay under the drivers seat. The last van I had a 1987, I had installed an optima using an aftermarket isolator and never had success with proper charge and drain (not to mention was a pain in the #$% to wire it in) Can anyone one point me in the right direction? I believe the existing relay has 5 pins (2 of which seem to have constant power) Do I make my own ground and splice into one of the other pins? and if so what gauge wire do I use as most starting batteries are about 8 gauge.

Thanks in advance

Tristen