I have installed a second Group 41 battery under the rear seat which has 1
AWG cables (+) and (-) directly to the starter.  They are both brand new
Interstate batteries.  The thinking was to disconnect this battery once at
the campsite and save it for the trip home.  Everything would run off of
the stock battery and wiring still under the passenger seat.  This set-up
is basically two batteries, in parallel, tied together at the solenoid (+)
post.  The second battery has the lever type disconnects and is mounted in
a box.
Then I got thinking...
I have a brand new VW (Hella) aux. battery kit sitting at home.   If I
connected only the second battery to the starter, it would become a start
only battery and would not have to be disconnected at the campsite.  The
stock wiring could be disconnected from the solenoid post--it would just go
directly to the alternator.  The second battery would be re-charged only
after the alternator is putting out voltage and closing the aux. battery
relay.  It would require running a blue wire to the relay, a new red wire
from the alternator to the relay and a red wire from the relay to the
second battery.
Advantages:  no disconnecting the (-) cable at the campsite.  Stock battery
would provide a full 12V to ignition system while cranking (would that make
a difference?)
Disadvantages:  only one battery available for cranking the starter (but is
has much shorter, heavier cables than the stock '85 wiring).
Anyone think of any other pros or cons?
AND THE KEY QUESTION IS:
What size wire would be suitable to run from the alternator to the new
relay and from the relay to the second battery?
I don't want to fry the relay.  I think they are good for about 30 amps?
Keeping the wire size to about 12 or 14 AWG might be OK?

                                              Jay '85 Westy (The
Hindenburg)