This topic can get complicated real quick. The simplest approach is to
use the existing relay and wiring as much as possible. This approach can
give adequate results if your needs are small and your wiring is in very
good shape. First you must determine if the existing wiring has enough
voltage available at the relay to even charge a second battery while you
are driving with the fridge running. Measure the voltage at the heavy
wires to the relay, with the engine RUNNING and the fridge TURNED ON to
12 volts DC setting. If you don't have at least 12.50 volts at the
existing relay under these conditions then drop the idea of using the
simplest approach. You will need to run a more direct charging wire to
get a high enough voltage to charge the second battery. 13.50 volts is
more like what you want the second battery to see.

I consider the relay kit sold by TBD to be a bit of a joke but it works
and is safe. Better to use a slightly bigger relay and heavier charging
wire IMO. The kit sold by TBD and other places is sized right for 60s
and 70s buses but your 88 has a much more robust alternator and
electrical system. VW used a much more capable second battery charging
arrangement on those Vanagons that came with second batteries so I would
duplicate that later stock method.

Mark

John Graham wrote:

>I have an 88 Westfalia with the full refrig, stove etc package.  I've
>finally decided that it would be nice to have a auxillary battery to
>power things when parked.  I have found that it's really easy to start
>the refrig on gas if it's been running on AC or DC for a little while
>so that the 'chimney' is warm and drawing a draft.  Being able to cut
>it on for a short on the battery while sitting still would be a nice
>feature.....
>
>