The battery relay for AUX. batteries is only for disconnecting it from
the primary battery in that way, that the starter battery can not be
discharged during camping. The AUX. is always switched into the
camper's circuits and the primary is always switched into the car's
circuits.

But there are two problems (at least on my camper):
The relay is powered by turning the ignition key. So the AUX. battery
is switched into the car's circuits during the engine start
process. The AUX. is discharging the primary during this time but
this is not the intention of the disconnecting relay.

Deep circle batteries should be charged by a limited current to
guaranty a long life time. A simple relay have no current limiter
and when it is powered the first time and connecting two batteries
with different charge there will be a very high charging current
only limited by the inner resistance of the batteries. That is why
you have to use a 40 Amps relay.

I prefer a electronic charging device which will operate during
'ignition is ON' and 'battery indicator is OFF' and charges the
AUX. battery with a well know current.

A tiny diode connected to the relay coil is for preventing electronic
drivers from high voltages which comes up in revers polarity during
unpowering the relay.

Helmut
'80 Camper
 ----------
>From: drew
To: Multiple recipients of list
Subject: Re: Battery Relay for Aux Battery.
Date: Wednesday, 27 Nov, 1996 21:22

I must interject that I read in horror about this relay every time it is
mentioned on this list because of the idea that list members will try to use
it to connect and disconnect their auxiliary batteries. I don't like this
relay for this purpose for about five major reasons, none of which I have
the time to comment on here a this time, but here is a really crudely
written, hasty try:

1. The relay is too small to switch in a big battery for starting purposes,
and also too small to operate auxiliary items that drive a lot of power,
like 12 volt chainsaws, winches, or killer stereos. I prefer to use 600
amp/200 amp relays from Wrangler and wire things up with 2 aught (spelling)
cable.

2. The relay hookup presumed with this thing is to disable the drivers side
battery when the vehicle is off, yet an optimal setup disables the
*passenger side* battery when the vehicle is turned off. Here really
important: the drivers side battery is the deep cycle model, and so should
be on duty when the car is not running, not the other way around.