Tom,
There are devices on the market to do this. A problem with the concept is
that measured battery state of charge (SOC) relies on voltage and
temperature. Voltage will drop as load is increased. Depending on load a
fully charged battery may fall below 11 volts and when the load is removed
the voltage will recover so a voltage only device may short cycle.

Personally, for the batteries most of will use, just consider them a
consumable and plan to replace them when needed. At best most will last
300-500 cycles. A cycle is anytime the SOC falls below 50% or it is charged
above 80% to the point of off gassing/mixing the electrolyte. This is one of
the reasons the VW charging system is set around 13.8 volt, (2.3 volt/cell).
This keeps the battery just below the gassing stage. While this can allow
sulphation, it is not much of a problem for a starting battery.

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
Tom Hargrave
Sent: Sunday, October 10, 2010 4:31 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Need Someone to test a Battery Saver

I know it's not Friday but it's Vanagon related.

A while ago we had a battery charge / discharge discussion. I decided then
to design a battery saver that would automatically shut off loads when the
house battery drew down.

I finally took the time last week to design the box. It will handle up to 20
amps, has a 20 amp input breaker, will shut off at 11 volts and turn back on
at 13.5 volts.

The idea is to have something that will shut off accessories but will
automatically turn back on when the battery is charging again, preventing
your house battery from going through a very deep discharge cycle.

It's a three wire design, one wire goes to the battery, the second to
chassis ground and the third connects to whatever accessory or accessories
you want to control.

The prototype boards will be in closer to the end of the months and it will
take me about a week to build and test them.

But I can only do so much testing in my shop, so I'm curious if any of you
are interested in testing these?

The price is right - I'll send you a pre-tested unit, you install it and let
me know. In return you keep the one I send you - for free.

The first 4 who say "yes" get one as soon as they are built and tested.

Thanks, Tom

www.towercooler.com