Date: Sun, 10 Oct 2010 23:33:09 -0500
Reply-To: Tom Hargrave <thargrav@HIWAAY.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Tom Hargrave <thargrav@HIWAAY.NET>
Subject: Re: Need Someone to test a Battery Saver
In-Reply-To: <BAY152-ds20E5263FB9C87259DE5B9CA0530@phx.gbl>
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Dennis,
I understand that there are devices like this on the market and I studied
how they work before designing mine. And I understand that it's not a
perfect solution, nothing ever is. But one thing I did do with my design is
set a wide span between switch off & switch back on voltages. I set the
switch off to 11 and the switch on to 13.5 volts, not far below the charging
voltage of just about everything out there.
What's left is some real world testing and this is why I asked for some
help. I need to know if a battery that's drawn down to a sustained 11 volts
will float back up past 13.5 volts by itself. The one example I have does
not without help from a charger.
Also, I designed some time delay into the circuit - the circuit does not
shut off immediately at 11 volts. This should not act as an oscillator
unless the load is high enough to drop your battery voltage 2.5 V when on &
then I suspect you are asking too much from your house battery or you have a
sulfated battery that should be replaced.
I also understand that anything drawing a fully charged house battery down
to 11 volts is drawing some serious current, or the battery is on its last
leg anyway. Even a starting battery is rarely drawn down to 9 volts & that's
with a current sucking series wound start motor spinning on the other end of
the wire drawing 60A to 150A, or 200+A if it's a diesel.
Thanks, Tom
-----Original Message-----
From: Dennis Haynes [mailto:d23haynes57@hotmail.com]
Sent: Sunday, October 10, 2010 10:33 PM
To: 'Tom Hargrave'; vanagon@GERRY.VANAGON.COM
Subject: RE: Need Someone to test a Battery Saver
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
