Date: Wed, 14 Apr 2010 23:25:46 -0500
Reply-To: Tom Hargrave <thargrav@HIWAAY.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Tom Hargrave <thargrav@HIWAAY.NET>
Subject: Re: VoltMinder-Is it a good thing?
In-Reply-To: <y2xeccfedcc1004141542i670c8a0agb73a0a4b4e1e661a@mail.gmail.com>
Content-Type: text/plain; charset="us-ascii"
A fully charged lead acid battery is about 12.6 VDC while a charging battery
will be 13.5 - 14.6 VDC at full charge. A partly discharged charging battery
can easily measure 11 VDC or less depending on the output of your
alternator, the battery's state and other accessories pulling current off
the alternator.
And each battery needs to be monitored separately. I'm not trying to be
facetious but you need to understand what the meter is telling you. You
probably don't want your starting battery to draw down too far but it would
be perfectly acceptable to draw your deep cycle house battery down to a much
lower voltage.
Why?
The experts will tell you that a car starting battery should never be drawn
down below 10.1 VDC and every time you draw your starting battery below this
voltage you are taking serious life from the battery. This is because a
starting battery cells are made of many very thin lead plates for more
surface area and more starting current. But deep cycle batteries are built
differently. Deep cycle batteries are made of fewer very heavy lead plates
that can take deep discharge without disintegrating the plates. They are
designed to be discharges as much as 80% time after time with no damage to
the battery - you are using a deep cycle battery as a house battery, right?
To further complicate things a battery's static voltage drops as it's being
discharged and the battery's internal resistance increases as its being
discharged.
This means your deep cycle house battery voltage is "up in the air"
depending on its amp/hour rating, if it's being charged, it's current charge
level and what you are actually trying to do with it or how much current you
are drawing from the battery right now.
Confused yet? If you aren't, I am.
The best solution is to mount a panel meter somewhere in your vanagon to
monitor your house battery. Then watch the battery voltage as things happen.
Pretty quickly you'll get an idea of how far you can draw your house battery
down before you need to start charging.
RadioShack has a 15 VDC panel meter for $12.99 - you would just wire it
across your house battery. The meter itself will draw very little current
from the battery but if you want to turn it on & off you'd wire a switch in
series with the meter.
http://www.radioshack.com/product/index.jsp?productId=2103957&CAWELAID=10759
4018
Oh, and if you installed a starting battery as your house battery be
prepared to replace it in a couple of years unless you plan on using it very
lightly & never draw its voltage down below 10.1 VDC.
Tom
www.kegkits.com
-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@GERRY.VANAGON.COM] On Behalf Of
Michael Sullivan
Sent: Wednesday, April 14, 2010 5:43 PM
To: vanagon@GERRY.VANAGON.COM
Subject: VoltMinder-Is it a good thing?
Since I am putting in the aux battery, a VoltMinder was suggested. any
reviews on this? How can I hook it up to monitor both batteries? Thnaks to
all.
Michael in San Antonio
91GL Weekender AT 2.1L 'Gringo'
73 Beetle
