Date:         Wed, 13 Feb 2008 08:17:51 -0800
Reply-To:     Michael Elliott <camping.elliott@GMAIL.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Michael Elliott <camping.elliott@GMAIL.COM>
Subject:      Re: Sure Power 1315 question > monitoring battery
In-Reply-To:  <86476e250802122223r49fe2249ne3c9c9be79d2f4e0@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

The state of charge of a battery can indeed be determined with a
voltmeter, but the battery needs to be disconnected from any loads for an
hour or so to allow the battery voltage to stabilize. If you read the
voltage while it is powering something then the voltage reading will be
dragged down by the load, and cannot be used to determine the battery's
state of charge.

For a generic flooded-cell lead acid 12 volt battery, here's the
relationship between no-load voltage and state of charge.

No-load Voltage         Percent Charge
 >12.6                  100%
12.45-12.6              75-100%
12.24-12.45             50-75%
12.06-12.24             25-50%
11.7-12.06              0-25%
<11.7                   0%

Measuring the specific gravity of the electrolyte with a hydrometer is a
more accurate method of determining state of charge. With totally sealed
batteries this is not possible.

A chart illustrating specific gravity and no-load voltage vs charge is at

http://www.semarine.com/store/home.php?cat=86

More information about measuring methods is at

http://www.rpc.com.au/products/batteries/car-deepcycle/carfaq9.htm#charged

They list, in decreasing order of accuracy:

1. Measuring charging current under specific conditions,
2. Measuring with a hydrometer after the battery has stabilized, then
"temperature compensate, and compare the average of the readings with the
battery's manufacturer's Specific Gravity definition of a cell in a fully
charged battery," and
3. Measuring with an "accurate (.5% or better) digital voltmeter" after
the battery has stabilized, then "temperature compensate, and compare the
reading with the battery's manufacturer's [no-load] definition of a fully
charged battery."

When we camp the battery is either being charged (by solar) or running
something so the battery voltage or specific gravity are both pretty
useless for determining charge.

So given all that, we use a battery monitor which seems to give me
reliable readings. It monitors energy going in and coming out of the
battery and displays how much capacity is available, like a fuel gauge.
Here's the one we use --
http://www.xantrex.com/web/id/96/p/1/pt/7/product.asp -- there may be
others. Not inexpensive, and a bit of wiring is needed but works for me.

Battery monitors (AKA "coulomb counters") are not perfect because modeling
the complex electro-chemical characteristics of a lead acid battery is not
trivial, but with appropriate programming (they are programmable) they are
darn accurate,* much more accurate than using a voltmeter, and miles more
practical than probing around in a sulfuric acid-filled battery with a
hydrometer in one hand and a flashlight in the other: the electrolyte in
question is also called "battery acid" -- I'm know I'm too spastic to be
trusted around that stuff.

============
* I've programmed mine to know that the battery is a 130Ah one, and that
it is fully-charged when the charge voltage is 14.4 volts or more and the
charger (solar or tender or engine) is able to hold that voltage with less
than 200mA of current for more than 1 minute. Once those conditions have
been met, the monitor resets itself to display 100% charge. Before it
resets itself it's only off 100% by a percentage or so, so the modeling
seems to be close enough. When the battery is powering stuff, the monitor
measures discharge amps and time, curves the results and calculates
amp-hours remaining.

--
Mike "Rocket J Squirrel" Elliott
71 Type 2: the Wonderbus
84 Westfalia: Mellow Yellow ("The Electrical Banana")
74 Utility Trailer. Ladybug Trailer, Inc., San Juan Capistrano
KG6RCR



On 2/12/2008 10:23 PM Loren Busch wrote:

> My aux battery is a 110 AH AGM, under the back seat.  I've had no sign of
> problems because my starting battery is a stock Group 41 lead-acid battery.
> The volt meter seems to give me all the info I need so far.  A more
> sophisticated (and much more expensive) battery monitor that shows actual %
> capacity would be nice but I didn't go that way.  Also more complex install.
>
> On Feb 12, 2008 10:05 PM, Jeffrey Vickers <jeff@vickersdesign.com> wrote:
>
>> Thanks, Loren. What sort of aux battery do you have? I'm guessing that
>> since I'm not using an AGM type of battery for my aux, the 110V charger
>> won't care too much that my two batteries don't match. Are you finding that
>> a voltmeter is sufficient to tell you how much juice you have left in your
>> aux battery?
>>
>>