Date: Wed, 13 Feb 2008 10:24:22 -0800
Reply-To: Loren Busch <starwagen@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Loren Busch <starwagen@GMAIL.COM>
Subject: Re: Sure Power 1315 question > monitoring battery
In-Reply-To: <47B3182F.9040003@gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Mike, great writeup. Have you put something like it in the Wiki?
On Feb 13, 2008 8:17 AM, Michael Elliott <camping.elliott@gmail.com> wrote:
> 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?
> >>
> >>
>
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