I like your test, good idea.  Won't the resting voltage of the
battery tell you it's state of charge?  or is that less accurate?
Edward

At 12:24 PM 8/14/2011, Tom Hargrave wrote:
>Charging then putting a load on the battery for a specific time won't tell
>any more than the CC test will tell.
>
>But here is a test that will tell you something.
>
>Fully charge the battery.
>Put a known load on the battery - something like a light that draws 20 amps.
>Watch the battery until the voltage drops to 11.8 volts.
>Subtract the start time from the stop time then multiply the hours X the
>load.
>The result will be ampere-hours delivered.
>
>You need to do this test on a new battery right after you install it &
>record the result as a baseline then do the exact same test with the exact
>same load later when you expect a problem. And don't expect the results to
>be the same as what's marked on the battery. If you test again a couple of
>years later and the time is 20% less than your capacity has dropped 20%.
>
>Also, it's extremely important to use the exact same load. As load increases
>the ampere-hours delivered decrease because more energy is turned into heat
>inside the battery.
>
>Thanks, Tom Hargrave
>www.stir-plate.com
>www.towercooler.com
>www.kegkits.com
>
>
>-----Original Message-----
>From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
>Edward Maglott
>Sent: Sunday, August 14, 2011 10:26 AM
>To: vanagon@gerry.vanagon.com
>Subject: Re: Battery testing solution
>
>I have thought about this many times.  For our "house" batteries we
>don't really care about CCA, we are concerned about capacity.  I have
>gently used various "inappropriate" batteries for my house battery
>over the years, like starting batteries or SLA batteries designed for
>UPS systems.  I have found it hard to even find out the capacity of
>these batteries, they just advertise CCA.
>
>I have wondered what would be a good laymans way to determine the
>remaining capacity in these batteries as they age.  Maybe
>this:  Charge them up. Let them rest for a while to get a good
>resting voltage.  Put a known load of a couple Amps on them for a
>fixed time.  (Not discharging too low of course.)  Let them rest
>again.  Measure an ending voltage.  Calculate something.  There must
>be some Electrical gurus (like David B.) on this list who could
>devise a good method so that we don't have to buy that $3K analyzer.
>
>Edward
>
>At 10:17 PM 8/12/2011, Jim Arnott wrote:
> >"The purpose of battery testing is to estimate the end of battery
> >life. Automotive technicians are most familiar with CCA, a measurement
> >that reflects engine cranking but does not always predict the end of
> >battery life correctly. The leading health indicator of a battery is
> >capacity. So we ask, "At what capacity should the battery be replaced?""
> >
> >http://www.ecnmag.com/Articles/2011/04/Design-Talk/Battery-Testing-at-Garag
>es-Simplified/
> >
> >
> >Go check out the article. See what $3k+ gets you....
>-----
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