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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