All batteries designed for cycling applications are rated in amp hours over
some time period. They are usually considered 100% at 10.5 volts and for
longer life you only want to discharge them down to a 20% stat of charge.
The usual test is known as load discharge test and is usually done with a
load resistor bank.

Depending on brand 5 or 6 hours is the most common recommended discharge
rate. Some will list a 20 hour rate as this raises the total A/H delivered.

So for an example a 60 A/H rated battery should be able to support a 10 A/H
load for 6 six hours before the voltage drops to 10.5.

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
Edward Maglott
Sent: Sunday, August 14, 2011 11:26 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-G
>arages-Simplified/
>
>
>Go check out the article. See what $3k+ gets you....