On Wed, 24 Oct 2007 21:59:21 -0700, Richard Lane
wrote:
I have 2x SRM27 (85Ah) flooded batteries approximately 5 yrs old on my
sailboat. I wish to test their remaining Ah capacity by applying a fixed
current load and monitoring the terminal voltage over time.
Question: I believe the Interstate table of voltage vs state of charge
is open circuit voltage, if this is true then I would have to interrupt
the discharge periodically in order to measure the o/c voltage then
resume the load until I have achieved 1/2 remaining Ah.
Does anyone have a more direct test of remaining Ah capacity?
Dick

Battery capacity in Amp-Hours is usually spec'd at the 20 hour rate,
e.g., a 100 AH battery should be measured with a 5 amp discharge
current, and your 170 AH bank should be measured at a rate of 8.5
amps.

First step is to build a load that will draw about 8.5 amps. Some
combination of 12 volt bulbs will work.

Next step is to fully charge the batteries and leave them with no load
attached for 24 hours. The open circuit voltage at the batteries
should measure about 12.6 volts.

Now attach your 8.5 amp load and measure the time it takes for the
voltage to decrease to 11.6 volts (approximately 50% discharged).

Your effective A-H capacity is the time in hours x 8.5 x 2, e.g., if
it took 7.6 hours to reach 11.6 volts your capacity is 7.6 x 8.5 x 2.

That said, if your batteries are 5 years old it is almost certainly
time to replace them as you will probably discover when you do the
capacity measurement.