Testing AH of batteries

Battery capacity is measured using a defined minimum voltage. That voltage
may or may not correspond to the minimum voltage your glider equipment will
operate on. If you use diodes, that minimum voltage will move slightly.
Capacity also depends upon a starting voltage - which is significantly
higher than 12 volts.

I often need to test batterys used in our search and rescue radios and have
found that I can test battery capacity quickly without a full discharge. I
charge the batteries, then let them sit for a day. Then, I put a load on
them and simply watch the decay of voltage over time. In a very short time,
you can make a graph that will indicate the trend of the battery and compare
with a new battery. A battery with reduced capacity will drop voltage much
more quickly.

You can determine the capacity of the battery during charge, too. Capacity
is the ability of the battery to resist change. That applies either way.
it means the battery will drop in voltage or charge more slowly. So, you
can simply time the charge of two batteries and learn the comparative
capacity of each.

Colin