Mark Zivley wrote:
Are you interested in the current charge state of the battery or it's
ability to provide power for the duration of the day?

For the charge state you simply need to use a voltmeter. 12.5 to
perhaps 13 volts would be ok. On charge the battery should be getting
13.5 to 13.8 volts.

To elaborate a little on what Mark mentioned: This voltage would be
after it's fully charged. With a "good" charger, the voltage should be
bit higher, about 14.4 to 14.8, before it drops back to 13.5 or so when
charging is complete.


If you want to test your battery's capacity you need to connect the
battery directly to a large resistor and monitor the voltage on the
battery for a couple hours. A 10 ohm, 25 watt resistor will work and by
using a 10 ohm resistor it's easy to compute what the current is by
looking at the voltmeter. 12 / 10 = 1.2 Amps. Be careful, this
resistor will be hot! The standard "gel cell" should stay above 12
volts for several hours if it's "good". My two cents anyway.

A good technique, but for the usual 7 AH battery, I'd use a 25 ohm, 10
watt resistor instead, as the load would be closer to the typical
current draw in a glider. A fresh battery should stay above 11 volts for
about 13-14 hours. Be sure to charge the battery at the end of the test,
as it will degrade more rapidly if it sits around uncharged.

Your glider's equipment might not function as low as 11 volts, but the
test does give you an idea of how good the battery is compared to a new one.

--
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Eric Greenwell
Washington State
USA