And to elaborate a bit on both Mark's and Eric's notes.

The modelling fraternity often completes these type of tests on their
battery packs,
you may find a suitable discharge unit available from their suppliers? (I
have one from
years ago here in the UK)

It is good to create a graph of the results of each battery pack over a
typical
life span. You will quickly see then if the capacity is changing and how
quickly.

Remember, over charging is often the killer for all batteries.

Malcolm...

"Eric Greenwell" wrote in message
...
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.

--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA