"Tim Newport-Peace" ] wrote in message
...
X-no-archive: yes
In article , Malcolm Austin
writes
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


The is available an 'intelligent battery tester' that will quickly
measure battery capacity without the need to do a measured discharge.

Visit www.actmeters.com. The model I use is ACT-IBT, although the price
may put some people off (web site has prices in pounds and dollars).

Best regards,
Tim Newport-Peace

(No commection with ACT meters except as a user).

It looks like there's an opportunity for a data logger update here.

The logger is already in the aircraft. It's generally connected to the
battery.
An 8 bit A/D would give an accuracy of 20 mV across the 10-15V range.
Sampling once per minute or so should be adequate. Post-flight analysis
should give you a pretty accurate idea of the health of the battery..
Battery health information doesn't really need to be real time, since you're
probably not going to be able to replace it in the middle of a task, anyway.

Tim Ward


Tim Ward