Re: measuring 12V 12Ah Lead-Acid battery
From: aurgathor (spam-me_at_if-you.com)
Date: 12/14/04
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Date: Tue, 14 Dec 2004 12:37:12 -0800
Thanks for the replies.
> Well, you've proved for yourself that open-circuit voltage
> is no use as a measure of capacity.
Yes, I do know very well that the open circuit measurement
may not be very meaningful, that's why I'm asking for help.
I guess I'll just put up some load and measure how long it takes
run it down to 10Vas you said, and calculate capacity from that.
Although I think there exists some equation or formula which
allows capacity can be computed from a few measurement points
taken in time under some reasonable load.
> Sometimes, an APC UPS will give a false 'battery low'
> indication if you try to start it with a load connected.
> Disconnect all loads and thenstart the UPS
While some bad battery warning happeneed when I tried to start
my PC on a non-fully charged UPS, the irritating ones seem to
happen randomly after some time (usual;ly weeks) of running.
Turning everything off, then restarting the UPS first usually fixes
the problem.
> forget about measuring the capacity, and just connect it
> up to your load
This is simple, but it also puts in the conversion losses.
> If you depend on this UPS for backing up your important
> computer, then don't mess around. Get a new battery.
Nothing is very important that really require UPS protection,
but I have quite a few brownouts and dropouts usually lasting
for a few seconds, and they can be very irritating.
> it is probably not passing the load test that the UPS does.
That's very possible, but the thing I don't understand that
it can run for a long time just fine, then fail randomly. The
first time it happened was about a year ago, afterwards I
lessened the load on the UPS, but it happened again after
a while. I'd say the frequency is about 1 warning every other
month. If I had daily or constant warnings, yes, I'd say the
battery is probably no good.
> If you have $300 to spend, get this great battery checker.
Very nice, but I don't think I can justify it right now. :-(
> Most of my failures have been due to increased series
> resistance. Just replaced a set yesterday. 6V cells
> measured almost 7V open circuit and quickly dropped
> to 4.3V under load. Stick on enough light bulbs to match
> your load and measure the battery
Will do that, but I need to get a 0.1 ohm or so resistor to
measure current first.
aur
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