How to check my home UPS’ lead-acid battery’s potential with multimeter?
My home UPS just keeps on charging the battery for days on end (with its exhaust fan whirring away), and the battery gives a backup time of about 12 mins (!) on 75% load.
So how should i check its potential? I know i have to do it in parallel, but should i first put the inverter in backup mode and then insert the multimeter’s probes into the battery’s terminals going into the inverter; or should i check when its charging (though i think the latter is wrong)??
And what will happen if i check the potential with the battery disconnected? Will the multimeter blow up with a lot of current being drawn, kinda like a short circuit?
“Unplug it, watch voltage drop. If it drops gradually over the 12minutes to about 10V”
Thanks for the quick answer, so is it safe to unplug and check with multimeter? And should i not measure the voltage when under load?
Its a 12v battery.
Some details, Its 2 years old, used to give 100mins on ~100% load. 500 watts inverter.
Damn i’m so dumb at times… of course its fine to check potential when disconnected when in voltmeter mode as its resistance is high, so little current will flow :).
Thanks for the updated details wingstwo, i’ll carry out the checks tomorrow being careful not to touch the hot lines…you can safely assume i would be more than extra careful after a narrow escape once with a very bad shock!
October 10th, 2009 at 5:20 pm
If your UPS is older than 3 years, probably bad battery.
I would hook up multimeter on DC voltage to the two battery terminals. When charging, voltage should be about 13.5V for 12V battery, half that for 6V battery. If less than 13V, charger not working right or battery sucking too much power. Unplug it, watch voltage drop. If it drops gradually over the 12minutes to about 10V or so, then shuts off, device working OK. If 12min is too small, replace battery.
Edit:
Safety is critical, of course. First, anytime working with hot circuits take care not to touch the power line, or to allow any conductive material to touch it. Be especially careful when a piece of equipment is open. If you have not been trained with electricity, have someone do it that is.
When you unplug the UPS, the battery is under load. Most UPS’s draw a battery down to about 10V, then they quit.
Yes, multimeter on volts is very high impedence, safe to apply across the 12V. I use alligator clips to hook it up and keep it hooked up.
I’ve never seen a UPS fail. But I’ve replaced the battery many times. Most likely, bad battery. I just would do the function checks to make sure the charging system is functional. It would be annoying to replace the battery then find the UPS was bad.
Most likely, the battery won’t take/hold a charge. Second possibility is that charging voltage/current is too low. Third is that UPS is malfunctioning, drawing excessive current. This is not easy to measure. I would put a very small ( wingstwo
October 13th, 2009 at 8:08 am
Don’t trust that just because you buy a UPS that its not a piece of crap. Your battery shouldn’t be charging all the time. You shouldn’t be using a Lead Acid Battery because it consists of primary cells as opposed to secondary cells. Primary cells are disposables and have very limited ability to be recharged. Build your own. Don’t bother wasting your time testing it. Rick W