Battery Backup Guide

Patrick asked:


I had a 350W AC motor that did not work for this purpose. Do I need to have a DC motor? Also, what type of battery should I use? Obviously, it must be rechargeable so should I go with a lead-acid battery? This system will have a human-powered crankshaft, a motor, a battery, an inverter, and will need to produce 100W maximum to charge my laptop battery through the inverter.

StarryKnight asked:


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!

Harish Kumar Mysore asked:


i have an old UPS rated around 500VA. It uses a 12V 7.5Ah sealed lead acid battery.? I have a 12V 250Ah car battery also. Can I replace my UPS battery with car battery. I’ve heared that it may cause come problem with the charger. If I can not replace the battery can I use the car battery by charging externally?. This is for using the same as an inverter at my home(Any explanation, details, links or schematics wolud be appreciated).