UPS / Battery Query

[Feline]

OFSO - Seems to me that you are re-tasking your UPS.

In its existing form, your UPS is designed to provide you with enough time to close open documents and shut down your system should you have a mains outage. Depending on the design, it may also offer some power line conditioning as well (ie. it may protect your equipment from spikes and surges).

Hooking it up to a bigger battery will, on the face of it, give you the time to complete whatever you are working on - I have a UPS rated at about 500 watts output which runs off two 105 ampere/hour batteries. This gives me about 8 hours continuous operation of my PC and screen. Problem is that it's very noisy because of the fans installed.

The possible problem with using your existing system is the possibility of it overheating - it's designed to provide a certain amount of power for a few minutes - and using it for some hours might take it out of limits.

Secondly, a bigger battery may never get to recharge completely - the smaller units use a 7.2A/h battery which is designed to be recharged over 8 hours - so the recharging current will be limited to about 1 amp which is simply not sufficient to recharge a larger battery. UPS designed for long periods of operation typically have a more sophisticated charger which provides a high charge level (say 10 - 15 amps) when the battery is very low; they then step down to a lower charge rate (say about 5 amps) as the battery comes up (this lower charge rate is so that the battery doesn't overheat). Finally, when the battery reaches its fully charged state, the charger switches to trickle charge (less than 1 amp) to make sure they stay fully charged over a period of time.

Also note that you shouldn't use a car battery with a UPS: car batteries are designed to deliver heavy currents (300 - 500 amps) to crank your engine on a very low duty cycle (>1%). The rest of the time they are recharged by your alternator. UPS batteries (designated as "Deep Discharge" batteries) are designed to provide a much lower output current (typically about 10 amps) over a much longer period of time. A car battery will have a much shorter life when used with a UPS.