How long will my UPS last on battery power?

[pine-o-cleen]

Hi,

I have a UPS that is powered by a 12v 7Ah battery. What I want to know is how long (for example) a 25w 230v appliance will run for on that battery, when the power goes out.

How do I figure that out?

[Cato]

Should be a rating on your UPS?

[pine-o-cleen]

Doesn't seem to be any? Just has model, serial number and input and output voltage. I took it apart to change out the battery, and that's the only reason I know the rating of the battery.

[bob_doe_nz]

Plug it in and try it.

[KarameaDave]

In theory if the battery is new it should go something like this.....
12vx7ah=84wh less 20% or so for the inverter losses is 67.2wh
Divide that by your 25wh gives 2.668 hours.
I think.....?

[Paul.Cov]

I was trying to breathe life into a dead UPS a few weeks ago, and the research I did at the time was suggesting about 30 minutes if you're lucky, and depending a lot on the drain of the appliances running off the device.
Domestic units should really be considered as an opportunity to close programs and shut down normally before it dies, or as a remedy to those brief power glitches that are enough to cause the system to reboot, rather than an opportunity to carry on as normal during a power cut.

I'm too vague on the amp-hour bit to know if Karameas maths is correct, although my initial guestimate was exactly as Karamea has shown.

However, if we're talking about a PC with a 400W PSU as the appliance, then you've got a lot less time to react and shut down.

Other issue, if the appliance is not a PC, is that the PSU power-out connector is one of those female PC type power plugs, so you'd have to re-wire a cord to adapt it to an NZ power plug.

[R2x1]

Just don't try taking your battery much below 50% if you want it to have at least one birthday. Gel cells in particular do not thrive on any great depth of discharge.
Also, bear in mind that the 7 AH rating is measured at either a 5 or 10 hour rate, 7 amps is not going to be maintained for an hour. 700 mA for ten hours should be possible with a new battery at least once, but it will damage the battery. Life sucks sometimes.

[wainuitech]

A Lot depends on the size of the battery, how much stuff you have connected to it, PC, monitor, Speakers,Printer, Modem/Routers etc.

Remember they are only designed to keep power going in the event of a power cut and stop a PC from suddenly shutting down, if its a prolonged cut you have to turn off the PC.

The Ones in the lounge last about 30 minutes, they have PC, Monitor,Speakers attached. mine lasts about 20 minutes, it also has the Telstra Modem,Router & a Switch.

Larger UPS's are available to last longer, but usually cost Sh1t Loads

[pine-o-cleen]

Thanks for the info guys.

[porkster]

Your UPS will last 30 seconds shorter than you need it (A digital Murphy's law)