UPS runtime

August 20, 2010 8:10 AM

UPS/Battery backup question - I need a battery backup power source that will last as long as possible. My load is only 30w, so I understand that even a small UPS will power it ok - but I need to maximize the time that the UPS will continue to keep it going.

UPS's are typically rated by VA and Watts - they seldom quote battery total power capacity. Am I right in thinking that to get the maximum run time (for my small load), the VA capacity is basically meaningless; what I want is a UPS with lots of cells/batteries (and is that measured in amp-hours AH?).

I read somewhere that the typical $100ish consumer UPS isn't designed to provide power for extended period (internal components burn out?) - is this true? And if so, it is a danger that a 'small' UPS - if I get big batteries for it - might destroy itself if it's asked to run for hours (rather than the minutes many UPS last for)?

Any suggestion of US retail-available kit I buy to run my 30w AC load for hours? I hoping that a higher rated UPS (like a 1000VA unit) will have bigger batteries and therefore a longer run-time ...??

Thanks

posted by Xhris to Computers & Internet (8 answers total)

I've got a Server-room APC unit that would do that, but it's was not cheap. Generally, these are rated to run at 8 hours or so, which should be enough considering you're drawing considerably less than a rack full of servers.
posted by Oktober at 8:17 AM on August 20, 2010

APC has some calculators that will help you find a UPS based on draw and desired runtime.
posted by msbutah at 8:25 AM on August 20, 2010

I have run an ~20 watt load for several hours on battery before on some 1500va APCs. I have a feeling I'd get a lot more runtime out of a homebrew system or using DC directly, since there would be no inverter or unnecessary fans. (my load is a <1>
I think it's the SmartUPS XL line that has a connector on the back for attaching another chassis full of batteries. IIRC, they daisy chain, so you could have 3 or 4 extra battery modules.

The cheapest solution would be to put something together yourself with deep cycle lead acid batteries, which could get you any arbitrary runtime you're willing to pay for at the cost of the batteries, an inverter, and a good charger.
posted by wierdo at 8:53 AM on August 20, 2010

Hmm, that didn't work so well. My load is a 10 watt-ish Mini-ITX computer and a DSL modem.
posted by wierdo at 8:54 AM on August 20, 2010

We were without power for a week due to the May 10 tornadoes here in Oklahoma and I ran some basic necessities (radio, CFL lights for brief bits of time, etc) for many days on a 700 VA UPS.

I'm not sure what you're trying to power, but speaking from this experience you may want to consider an auto power inverter which costs about $30-50 and plugs into the car cigarette lighter. You can run a long extension cord into the house, turn the car on, and run it for days that way.
posted by crapmatic at 9:14 AM on August 20, 2010 [1 favorite]

FWIW, I think crapmatic is on the right track, in that if your requirements are for longer-term power support, a UPS is not what you want. UPS's, in general, are in place to do two things:

- "Clean" an uneven power flow.
- In an outage, provide enough power to a computer so it can shut down gracefully.

A higher-end unit will provide more power, yes, but the intent is that it provides power to more things, not more power to fewer things. A generator is a solution for ongoing, sustained power, I'd think, though you'd have to solve the problem of how/when to turn it on. How long-term are we talking about, and how much equipment? Is uninterrupted power as important as being able to provide power in a blackout?
posted by mkultra at 9:52 AM on August 20, 2010

Response by poster: Thanks for all the ideas.
The load is a set of receive-only VHF radios - they need to be on all the time and are un-monitored (by humans) . So I need something that will switch on or switch over automatically. The longest power-outage we've had was 3 hours.

They are AC powered. I'm not so keen in a custom built thing. They are on 365/24/7 - and I personally don't know if a $30 inverter is built for continuous usage (so I just don't know if a charger-battery-inverter always-on is the right setup?).

A generator seems overkill for 30w?

I figured a off-the-shelf UPS has all the technology I need - if it will last a few hours. Or if I can piggy-back a bigger battery (like a car battery), without it burning out the circuits on the UPS through multi-hour usage?

Regards
- Xhris
posted by Xhris at 12:08 PM on August 20, 2010

You'll get close with one of the 1500VA APCs. You might need to get a rack mount model that can take an extra battery, though.

I would be more specific, but the APCs I have overestimate the load and underestimate the runtime on my small load, so I can't just query my UPS and calculate from there. Since it's about a thousand miles away, I can't just go unplug it and time it, either. :(
posted by wierdo at 4:29 PM on August 20, 2010