Low power UPS? Also, how long will a UPS last?

[markbrannan]

I'm looking to add a UPS to protect some equipment from being powered off in the event of a power failure. The equipment in question comprises two small units which produce high voltage but at low current - the overall power consumption being something like 50W.

My problem is that as far as I understand these UPS units are most efficient at near full load, and that the backup time at full load is measured in terms of minutes - perhaps 15min at best. I am looking to keep these units running for potentially 2 days in order that if the power fails on a Saturday morning that nothing bad will happen by Monday morning.

Is a UPS my best option, or am I better trying to come up with some ultracapacitor solution?

Also, there is a whole lot of hand-waving and half-formulae on forums about how long a UPS will last under a given load - does anyone here have a decent method of calculating this? It doesn't need to be super-accurate, just something I can use to get from the generally specified figure of VA given a load in W to find out how long it will last (t).

[oldway]

Why do you need an UPS ?
With such a low power (50W), you can use a 100W inverter, with 12V car battery and an intelligent battery charger.

[markbrannan]

My knee-jerk reaction to "these power supplies can't be interrupted" was to choose a UPS, but when I started looking into it and ran into all these problems I figured there must be a better solution.

So in the battery+inverter+charger solution would the units be constantly powered by the battery+inverter and topped up from time to time by the charger (I guess this is where the "intelligent" comes in)? Would the charger inadvertently also suck some power from the batteries if the power went down?

[SeanB]

If they can run off 12VDC just use a big enough deep cycle battery and an intelligent charger to keep the battery at around 80% of full charge, so that it has maximum standby life. going to be a big battery to get 2 day run time, 50W is around 4A, so you need 4*48/0.7=around 300AH capacity. That is a close to 50kg unit, and the charger has to be able to charge at 30A peak to keep the battery electrolyte stirred.

If you use a SMPS from 48V you can use 4 100AH batteries in series, and a common telecoms battery maintainer, which will both charge the batteries and do switching from the mains derived supply to the battery along with having outputs to indicate status for the whole unit.

[Fsck]

there isn't a way to translate a VA rating to possible run time. efficiency, battery capacity and such play a large role.

throw ultracapacitors out the window unless you have an *insane* amount of space.

if space/weight isn't a concern, lead acid as SeanB says, if it is, you'll need to go li-on or some such.

[oldway]

With 50W, current will be around 5A /12V. (not 4A !) (eficiency low power inverter 12V : more or less 85%)

For 48h,  48x5A = 240Ah, you need a 480 Ah battery. (discharge to 50% of his capacity)
5 lead acid batteries 12v 100Ah in parallel.

Inteligent battery charger: min. 10A
Floating: 13.8V
deep charge: 14.4V

[denelec]

An UPS that will last 48 hours will be expensive.  Even if the load is only 50W.
http://www.apc.com/tools/ups_selector/US/en/home/load

At 50 watts, an ordinary 500VA APC Back-UPS 500 will only last an hour.