Why do UPS have such tiny batteries?

Chris Green · 2025-11-10T14:32:06+00:00

I'm looking for a UPS (I might DIY, depends on what I can find to buy)to run our VOIP phone, ONT, router and some other bits and pieces ifwe get a power cut. As we are in a rural location power cuts aren'tall that rare and can last a few hours sometimes.Most commercial UPS systems seem to have what I would reagard as**tiny** batteries. For example this Armour 1000 watt/1500 kva UPShas 2 x 12V7AH betteries. If it really can supply 1000 watts that'saround 80 amps at 12 volts. That would discharge the batteries inonly a few minutes (if they coped at all). The data sheet claims"15-20 minutes (dependent on load)".This is a UPS from CPC/Farnell. Most of the other ones they offerseem to be quite similar in only offering very short run times.I don't need very much power, probably only a few hundred watts so aUPS of the power of the example above would be fine except that I needit to run for hours rather than minutes. I do actually already havesome suitable batteries which came from our boat which has solarpanels and batteries for use when we're away from mains power. Thebatteries I have are 12v 100Ah deep cycle ones so would provide powerfor some hours at the sort of consumption I'm looking at.Are there any UPS out there that allow one to connect externalbatteries? That would be the ideal answer for me as a self-containedbox with the required monitoring and electronics would be nice (andlooking at prices of ones like the above quite cheap), I'd just haveto provide and maintain the batteries.

Nov 10, 2025 Last reply: 9 months ago 44 Replies

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John R Walliker 10 months ago

The telephone industry is all based on -48V supplies and PoE has in some cases inherited those conventions. I will check a few of my PoE switches and report back... John

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Sam Plusnet 10 months ago

I was just reading this thread when the power went out. Only a 5-10 second outage - but I'm blaming all the people in this thread for causing it.

When we had a similar length cut a couple of years back, it cost me a new PSU for my desktop PC. The NAS was unaffected, but I bought it a UPS (to allow it to close down in an orderly fashion) as a 'one bitten twice shy' precaution. The PC isn't supported by the UPS, but it does have a clean power feed from it to get rid of spikes.

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John Rumm 10 months ago

As a side note, something to watch when running a CH boiler from a UPS or genset, is making sure that the neutral of the power supply is tied to earth at some point. Many inverters will provide an output where the legs are separated by 230V, but both are floating wrt earth. Any boiler that uses flame rectification to sense when the flame is lit, may trip out without a supplied referenced to earth, because may not be able to sense the flame properly.

You could also run a SIP VoIP client on the phone to access your "landline" number. (assuming VM will tell you the details!)

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