High power long-life UPS, homebrew?

Identify the load and do the math. Large loads through time require LOTS of batteries.

I disconnected a 125 kva UPS last week. There were 120 batteries in a array.

please determine max load in watts.

if you shop around you can find new 2 kW 12 volt inverters for about $150 USD. it will be difficult if not impossible to rig them to start and changeover automatically the way a good UPS does without a brief interruption.

my largest UPS is 15 kW and provides about 30 min backup for 9 studios plus an equipment/server room. normal load is about 50% of 15 kW. it is continuous conversion 3 phase. it cost more then my Jeep the back up generator auto starts in 15 seconds.

continuous duty units must be built more robustly therefore will cost more.

many of the budget UPS units need only last 5 to 8 minutes at heavy load. you can cut all kinds of corners when you don't have to worry much about heating effects.

unless you enjoy having a bank of batteries about the volume of a refrigerator (which will need replacing every 3 to 5 years) reduce your load, reduce your runtime, or both. invest in an automatic generator and transfer switch. make sure the generator is stable enough that the UPS units wont engage and run themselves down anyway.

I just wanted to explore the idea before getting specific, but we're talking about 5 PC towers, a couple of 21" monitors, assorted comms kit, a couple of laptops, and a back projection TV (not so much because we want to watch TV when the lights go out as we don't want to be left with out the cooling fan if it was switched on). About 5kva, ballpark.

The problem is really the run time: if I don't want to invest in an autostart generator -- expensive -- I need something that can power the basics for a minimum of a working day whilst I'm out of the house.

The extended battery packs on the APC 5kva units suggest they're rated for longer use than just shutdown time. Is practical to disconnect the internal charger and use an external unit with a deep discharge battery?

That's a good point. What kind of issues would make the generator unsuitable for powering a UPS?

Ian ...

You said you want to run the UPS for a day without external charging by generator. For a whole house system, that's a pretty big demand unless you are strictly talking about running a radio and a few lights.

What kind of loads are you connecting? Are any of these going to be motor loads?

Air conditioners? Air Handlers? Microwave ovens? Washer/Dryer? Dishwasher? Clocks? Sensitive Electronic Equipment? Refrigerator? Garbage Disposer? Any heating appliances like coffee makers, grills, toasters?

Motor loads may have starting current issues. Your inverter and battery system must be capable of supplying the starting current and then some. This is where it can get expensive.

Beachcomber

Hi Beachcomer --

You may have missed this:

... but we're talking about 5 PC towers, a couple of 21" monitors, assorted comms kit, a couple of laptops, and a back projection TV (not so much because

we want to watch TV when the lights go out as we don't want to be left with out the cooling fan if it was switched on). About 5kva, ballpark.

That's max of course. If I'm out when the power went off it would be whatever PC were on and the comms kit.

Ian ...

To answer your question about what problem you can run into let me tell you a true story. A radio broadcast company had a couple of stations located in the 18th story penthouse suite of a new bank/office tower. They used numerous small UPS units (up to 1.5 KVA) some used 15A twist lock connectors on the cables (with standard nema 5-20r on the units so they could not easily be bypassed). There was no installed generator but there was a manual transfer switch. One day the building management announced that they were going to do an 'annual' electrical tighten down on Sunday. This would require complete turn off of power (except elevators I believe) for a whole day. The stations engineer went down to the U-rent-um tool place and picked up- whatever generator was available. come D-Day much to the shock and dismay of everyone when the power went out and the generator (out on the balcony) went on, the frequency stability was so poor that the UPS units remained engaged. The batteries ran down and in fact I believe it was so bad the computers would keep locking up even when plugged in direct with extension cords. The enterprising engineer managed to fine a big roll of romex wired plugs to it and lowered it down 18 stories to a parking garage.

Now it turned out that there was a way to loosen the tolerances of these particular units by getting step by step directions from the makes tech support. it involved pushing combinations of buttons on the front panel in bizarre ways.

Of course this anecdote wont apply to all situations. A continuous conversion (AKA double conversion) UPS might just take any kind of power it sees and be happy. if your running a home business with web hosting or the like it may be well worth the investment.

I suspect you are overestimating your load. Invest in a kill-a-watt meter (or the like) and measure your real load.