Can I chain in series two uninterruptable power supply devices?

At the APC website, I am told that the most appropriate UPS for my setup would be 725VA or more. These run about $100, but I can get a 500 for under $50. If I can run them in series, I’ll have a more flexible setup (can always use at two separate sites in the future).

I don’t think this will be as good as a 1000VA device, but according to their numbers,
500VA 3 minutes
725VA 11 minutes
800VA 18 minutes
so I figure two 500 devices should give me at least 6 minutes.

Any flaws in my reasoning?

There’s no obvious reason why you can’t use two UPS in series. When the power fails, the first one connected to the line power will discharge, and then when that one expires, the second one will discharge.

It appears that the numbers you are giving are not the storage capacity of the batteries, but the maximum rating for the equipment attached. So by purchasing two 500VA, you are not getting 1000VA of capacity… you still may only run 500VA of load… and probably less, since you need to retain some margin to allow for recharging of the second UPS.

I strongly recommend you consult the manufacturer for any precautions you may need to take while operating these devices in series. There may possibly also be a hazard of some sort of crosstalk between the two devices’ inverters.

It should work, I think - as long as the first one can supply the normal power demands of the second; when the power goes out, the second in the chain won’t notice until the first one runs out.

But it isn’t a very efficient way to work things; you have twice as many components in line to go wrong and (depending on the design of the units) you could be stepping up and down between mains voltage AC and DC battery voltage repeatedly.

Some things to consider:

It may be better to plug some things into the 1st UPS (computer) and other (Monitor) into the 2nd UPS.

USP’s do more then just provide power during a outage, but actually converts inperfect power into a more acceptable steady power. I don’t know how one UPS would ‘read’ the 2nd USP’s voltage and may try to ‘correct’ it which would require power from somewhere.

Also I have an older UPS which won’t turn on unless it has AC power to it (the idea is once power is cut you shut down and turn it off and wait till it’s restored). I tryed to fool it by powering that UPS with a active UPS so I can turn it on, it wasn’t fooled and wouldn’t turn on.

Another option is to boost the battery capacity yourself, this is handy when you have a old UPS w/ a dead battery. Instead of paying $35 (or whatever) for a new small battery, buy a deep cycle marine battery ($50 at BJ’s) and connect it up (requires some basic skills and make sure you have the polarity correct). The old USP would last about 15 min w/ my 21 in monitor, but w/ this one I have run the computer for over an hour on battery.

Good for you that you go it to work, but it strikes me that plugging a super heavy duty battery into a charging circuit designed for a lighter duty battery might be somewhat dangerous.

Another thing to consider is the time ratings are only good for a new, fully-charged battery. Battery capacity declines over time. I haven’t been involved in this field for a while, and batteries may have been improved, but when I administered a large office-building network, I considered, as a rule of thumb, 1/3 of the original battery capacity to be lost per year. Not only did I replace all batteries after 3 or 4 years, I took the decline into account for sizing the original unit purchase. For example, if I needed a 500VA unit to hold equipment up for 10 minutes, I would purchase a 1500VA unit, anticipating the third year of battery life.

Sure, I’ve had gel-cels last much longer than 3 years, but reliability was more important to me than squeezing out a few dollars of savings by taking a greater risk.

There’s nothing worse than expecting the unit to perform only to find out it is seriously underrated when something happens. And don’t forget that multiple power outages a few minutes apart won’t give the charging circuitry enough time to restore the full capacity, either.