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Build your own UPS at home, for cheap (instructables.com)
14 points by profquail on Aug 31, 2009 | hide | past | web | favorite | 17 comments

A note of caution. The power output from a UPS while running on battery is not a pure sine wave like line AC. Its an approximate sine wave consisting of a collection of square waves. This output is generally very noisy (i.e. has a lot of high frequency elements). The output is "close enough" to a sine wave for most electronics to run but the high frequency noise is damaging to the input circuits because they generate a lot of heat. This problem is worse on the low-end UPS's because one way to cut the costs is to save money on the DC-to-AC converter and accept more noise on the output line when running on battery. The higher-end UPS's (particularly models that are designed to run long term) have higher quality DC-to-AC converters which is why they are more expensive, not due to just larger batteries.

One final note, this project seems like a really uneconomical use of time and money.

One thing you could do to smooth the waveform, since you're building the battery pack anyway, wire it for half or 2x the voltage (with corresponding decrease or increase in amperage) and run it through a transformer to the correct voltage. You will lose some power in the transformer, but the waveform will be much smoother.

This is easily the most dangerous post I've ever seen on Hacker News. I really hope no one gets hurt trying this. A total lack of safety advice in the section involving connecting the batteries in series is particularly disturbing.

My apologies for not including the safety information. There is another very similar article that includes some safety tips:


I have done this for years. I currently (no pun intended) have two such setups operating. Hooking up batteries is no more dangerous than jump-starting a car, and we are all adults here -- just think things through carefully.

Car batteries, or if you want to go even cheaper, the "lawn and garden" riding mower batteries from Walmart, are not made for repeated deep discharge, as some other alarmist posts in this thread have pointed out. However, you start your car several times a day, but the power goes out much less frequently, and every power outage doesn't run it all the way down (if so, the UPS isn't helping much).

A few additional points:

1) The wires that attach to the battery should be big. Canabalized lamp cord or such will not work; worse, it will appear to work, until the compture runs off the battery for an extended amount of time, and then it can get hot enough that the plastic gets bubbly and then bursts into flame. (Not my personal experience, I use super heavy wire from old well pump wiring, that is about the size of the leads to a starter motor on a car -- about 12 or 4 gauge).

2) If a cheap UPS says it will keep the computer up for 15 min, it is likely not cooled enugh to run continously for hours without burning out. Look at refurbished computer places, or Goodwill Computer Works in Austin, and get the APC brand UPSs or similar, and look for those that have fan to actively cool them when they run on battery.

3) For reasons 1) and 2) above, TEST THE THING outdoors, giving it a full change and then putting several 100 watt lamps on it and letting it run down.

4) I put the entire thing in one fo those big plastic tubs, that looks like a big tuperware container, to keep acid from the non-sealed battery from ever getting on the floor. I leave the lid on loosely, so the power cords can get in and so hydrogen gas can not accumulate.

Another thing, is you can put jumper cable style clamps on the battery wires of the UPS, not close it up, and use it to make AC current for running saws and such where there is no electricity. If you do this wtih cheap UPSs from garage sales, you will swiftly find that you can light the cheap brands on fire, and that APC rocks.

A battery that may not be good enough to start your car on cold mornings may keep your computer up through occasional 3 min outages for several years.

Title is a little misleading. More like, "replace your UPS battery at home, for cheap"

I would be more concerned with the charging circuitry in the UPS. They are designed for use with the batteries they install at the factory, not large capacity batteries. You may run into problems with it not being able to detect battery capacity and as an extreme you may unduly stress the charging circuit with the longer charge cycles needed for the big batteries.

Assuming it's a current-limited "float charger," which is I think what almost all UPSes have (they just float some voltage, like 13.8V, over the batteries with some current limiting circuitry so they don't charge them too fast), how would you do that?

If anything, by using larger batteries for the same length of outage that you'd use small ones for, the batteries wouldn't be run down as far. That means while charging that they wouldn't draw as much current initially (although they'd draw it for longer), which might make them easier for the UPS to charge.

What I'd imagine you might want to do is charge the batteries initially with a fast/slow charger, and then hook them up to the UPS to keep them topped off.

Auto batteries make terrible UPS cells. They're designed to survive very shallow cycling at very high currents, whereas the cells in your UPS will be very deeply cycled at very low currents.

I don't know about the marine batteries he's using. I expect they're still designed for high current output, which is a needless cost.

You will probably be better-served to find a local supplier that specializes in battery supply, not an auto parts warehouse or a costco. Big lead-acids can be pretty cheap, if you don't buy them from UPS vendors.

Secondly, don't use a consumer piece of trash as your "scrap" unit. Large, commercial-grade UPSes are cheap as dirt on eBay, because their batteries are invariably shot. Just persuade the seller not to mail you the batteries and shipping becomes reasonable.

Wow. You admit you don't know anything about marine batteries and then you go on about how horrible of an idea this is? Do some research before you pull the trigger there, tiger.

Marine batteries work well in this application. I've been using them for years.

I just noticed the "Costco" label on his batteries, and that raised my suspicion. For me, specialized suppliers have been cheaper for this purpose.

The important bit in my comment is unfortunately in the second part: Don't use consumer trash when you can get the really good equipment for cheap.

Er, deep discharge marine batteries are exactly the thing to use. They are designed to withstand repeated deep discharge, hence their name. Their current output, measured on an instantaneous basis, which is how "starting current" is measured, is often less than an auto battery. Auto batteries purpose is to work very hard for about 12 seconds.

Marine batteries are deep-cycle batteries, and are good batteries for this use.

There is a trio of people claiming marine batteries are good for this use. It would be unfair to reply to just one.

Marine batteries are fine for this use. A typical marine battery might be good for 350 cycles. That should be more than enough for a UPS. (It isn't clear from the battery literature what they consider a "complete cycle", it seems like a high number for a full discharge, but even if overstated it is still plenty for a UPS).

Marine batteries do have a higher cost per stored energy compared to deep cycle batteries. I don't know if that is from the higher available discharge rate or having the word "marine" in front of them.

Renewable energy folks shun marine batteries because the real deep cycle batteries are good for 3000+ cycles, but you just don't need that for a UPS.

Most "marine" batteries are sort of a hybrid starting and deep-cycle battery. I think they're essentially a starting battery with thicker plates. They're built that way because, on a boat, you need a lot of current to start the engines but you also use the batteries as your sole 'housekeeping' power source from time to time (particularly on sailboats when the engine isn't running).

Real deep-cycle batteries would be better, but they're more expensive. Something like this http://www.usbattery.com/usb_solar_p1.html would probably be optimal; it's designed for use in alternative energy systems where you charge during the day from solar cells and then discharge during the night. Very heavy plates.

A cheaper alternative, which I've seen some people use in UPS systems, are wheelchair or golf cart batteries. Both of them are true deep-cycle designs, made for sustained loads.

I'm going to feel real dumb explaining this to the firemen.

Not as dumb as your loved ones explaining this to the hospital.

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