Upgrade your UPS!

(with some car batteries...)

First published: 27 June 2006
Last modified 03-Dec-2011.


A while ago, someone asked me how much my DIY UPS cost.

The answer ended up being a bit long, so I'm giving it its own page, rather than putting it in one of my serried ranks of letters columns.

Ghetto UPS!

Something as beautiful as my original DIY UPS would, of course, be cheap at any price, but the thing itself didn't cost me much. I had almost all of the bits sitting around already, and a local battery place gave me the slightly used fancy battery.

This knowledge, of course, doesn't help someone who'd like to build something like that for themselves. But that piece is five years old, now; the build-from-scratch price today would be cheaper than it was then. And, because proper UPSes (or SPSes, anyway) are now also cheaper than they used to be, there are some good options out there for happy hackers who'd like to get a good standby power solution together on the cheap. The "DIY UPS" was half tutorial on what actually makes up a UPS and half contraption-for-its-own-sake; it wasn't meant to be a practical device. But you can make a practical standby power unit on the cheap, if you start with a standard UPS with dead batteries.

The inverter I used for the DIY UPS was an old-style big-transformer unit, which was somewhat outdated even then. Today, lighter, cheaper and more efficient switchmode inverters are standard electronics-store products.

You don't have to worry about an inverter's output waveform for computer-powering purposes, because computer power supplies don't care. Any inverter that doesn't have really awful spike or voltage problems will drive other switchmode PSUs, like PCs and modern lightweight AC adapters, perfectly.

500-watt-continuous sinewave inverters (suitable to drive anything, including all motors) are down to $AU330 or so nowadays ($US250, say); 400-watt-continuous modified square wave inverters are half that price. I wouldn't bet on a cheap "400W" inverter actually enjoying delivering more than 300 watts for very long (and power factor should be taken into account; a load with a PF of 0.7 that draws 300 watts needs a "430 watt" inverter), but you can still get quite a lot of reasonably well made inverter for not a lot of money these days.

Power supply prices haven't dropped as much, but all you need is something 12-volt-ish that can charge the battery.

Like... a battery charger.

Smarter chargers may freak out if there's an inverter as well as a battery across their outputs, and the output from cheap chargers can be pretty hideous - but the main problem with them that the average cheap Chinese battery charger only has five to ten amp peak output, which isn't likely to be enough to run many normal PCs. Chargers with output above 25 amps get pretty expensive.

For the price of a 25A charger, you can get a proper bench supply with the same output capability, but you won't get the automatic charging circuit you need for long battery life (just hooking a 12V lead acid battery up to 13.8 volts will be OK for a while, but the thing'll need serious reconditioning, at least, if you keep that up for months on end). A medium-sized 12V charger should be fine to run a modest PC - which, with the arrival of low-wattage high performance CPUs like Intel's "Core" models, doesn't mean something that can't play games - and if you need more, you could go to a 24 volt system with a truck battery charger and 24V inverter that won't cost you a whole lot more, but'll raise the power ceiling considerably.

The battery is no big deal, either. A cheap "maintenance free" car battery will do ("maintenance free" batteries are just like the regular kind, only you can't unscrew the cell caps to maintain them; they're less likely to leak on your carpet, though), give you tons of capacity, and last well too as long as you don't run it flat (and even then, it can probably be rescued - the recently popularised "desulfator" gadgets really do seem to work). Or you could use the ubiquitous 7Ah sealed lead acid (SLA or "gel cell") "bricks", which are also cheap, though not nearly as cheap per unit of energy storage as basic wet car batteries.

If you've got a UPS that runs from some variety of gel cell that you can't easily buy locally, the odds are very good that it'll also run A-OK from some other lead acid battery with the same terminal voltage (which almost always means one 12V battery for smaller UPSes, and two 12V batteries in series for bigger ones).

What could possibly go wrong?

Herewith, a humble Opti 1000ES whose two 12Ah SLA bricks died of old age. Replacements would probably have cost me $AU40 apiece, ex delivery (the first US price I found is about $AU74 delivered). So this UPS is now running from a pair of K-Mart's finest cheapest car batteries, which were $AU45 each (full price - I committed the cardinal sin of Buying Something At K-Mart That Wasn't On Special).

The above setup is shown before I added a protective few layers of tape on the exposed terminals. The reason why I needed to do that will be familiar to anybody who's ever dropped a wrench across the terminals of a car battery. I may also upgrade to bolt-on contacts for the output terminals instead of the cut up old jumper leads I'm using here, but I do like the look of the big butch alligator clips.

Cheap jumper leads are, by the way, great for this sort of thing, because they've got enough copper under their heavy insulation to easily pass constant current in the tens of amps, but they're not so ludicrously thick that you need a plumber's kit to solder to them, or simply have to use crimp connections.

(Crimp connections are electrically and mechanically excellent, and it's not very hard to make a good, if ugly, crimp with hand tools. Adding solder to a crimped connection gives you corrosion resistance, but nothing else - if you're not worried about rust, the crimp itself is all you need. There's still no good way, though, to crimp a lug made for 14 gauge wire onto the shiny new two-gauge Behemoth Cable you just bought at great expense from the local Boom Car Barn. You can give the thick cable a haircut until there's little enough copper left that you can attach the smaller lug, and there are few electrical hackers who haven't done such a thing at some point, but subsequently sitting and looking at what you have wrought should be enough to put you off such butchery in the future.)

I soldered spade lugs onto the ends of these cables so I could plug them into the Opti's battery leads, which I threaded out of a hole drilled in the UPS's front panel. So, if I want, I can easily reverse the modification.

The down side of thin-ish cable for jumper leads, of course - these ones are about 9AWG - is that they're not actually all that terrific for their intended purpose. These leads successfully jumped several four cylinder cars before being replaced by nicer ones, but a big engine with a beefy starter motor won't turn too well if its starting current has to pass through six metres of 9AWG.

Getting back to the batteries: Because they're cheap car batteries, I didn't bust a gut hauling them home. There's more lead in a good battery. But these ones still have a Reserve Capacity (RC) rating of 50, which is pretty much on par for small-car batteries like these.

Car batteries often don't come with an amp-hour rating on the sticker, which is fair enough since discharging them even half-way can do them damage (desulfators aside) if you don't recharge them soon afterwards. They expect to start a car then be topped up by the alternator, after all; ordinary starter batteries are only as hefty as they are because a tiddly little battery can't deliver enough current to turn a motor.

(OK, and also so that you can leave your headlights on for half an hour, or drive for a while with no alternator, and not end up with a stone dead car.)

The RC rating tells you for how many minutes the battery, with an ambient temperature of 80 Fahrenheit (26.7 Celsius), is supposed to be able to deliver 25 amps of current without falling to 10.5 volts (at which point you'd better charge it, pronto).

For these batteries, the 50 minute RC rating means they've got a capacity at that current level of more than 20 amp-hours - about 1.7 times the capacity of this UPS's standard gel cells.

Better car batteries - like, for instance, the one I've got in my car - do not generally have much higher RC ratings than cheap batteries of the same size. What they generally have, besides a better chance of remaining somewhere near their specs after a year of service, is a higher Cold Cranking Amps (CCA) rating.

CCA is the car battery statistic that matters to people who're using them, you know, in cars. It's how much current the battery is supposed to be able to deliver for 30 seconds at zero degrees Fahrenheit (-17.8 Celsius).

Engines are hardest to crank when it's cold, and lead acid batteries lose current capacity when they're cold, so even if you're an Australian who doesn't ever expect to be starting his or her car when it's colder than 5C, it's nice to know that your battery's rated to do it.

(There's also sometimes a "Marine Cranking Amps" or MCA rating, or just a "CA" rating; that's supposed to be the same thing but at 32F/0C. It may or may not bear any relation to anything in the real world.)

My two cheap batteries are rated at a possibly-optimistic 260 CCA, versus 300 to 330 for high quality batteries of the same size.

For UPS-powering purposes the CCA rating's meaningless. If you're shopping for car batteries for a UPS, you're interested in RC-per-dollar.

Batteries for big passenger cars are the biggest ones you'll find on the discount shelf, and the good versions of those commonly have an RC of about 100 (and CCA ratings of 450 and up). The cheap ones still offer something like RC 80 (and CCA 410), though, so they too could well be of interest to UPS hackers.

("Masse" batteries, by the way, are Exide's budget brand here in Australia. They've got an RC80, CCA410 battery in the same line, presumably for the convenience of V8 owners whose fuel budget now rules out buying a better battery.)

For comparison, a little 7Ah SLA brick will be lucky to manage an RC rating of 10. It's cruel to ask for 25 amps from one of those little tackers, of course, but a mere "300VA" UPS that runs from one brick will, if it's fully loaded. Do not expect your typical off-brand brick to survive a lot of that sort of treatment.

A serious emergency or alternative power system should, of course, use deep cycle batteries, which'll be much happier in this sort of application; they're built for capacity and durability, not cranking capability. But on a dollars per amp-hour basis, there's still a lot to be said for basic car batteries.

A 12 volt UPS with, say, 90% inverter efficiency (modern modified square wave inverters are likely to do a bit better than this, sine wave inverters a bit worse), drawing 25 amps, will be outputting 270 volt-amps (VA), which ought to be enough to power around 200 watts worth of computer hardware (power factor, again). That adds up to one quite large and hairy PC, even if it's got a CRT monitor. Yes, many modern PCs can suck down quite a bit more than that if they're working hard, but I'm assuming that when the lights go out, you are at least willing to quit your 3D game and pause the cow.

A 24 volt UPS drawing the same 25 amps will, of course, be delivering twice as much power. A 1000VA 24V UPS loaded to the limit will be drawing almost 42 amps. In reality nobody sensible completely loads up their UPS (if you do, the thing'll be beeping at you any time you happen to go juuust over the limit, and will of course just shut down if you cross that line when the power's gone out, which defeats the purpose of having a UPS in the first place), so we're probably talking a ceiling more like 35A, which is well within the capabilities of cheap car batteries (and a rather alarming load for humble little gel cells).

Note that even a "maintenance free" wet battery that's sitting on a nice stable floor, rather than in a moving vehicle, can leak acid, particularly if it's overcharged and starts bubbling a bit. The acid in question is sulfuric deliciously seasoned with dissolved lead, which is not instant screaming death on a stick, but you obviously still don't want it sitting in a puddle on your computer room floor.

For that reason, it's not a bad idea to sit your batteries on a tray with a lip around it - a baking tray will do. Pour a powdered non-toxic base into the trays - bicarbonate of soda will do nicely and cost you close to nothing from the supermarket - and any drips and dribbles will fizz away harmlessly before they even hit the tray. This also means it doesn't matter what the trays are made from - aluminium won't last long against battery acid, but aluminium with bicarb on it won't touch the acid in the first place.

Charging a lead acid battery also produces hydrogen, which can be an explosion hazard in some situations but is exceedingly unlikely to be one in a home or office, because it'll just rise to the ceiling and find its way out of the building via whatever tiny cracks exist. UPS chargers shouldn't ever push car batteries hard enough that they bubble much, anyway.

This project is, therefore, quite safe, quite straightforward, and offers very good value for money.

My usual disclaimer applies, though: If your ornamental Uruk-Hai sword falls off the wall, bridges your battery bank and burns down your house, or if your actions after reading this in some other way result in someone or something being crushed, dissolved or barbecued, you can't say you weren't warned.

And here's another warning, which didn't occur to me when I first wrote this piece:

Small, cheap UPSes are not expected to run for long periods of time at full power, because their standard batteries don't have enough capacity to run them for long periods of time at full power. In other words, they have a relatively short "duty cycle".

If you swap in batteries with much higher capacity, a UPS will be able to run for much longer. If it's not having to deliver a large fraction of its rated power then this is unlikely to be a problem, but if it's heavily loaded, it may overheat and fail, or even catch fire.

This shouldn't be a problem with any UPS that has a cooling fan, like the old APC Smart-UPS 1400 that's hooked up to most of the stuff in my office. It also shouldn't be a problem with any UPS that has add-on batteries available as an optional extra. But bear it in mind if you're connecting car batteries to cheap UPSes. Err on the side of caution, and keep the total load well below the UPS's rating.

Give Dan some money!
(and no-one gets hurt)