From SLA to car

A reader writes:

Having read this...

What could possibly go wrong?

...I became inspired to upgrade my UPS as it's time to replace the 5.5AH gel cell, so why not kill two birds with one stone.

Unfortunately, I don't know a heck of a lot about the ratings and other tech jargon behind what will make this all work, so I am sending this email in the hope that perhaps you could take a moment to take a look at what I have and let me know if it seems likely that it will work for a start and then what I should go out to buy to make it happen. I should at this point mention that I live in Thailand, the land where no matter what you want to buy, you can't find it. But still, given that I have a UPS unit and access to a place that sells cheap car batteries, I figured there may be hope.

Firstly, this is what I have. (The specs are in English at the bottom of the page.) The gel cell inside is a "Model AC-1255" rated at 12V 5.5AH/20Hz in case that means anything to you.

Does it seem likely that if I connect a car battery (or two) to this device I will be able to achieve similar results to what you did in your article? () Or is this UPS just not up for the task of keep a car battery or two charged and ready for the task at hand.

Out where I live power is OFTEN interrupted, but rarely more than 5-10 minutes at a time (90% of the time it's just a few seconds), but of course those few seconds are the ones immediately preceding my clicking "submit" on a 2 hour email type-up marathon. I NEED to have some form of UPS going but am not looking for hours of use after power-out. Just enough time for me to shut down the system gracefully.

I would appreciate any insight you could offer to my options and if you need any further information on the bits I have here, just let me know.

Many thanks


Fortunately, this is a pretty easy job. If you screw up, though, it can be quite dangerous.

Here are the ways in which you can get it wrong when hooking up new batteries, especially bigger new batteries, to a UPS:

1. A given UPS runs from 24 volts, so it wants two 12V batteries in series; you give it one, or two in parallel.

Danger: Possibly high, if you thus barbecue the batteries with too much charge voltage. You'll probably just get loud complaints from the UPS, though, and if you're not completely daft you'll disconnect the batteries before anything can go pop.

2. The opposite of the above; it wants one battery (as your particular UPS, like most small UPSes, does), but you give it two in series. (Two in parallel would be fine.)

Danger: Will probably kill the UPS. Probably will not set it on fire.

(Home/small-office UPSes are almost always 12V or 24V on the battery side, meaning one or two 12V batteries. Big serious UPSes may run more batteries in series - possibly built out of individual two-volt cells that are each bigger than the whole 12V battery in your UPS - because the higher the voltage the lower the current for a given power output, and big serious UPSes can usually deliver a lot of watts. Lower current is desirable because it means thinner wires and cheaper power transistors and other components. This is also, essentially, why big long-distance power lines run at such high voltages.)

3. You connect the battery or batteries backwards.

Danger: May or may not blow up the UPS. It's quite easy for the designers to guard against this mistake, but I've no idea how many do.

If your new battery is the same type as the old one, you have to be pretty seriously dedicated to screwing up in order to connect it backwards. It'll probably be connected with two spade lugs of different sizes; getting them the wrong way around can only be achieved if you're the sort of person who hammers a USB plug into a VGA socket.

If you're connecting a UPS to a bigger battery that has different connectors, though, it's usually quite easy to connect it backwards.

Whatever happens, this particular mistake probably won't set anything on fire.

4. You accidentally short out one or more of the batteries. Even little sealed-lead-acid "gel cells" can deliver a lot of current into a dead short, and very high current delivery is the major design goal of car batteries. The worst possible way to do this is to have a couple of batteries you're trying to connect in parallel, and to accidentally connect one of them backwards. (This is also what happens if you get the leads mixed up when jump-starting a car. In that situation one of the batteries is usually pretty flat, but a quite stimulating physics demonstration may still ensue.)

Result: From alarming to spectacular. Red-hot wires. Smoke and possibly flame. If you break the short-circuit quickly, though, the batteries themselves should be OK.

If you're building a battery pack for a cordless drill or R/C car or something, you can do it with discharged cells, which makes accidental short-circuits harmless. You generally can't do that with lead-acid batteries, because running them flat damages them. On the plus side, if you're upgrading a UPS battery you're probably not soldering any cells or batteries together; on the minus side, while you're running longer wires to connect a bigger battery outside the UPS, there are many opportunities to short the battery out.

(If you've got a liquid-electrolyte lead-acid battery, you can drain it of electrolyte while you work, which makes it harmless, just like building a battery pack from flat cells. The best solution if you're going to be fooling around with wires connected to a high-current-capacity battery is to buy a brand new battery that comes "dry", and buy your electrolyte separately. Note that lead-acid battery electrolyte is roughly 30% sulfuric acid, and should be treated with respect; battery acid won't melt the flesh from your bones, but it is still not your friend. This is all overkill for what we're talking about here, but I want to be as exhaustive as possible in writing about this stuff for the benefit of readers whose situation is not the same as yours.)

By now you are probably just about ready to throw up your hands and trade your computer for a manual typewriter, but I really did mean it when I said this job is pretty easy. You'll very probably be fine. Take your time, do not mitigate any uncertainty you feel with alcohol, and keep track of which wire's meant to be positive. If you do not own a cheap plastic multimeter, buy a cheap plastic multimeter. Some basic soldering ability will also be handy for extending power wires, but you'd get away with using wire nuts or something. (You'd probably also get away with twisting wires together and then mummifying them in leccy tape, but doing so makes the ghost of Nikola Tesla cry.)

And now, finally, specific answers to your actual questions.

I don't know whether your UPS will actually be happy running from a car battery, but it very probably will. I used to be less confident about this, but I've done it more times myself now and corresponded with plenty of other people about it, and it really does seem that most, if not all, consumer-market UPSes will work fine from much bigger batteries. They don't charge a big battery very quickly, but unless your local electricity is a ten-minutes-on, two-hours-off sort of deal, that's not a problem.

Car batteries are not an ideal choice for running UPSes, because they've got less capacity per kilo than batteries made to run, for instance, golf carts or fishing-dinghy trolling motors. Car batteries also don't like being run flat. But the price/performance ratio for low-end car batteries is much better than that of fancy deep-cycle batteries, and car batteries' shortcomings are largely irrelevant to someone like you who mainly just wants to ride out short power interruptions, and doesn't anticipate running from battery power for any great length of time.

(It also seems pretty definite now that lead-acid batteries that've "sulfated" because they were run flat and left that way can be rescued, with "desulfator" gadgets. I haven't done enough research of my own to be able to speak authoritatively about this, though.)

The specs on the side of your battery only matter if you're trying to buy a new one that'll fit inside the UPS, without having to know the exact dimensions of your old battery or the one you're buying. There is unfortunately no standardised naming for SLA batteries, so the "Model AC-1255" on the sticker is not helpful.

The most common battery in small consumer UPSes is a brick-shaped 12V unit with about a seven amp-hour capacity; the battery you've got is I think probably this size, but it doesn't matter since you're not after another weedy little gel cell.

(I've no idea what the "20Hz" on the sticker means, by the way. Batteries are not alternating-current devices, so whatever that is, I don't think it's meant to mean 20 cycles per second.)

One cheap car battery will probably do the job for you just fine. If you needed longer run time then you could add one or more extra car batteries in parallel (preferably identical batteries, by the way, though in relatively low-drain applications like this you can get away with all sorts of unsightly alternatives), but that doesn't seem to be the case.

Get a set of cheap jumper leads along with your cheap battery as I did, cut 'em up and splice them onto the UPS's existing battery leads, hook it up, and enjoy some relatively reliable computing.

13 Responses to “From SLA to car”

  1. Oli Says:

    Probably worth mentioning that it's advisable to charge all cells to the same level before connecting in parallel (sorry if you did mention that)

    Given my experience in designing SLA charge systems, if they've used a charge controller (they almost all do) it will have active current limiting (commonly used is the BQ24450 from unitrode, now TI). The only real problem might be connecting lower capacity cells that can't be charged at that rate, in which case they will gas slightly during the charge process - shortening the life of the cells.

  2. Chris Lineker Says:

    Here is what can happen if you hook up your jump leads wrong :D

  3. james.k Says:

    I think the 20Hz is probably actually 20Hr, so that particular battery is a 5.5Ah capacity when measured at a C/20 discharge rate - as most of them are.

  4. Jens Says:

    I found it is best to just use the cheap and cheerful car batteries from an economical point of view. All kinds of "specially foor the task" batteries in my boat had similar life spans, similar capacity. Unfortunately the price for them was not similar. Just make sure your UPS has a protective circuit that prevents the batteries from running flat (usually cuts them of at about 11 resp 22 ish Volts)

  5. Anne Says:

    It's not always as easy as just adding a diode to prevent zapping a device by putting the battery in backwards. The problem is that diodes drop a substantial fraction of a volt, forwards. For this application that might not be a problem - though if much current is drawn from the batteries, this diode might have to dissipate a fair amount of heat - but for small gadgets powered by one or two AA/C/whatever cells, that 0.7 volts might be wasting half of the power and dopping the voltage below what the gadget needs. I think a more common safety setup is to set up a diode in parallel with the load, so that if the polarity is reversed, the full current goes through the diode, blowing a fuse and cutting the circuit. Even more common in portable gadgets are battery compartments that just won't make contact with a battery that's the wrong way around.

  6. Oli Says:

    @Anne: there's always the use of an 'ideal diode' aka a repurposed MOSFET to reduce forward voltage drop while giving similar protection

  7. verstapp Says:

    >2 hour email type-up
    Most email progs will, at a keystroke, Ctrl-s is common, save a draft of what you've typed so far, so after a power outage you've only lost what you've typed since the last save.
    Many, eg Outlook, will do this for you automatically every x minutes, once you've set 'x'.
    None of this suggests that you shouldn't fix your ups, just that there are options out there that will mitigate the problem with or without a ups.

  8. snakeboy Says:

    @verstapp - Online forums and "BBS's" have no such functionality as you are composing your text server side. I have lost many a private message or lengthy post on a forum for this very reason.
    Yes, one could compose in a text editor locally, saving frequently then copy and paste it, but BBcode and smilies present on that particular board may not be available to you . And most people cannot type with BBcode "on the fly" anyway. The easiest solution: Avoid power outage with your UPS and use your computer as it was intended. ;)

  9. Fallingwater Says:

    @snakeboy: there is a Firefox plugin that autosaves form contents and restores them in the event of a crash. I personally just run the text editor when I notice a post is getting long, and that autosaves all by itself, but if you don't feel like doing that the plugin will work. I don't remember the name, but a quick google search will probably give it to you.

  10. RichVR Says:

    The Firefox plugin is called Lazarus. At times it can be a sanity saver.

  11. WSmart Says:

    We may be on the verge of mitigating ourselves out of existence, speaking of alcohol.

    Wondering how hard it is to get a desktop to run directly off DC the way a laptop does. Four 32650, D cell size lithium secondary cells will give you 12v @ 5500 mah I believe. That would provide enough time to save and shut down,no 50 LB lead battery or mystery hardware required.

    Beer is queer: be real, be sober.

  12. Anon Says:

    A desktop computer will expect +12 V, +5 V, +3.3 V and a little bit of -12 V all in DC so you can certainly run one off DC (as my computer is doing right now, largely due to having an internal power supply converting the 230 V (±10 V) from the wall into those various voltages.

    To run directly off batteries you'd probably be looking at buying a DC to DC converter (which should also provide voltage regulation). I should also note that a lot of ITX based systems and similar small form factor desktops run from power bricks (and so can already take DC).

  13. Eric Garner Says:

    "line interactive" type UPS like the one in your post are not intended to provide power off of battery for long periods. the limiting factor is not usually the battery capacity in the system, but the ability of the transistors in the inverter to get rid of heat. if you are looking for extended runtime off of large batteries, consider using an "online" or "double conversion" type UPS

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