Diy Dual Conversion Ups

diy ups

for a while now, i’ve been grabbing cheap used ups boxes to run my home servers off of.  problem is, they are just about useless.  i can run an old p2 (no monitor!) off of one for about 5 minutes before it fails.  even if i were to chain a few of them together, the puny sla batteries in them typically go dead a month or two after i buy them anyway.  consequently, i’ve been going through them like candy.

searching for a solution, i came across this article for building a diy ups.  with an inverter, a power supply or charger, and a car battery you can have yourself a pretty solid dual conversion ups that is expandable to meet your downtime needs.  it’s worth a read even if you just want to know how a proper ups works.

29 thoughts on “Diy Dual Conversion Ups

  1. Mmm, strange… I’ve been buying used UPSes too, and I’ve had no problems reviving them. All you need to do is to replace the sealed lead accus with completely new ones (which can be a sizey investment – I know someone who has a nice 19″ APC UPS but has to shell out at least EUR80 to get new but el-cheapo batteries) but then the UPS will be as good as new. I still have an APC here, which can power my server (P3-1000, 4 HD’s) for at least half an hour. The batteries in them are almost 3 years old by now btw.

  2. Doofus, if you’re going to use a car battery, just run some lines from the pos and neg contacts on your UPS to that car battery, and you’re good to go.

    And be careful of acid spills and hydrogen gas, stuff like that. Don’t examine the water level using your trusty bic.

  3. One thing with UPSs… due to the way they convert DC->AC when running of of battery power (square wave vs. smooth sine wave), it’s not always so swift to daisy-chain them as the imperfection of the waves gets magnified running through them all. Otherwise – really cool hack – I’ve considered something similar using a power inverter and some sealed lead-acid batteries I got from work. Hadn’t thought about throwing a charger in to make a full-fledged UPS tho – just planned on making a nice little portable 120AC supply for stuff.

    Keep up the good hacks!

  4. I’ve known about this method for a while and have even experimented with it.
    My server(250W max PS) runs great off my cheapo inveter, although the 6A battery charger is just a little too small, over a couple hours the voltage slowly drops a few millivolts, and the charger gets warm because it has to work so hard.

    Easily fixed if I buy a second exact model charger or a larger one. :>

    Remember everyone, do NOT run batteries in parallel!

  5. What do you mean ‘don’t run batteries in parallel’? Batteries work FINE in parallel! As long as they’re the same model, etc… just add together the A*Hr ratings, and you’re golden. Don’t fuck up, however. Dissimilar batteries, or batteries of different inital charge can do bad things. Use two (or more!) identical batteries.

    Also, a beefy capacitor or two could be added to the circuit to make up for sudden current draws like a degauss. just connect it like another battery, and it should work a trick.

  6. the power supply in your pc uses 12v and 5v why don’t we work on designing a case/powersupply with a battery in it seem like it would require a lot less equipment
    use the transformer in the ps and you wouldn’t need an inverter just a transistoer or resistor to drop the power to 5v

    this could run your monitor but would then need the inverter and an outlet to plug the monitor into

    a great solution for those 2-3 second black outs the people that live a ways from the city experience every time the wind blows

  7. hey who out there can answer this one
    if you use a transformer to covert 120ac to 12v dc
    at 1 amp or two and then inverting it to 120 ac again
    are you drawing less kwh from the power company ?

  8. Wouldn’t you still be drawing and using the same amount of amps regardless of the transformation? Hell, wouldn’t you be using more since you need to power the transformer as well?

  9. Hmm, I’d like to do something like daisy chaining a bunch of car batteries in an outside small enclosure to provide a very huge UPS for an office that will have many workstations and some servers that could have a huge power draw. The goal would be able to keep working an hour or more and then have a generator kick in if the battery power reaches a certain threshold. Any suggestions or experiences building this?

  10. Go ahead and use a UPS, just put bigger batteries on it!

    I pulled the 7ah SLAs out of mine and replaced them with two outboard 35ah gel cells.

    The whole thing’s been running for years flawlessly.

  11. A far cheaper solution is to do what I did: get yourself a cheap “off the shelf” 12V UPS, and simply replace the battery with something not quite so pathetic. I bought an APC 12 volt UPS (about $60 US), and a deep-cycle marine battery from Optima ( The battery is a lead-acid battery, but the lead-acid is in the form of a thick paste, so no leaking. Then I simply invested in a few connectors and a length of (very heavy) wire, and drilled a hole in the UPS case for the wire to go through (remember to put a rubber grommet in it!). I then soldered up the connectors to the wire, and had myself a great UPS (made for that purpose) only with a beefy battery on it.
    My setup is 2 rackmount PCs, a KVM switch, a wireless router, 2 17″ LCD monitors and a large stereo amp. Last year the trasformer behind my house blew up, and this UPS ran my entire setup for 2 days. (1 PC was on 24/7, 1 was on only during the day)

  12. #16
    Current isn’t SENT through a system it’s PULLED through the system by the device drawing it.

    Batteries hooked up that way will merely have more capacity for doing work: It will run longer, not use more energy.

    how’s that?

  13. #14, I agree 100%. I swapped out my 7.5 AH in my belkin battery backup for a 33 AH sealed lead acid and have had no problems for the past 6 months. In tests I went from 30 minutes before the low battery light came on to over 2 hours. All I did was get more connector and make “extender” wires for the new battery sitting behind the UPS.

  14. It is best to use “deep cycle” batteries, as car batteries aren’t meant to be completely discharged. Car batteries are meant to provide a ton of current for a short period. Discharging them completely damages them. Deep cycle is the opposite.

  15. strider_mt2k: Current isn’t “pulled” from
    batteries. A chemical reaction in the battery
    causes electrons to build up the surface of one
    electrode. The other electrode ends up losing
    electrons as they’re moved by the chemical
    process. The excess electrons on one terminal
    repel each other so they spread out. They spread down the conductive plate and
    out of the negative terminal of the battery.
    On the other terminal the reverse happens.
    The chemistry in the battery drives the whole
    thing. The load does nothing to drive this

  16. what do you guys think of this: i have a 800va belkin ups with a worn out battery running my computer network. i want to replace the worn out (gel-cell?) battery with a good optima sla and also attach a battery float charger to maintain the optima battery. anyone see any problems with running my ups this way?? thanks

  17. Re: 24 You can usually replace the internal UPS battery with a bigger one without problems – although the UPS may wonder why the new battery is taking longer to charge, that should be OK. DON’T attach any extra shit (battery float charger), it may confuse or damage the UPS.

  18. I have one of these setups running.

    A heavy duty car battery charger (you know one of those 3′ tall standing models with all of the overdraw and overheat protection built-in)
    6 1000 Cold Cranking Amps (850 Amp) lead acid 12 volt batteries connected in parallel with an isolator on the charging end.
    2 inverters 450 and 400 watt.
    2 3.06 GHz PCs with 450 watt power supplies.
    1 toshiba A70 laptop running 3.2 GHz mobile P4 (draws just over 2 amps @ 120 volts charging while on)
    I have a few meters on the rig at all times, overheat protection on the wiring, charger, and inverters. Low-voltage shutdown.
    So far the longest unpowered test of the system was 2 days.
    My next upgrade I think will be finding a good way to connect the thing to a computer or all of the computers so they will shutdown, or at least hibernate when the voltage gets near the danger zone with the CRT off. As well as maybe an audible warning for heat or voltage related shutdowns.
    Only thing is I need to watch the charger and periodically check the temperatures and acid levels of the batteries and top em up.
    I would have used deep-cycle batteries but at the time I made my rig the 1000 CCA car batteries were free for me from work.

    So far no fires or overheats.

  19. I’ve got a good UPS setup for my servers.

    First I have 8 350A/hr batteries. Then I have an Iota 12vDC/75amp charger, with the smart IQ module. Then I purchased a bunch of 12vDC power supplies from Mini-Box.

    I pulled out the power supplies that were in my servers, then put in the new DC power supplies (each PS is 100Watt). I have 7 servers that can run for about 10 hours without power.

  20. I like #9’s idea a lot. I’ve wondered myself how to get something together that would do that – and it wouldn’t have to last very long, either – 5 mins at the most, screw the monitor. I just need enough directly fed power to hit ctrl-S on my keyboard and then hit hybernate. Can anybody point me in the right direction? I can’t be only the second person to have ever thought of doing this.

  21. @2: nope, for a heck of a lot of reasons, namely, all the power losses. power factor losses (current and voltage not in phase on input or AC output stage), core losses and winding losses in the transformer, switching losses in the inverter, the list goes on and on. if you spent a good deal of time/money, you *may* be able to get above 80, maybe 85% efficiency. If you’re really good, know the theory, simulate and build a really solid design, that efficiency can get up past 90%, maybe up to 92-93% (as with top-of-the-line UPS systems, which are mucho expensive).

    @3: for the above reasons, you would be using more volt-amps then you would be getting out.

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