Make your computer UPS last for hours instead of minutes

Picture of Make your computer UPS last for hours instead of minutes
For what would appear common sense to me, but perhaps not to all, I have all of my computers on UPS battery backups. After getting frustrated when the power flickered one day, I immediately went out and purchased a UPS. Well, shortly after, the power went out for longer than the battery could keep my computer afloat. I needed a better solution!

I wanted my UPS to be able to last for at least 60 minutes in a power outage. I needed more power! My solution: Car batteries.

UPS that is rated at least double what you plan to draw (see step 8 to understand why).
Wire (12 awg or larger; two different colors)
Heat shrink tubing
Car battery with terminals on the top
Adapters to go from the car battery terminals to threaded rod.
Wing nuts the same size as this threaded rod
Wire crimp terminals that will fit over the threaded rod.
Plastic case for your car battery
Inline Fuse holder (radio shack)
30 amp fuse for holder (any auto store)

Wire cutters
Wire strippers
Soldering iron
Scissors (optional)
heat gun or alternative
Drill bits
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Step 1: Evaluate your needs

I was trying to power two computers (desktop and file server), and two flat panel monitors. My total power consumption was roughly 500 watts peak. (yikes!) Currently I was running on two 300 watt UPS's (NOTE: VA is not equal to WATTS. Find the WATT rating) with one computer and one monitor on each. Even though the two monitors were hooked up to the same computer, I needed to distribute my power load more evenly to get longer battery life out of my petty UPS's.

I discovered the hard way after nearly starting a fire and destroying a UPS that you need one that is rated at at least twice the wattage you are consuming. They can't handle being run for longer than a few minutes at this rating, but the batteries die before it's a problem normally.

So I now knew I needed 500 watts, and I wanted 60 minutes of power.. that means:
P / V = I
500 watts / 120 volts = 4.16 ampere hours (at 120 volts)

UPS batteries are usually 12 volts, but some are wired with two batteries in series. Check yours out first to make sure you won't need two car batteries.

So, assuming 12 volts, that means that, after adjusting for the voltage differences, I need a battery with at least 41.6 ampere hours. (yeah, I know there's inefficiencies in the UPS, but lets keep math easy)
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ErnnieS7 months ago

i think you should buy the 600 watts Ups for the comfort on handling it.. it is easy right? rather than buying two UPs

maintenance free battery is much better...

questlima.7 months ago

hi there a gr8 tut i am going to have lots of fun building one btw i wanted to know if it is possible to connect two computer ups and make it act like one ups, i know it can be done with an inverter, but not sure with the ups, if it is possible then can you do a tut on how to wire two PC ups thank you

zardiw11 months ago

All this talk about H2 buildup is BS. There's already H2 in the air.....and the minimal amount that is generated when this low power charging occurs isn't going to generate enough H2 to matter, unless you have this system in a small box or something.........

Great plan only I use gel cell batteries so have no issues with adding water or any other thing. Maintenance free only way to go. Do you have any issues with over charge on the battery? My UPS doesn't like any battery voltage above 13 volts but yet puts out 13.5 to charge the battery. My battery levels off at 13.4 volts and the UPS when turned on goes into fault mode and blinks red light and beeps. Once I lower the voltage at the battery down to 13 volts or less then all is good. Any ideas??

What's the make of that UPS?......So I'll make sure not to buy one.......z

I've got an old UPS - just a 750va which contained a 12V 7a/h battery.

I live in a rural town in Cambodia, so obtaining a replacement battery is impossible.

I bought a small motorcycle battery which is 12V 5 a/h and sealed.

Are these safe to use in the current UPS?

Power only goes off a few seconds at a time - and the ups will only power low voltage devices (eg. router) - so actual current draw will be low.

As its a smaller battery (5 a/h vs original 7/ah) - i shouldn't have any issues of the invertor overheating?

Only thing i'm not sure off is the charging - will the current UPS know when the motorcycle battery is fully charged?

And of course i'll need to keep the ups inside but in a more open area?

Hi Trazfusion,

It is ok to use a higher AH battery in place of rated battery for UPS, but lower AH battery should not be used with it as the Charge Current in UPS is set according to the rated battery and that could overheat and damage the lower AH battery.

zardiw yourdreamz11 months ago

Why in the world would anybody use a LOWER AH battery?.....lmao.........z

zardiw11 months ago

They make little 'maintenance' chargers that will keep a battery topped off and also will charge them when they go below max. They are totally automatic and can be left hooked up to the battery.

This will solve any problem about the UPS charger getting overworked on charging a larger battery.

Also, I would use Deep Cycle batteries.


J_R_S1 year ago

One final note (I can't seem to find any way to edit my original comment below):

I made a mistake about how much power you can get out of the 35ah (amp hour) AGM battery. I was thinking in DC terms (my background is in Solar Power systems). If looking at your orginal calculation:

500 watts / 120 volts = 4.16 ampere hours (at 120 volts)

...and converting your 4.16 amp hours @ 120 volts to the DC amp rating of the AGM battery @ 12 volts, it comes to 41.6ah DC amps @ 12 volts. So, the 35ah battery AGM battery comes up a little short, you can still draw 500 watts peak out of, it will just drain the battery a little faster than a 45ah battery.

J_R_S1 year ago

This is an incredibly well-done article and totally accurate for
everything except one Very Important Warning: The batteries inside
computer UPS units are "sealed lead acid" AGM batteries. These are the
only kind of battery that can safely be charged inside the house or any
enclosed area. Other types of lead acid batteries (such as standard Auto
and Marine batteries) -- even if they say "sealed" or "maintenance
free" or "deep cycle" always put out some hydrogen gas as a by-product
of the chemical activity inside the battery while it is charging.
Remember the Hindenburg? Hydrogen gas is explosive, odorless and
colorless. It is lighter than air and will rise to the top of an
enclosed space where it will build up if there is not really good
ventilation to the outside at the top of the enclosed space. At some
point the build-up can become dangerous and can be ignited by a spark,
resulting in fire or explosion.

This is something people who use
Solar Panels to store electricity in multiple battery banks find out about
early on. In this application, the hydrogen gas wouldn't be as much of a
problem because unless you have power outages very frequently you won't
be re-charging the battery all the time. But still, it's simply not a
good idea to charge any lead acid battery in an enclosed space other
than those identified as "sealed lead acid AGM".

batteries are more expensive than standard auto batteries, but they will
last 2 to 3 times longer, so in the end the total cost is the
same. You can buy a 35ah AGM battery online (or at Harbor Freight in their solar section) for about $65 that will provide 60 watts for an hour (the model UB 12350) -- and of course there are 55ah AGMs and much larger available as well.

The other option would be to place the automobile-type battery outside next
to the wall of the house with a small roof over it and maybe some wire screen
around it to keep it out of reach and then run the wires inside to the
UPS units. You would use more wire, but if you use the heavy gauge #10
or #8 wire your voltage drop won't be significant.

fretted2 years ago
"mintues" Darn Goonies
ben.mcfadden (author)  fretted2 years ago
Fixed. Thanks.
Happens to the best of us to me more than i'd like

Your Welcome
callones3 years ago
I have been interested to try this but reading the comments made think twice. I think it would be better not to modify the UPS, buy a car battery and an inverter, then plug the UPS to the inverter. It's like tricking the UPS that it's still on the mains.lol
To elaborate on Ben's response, doing your method, while being technically a safer for permanent installation would give you battery power to a device it would not be an automatic switchover when the power goes out. Thats the main advantage you get by using an actual UPS is the failover. Though you can make your own but you kind of lose the safety side lol. However, though the OP didnt do it, what you can do is close this stuff up a little better. If you get a plastic bin with a lid (larger tuperware) you can put the battery inside this case to protect it you just need to put air holes or a vent slot into the box so the battery can 'breathe' That way once you get passed the initial wiring the setup is much more safe again accidental touching, etc. I am aware this post is old but hopefully this helps someone else too :)
Additionally, if/when I do this I will be adding a small 12v fan inside the UPS units themselves to cool the circuitry and transformer. Since the devices 'cant' last very long they are not built to be run constantly on battery so if you are trying to greatly extend this time make sure to take precautions so you dont hurt yourself or the UPS or worse yet, start a fire. Which I can tell you from personal experience that electronics have amp/watt ratings for a reason.

Check temps/voltages of all parts of your unit while running on utility power Vs battery to make sure nothing will happen. The last thing you want is to find your DIY UPS set your house on fire while you were out of town because you didnt give something enough cooling or used wiring with too low amp rating ;)
ben.mcfadden (author)  callones3 years ago
Interesting solution.. I like the safety in your concept, but it involved a manual switch-over, which unfortunately wouldn't work for what I was doing. Good thinking though!
Hi Ben or whoever can help,

If a computer fan was used to blow air on to the transformer and if the case of the UPS was left open to allow heat dissipation would that enable using much more than 50% of the UPS’ rated capacity? I have an Upsonic Domestic DS600 600va UPS described at http://www.upsonic.com.au/pdf/domestic-sme.pdf . If a large battery is connected, do you think it could handle an average desktop computer and old monitor? How many watts could it probably run at if I connected a 100 amp hour deep cycle battery and used the fan as explained? I could measure the temperature of the transformer on the UPS with a non contact infrared thermometer every few minutes when I first tried it. What would be the maximum safe temperature it could reach?

Regards Richard.
garfnodie3 years ago
I have a UPS that I'm needing to replace the battery on, I'm thinking of getting two, or maybe even three replacement batteries for it, ones that are designed for this particular UPS (12v, 4AH, 20HR is printed on the existing battery). Can I hook them in series without any issues? I'm actually not even going to be using it for a computer, but for a set of decent speakers with my ipod hooked up and an alarm clock so that they all continue to work for hours even if the power goes out during the nasty storms we can get here in the midwest. Three batteries may be overkill, but they're pretty cheap and I want many hours of power, and these aren't high current devices, so I'm hoping it'll work just fine. Will the charger in the UPS be able to charge all three batteries just fine, even if it take a few days, that's fine.
ben.mcfadden (author)  garfnodie3 years ago
You can put the batteries in Parallel, but NOT in Series. Otherwise, that sounds perfectly reasonable. Make sure you don't get a buzz in your audio equipment on the inverter before you spend the money on the batteries. I've had less than stellar experience with power inverters and audio equipment.
shastalore3 years ago
Just be sure to spot check and take electrical and temperature measurements as you modify your rig, as it was never intended to function more than a few minutes.

I recently had the idea of converting my APC Back-UPS BK300C into a handy portable 120VAC power station, by replacing the internal OEM 7amp-hr battery with an external 36amp-hr deep-cycle battery pack, adding a carrying handle on top, and converting the wired-in 120VAC power cord to a 120VAC chassis socket with a removable 120VAC power cord.

But I soon discovered the the OEM internal battery is intentionally in a self-sacrificing mode during power outages: The internal circuit sounds a continuous alarm when the internal battery discharges down to 11.4 volts -and will continue to discharge until the battery is dead. Any 12 volt lead-acid battery that discharges down to 11.0 volts is generally considered dead anyway, and should be replaced with a new battery.

Also, the unit is not fan cooled, so someone's suggestion to run the unit at no more than half the rated output wattage (when in extended emergency backup mode) is sound advice. But the large internal transformer is mounted to the outside steel case, which still functions as an effective heat sink.

After some deliberation, I simply decided to put the perfectly functioning unit back to together again, with no modifications done, and left it on the street curb with a "FREE & WORKS" sign, where it vanished after a couple of hours.

It was a pity, as the internal circuitry is both efficient and smart, discharging the 12VDC battery in direct proportion to the 120VAC load. And the two heavy-duty 120VAC electrical outlets would have been more than welcome in the field.

I'll just have to pick up another Vector iMobile Microslim Power Inverter VEC414S instead, for two 120VAC outlets in the field.
somoney3 years ago
Nice DIY. I think your pushing those particular UPS's beyond red-line. I wouldn't want that running on battery when I'm not around to watch it (ie. sleeping). DIY'ers getting ready to duplicate this work may want to look at APC XS series UPS's as they come pre built with cooling fans for the inverter.
adam 1014 years ago
Take this as a grain of salt, I have no real world battery experience to back me up, but I thought that car batteries were for short high amp discharge (starter motors) and that putting this kind of load on them would quickly destroy them. Furthermore, deep cycle marine batteries should be used for "low" amp "long term" power draws. I am pretty sure I learned this while reading about a robot that needed two marine batteries to haul it's 300 pound frame around.
Zerocool8184 years ago
I would ( if i do this) use sealed lead acid batteries you know they have a reason to use sealed ones: they are smaller lighter and safer and can give you more charge cycles and longer run time and they are cheaper you can get some sealed scooter batteries for 20 dollars online and they are like 50 amps each and two of them can fit in the place of one of yours and have you considered using computer fans to cool the inverters so they dont get damaged? wow that was a mouthful. nice setup anyway!
hey ben did u connected two UPS's to a single battery. I am worried about the charing of the battery cos i was told that UPS cannot charge a Car battery. do u have any charging issues in this instructables. i am waiting for ur reply.
also i have 3 PCs two with CRTs and one with 15inchs TFT how much power do i need for the whole, i am currently using 2 USPs, 1 550 VA and other is 500 Va with which i am using one to power Just monitors and others to power TFT and 3 CPS. and its working fine. (I think) i am fed up of the backup its less that 5 minutes.
please guide me.
ben.mcfadden (author)  gagan98971759664 years ago
I have been using this system for 2 years now and it works great. The two UPS's do charge the batteries, although it does take a full week to get out of the "quick" charge and into the trickle charge. If you need to charge it faster, use a car-battery charger after an extended outage. Having two UPS's has shown no adverse affects. If anything, it just charges the batteries faster.

To determine how much power you need, I recommend using the Kill-A-Watt meter (http://www.thinkgeek.com/gadgets/travelpower/7657/) to determine how much power each item draws. Be sure to measure VA, as that's what most UPS's are rated in, although Watts are useful too.

Remember, if you are going to make your UPS system last for more than the time a stock battery will last, be sure you are putting less than 50% of their rated capacity on each UPS.
Thanks Pal
nothing will happen dear friends. what do u think about almost everyone in the world is using inverter systems (dont forget with battries) inside there homes or in the basements of there homes. what will u think about it.
Hi, noob on this board but not to DIY stuff; Question; Is the charger on the UPs callibrated for the batt size that it came with? I'm using a 480VA UPS and used it to charge to charge the small non-deep cycle battery on my motorcycle. Didn't leave it on long for this reason: How does the internal circuitry know when a 12v battery of a given size is 100% charged? What happens if you replace it for a larger A/H unit? I want to increase the lifespan of the UPS. Thanks!
A few points and bear in mind I have only been learning about this recently.
1. Its the voltage that tells the charger how well its doing.  For standard Lead acid batteries they are full when they read between about 13.8 and 14.7 volts.
2. Calcium-infused batteries apparently are full at a higher voltage so you need to be aware of that - you might be over or under charging.
3.  I think that so long as you are using the same type of battery with the same charging needs as the original one it should work, it will just might take longer to top up.
4. It won't be ideal but should get you there!
id go with a deep cycle if i were you 
ben.mcfadden (author)  superfreak776 years ago
The charging circuit checks the voltage of the battery to decide when it's charged. It isn't tracking the Ah that it is dumping into the battery. In other words, putting on a higher capacity battery (more Ah), so long as it is the same voltage, will work fine.
Cool, thanks. FWIW; Here's some data gathered in tonight's hook up of the 10 amp/hr bike battery. Note that by the middle of the test, the UPS kicked in to compensate for a low voltage coming in from the mains. I don't really know how it works but it does appear that it stopped charging and may even have drawn from the battery. I left the serial line attached to see the % charge on the pc screen; With a voltmeter hooked to the bike battery; 12.67v at first charge, -mains v. 123volt, pc displays batt at 92% charge. 12.99v after 10 mins, -mains voltage up to 133v , pc display at 96% Turned the bike switch with lights on; -Bike switch on 60 secs, charge dropped to 74% -Bike switch off, and 60 secs later, charge now at 86% 5 min. after switch off, charge now at 92% 20 min. after switch off, charge at 96% -Same as when it stopped charging in the first stage.
abadfart5 years ago
could you beef up the transformer with one from a microwave? 
farna5 years ago
If you don't need quite as much power as a car battery, consider a lawn tractor battery. Same type as a car, just smaller. A motorcycle battery would work, but most of them have about the same power as the original UPS battery.

The main caution with ANY automotive type replacement battery is that hydrogen gas is produced when charging (only small amounts when trickle charging like this) and they have liquid acid inside. Make sure they don't get tipped!!
vtsnaab5 years ago
I have easy access to used UPSs and have been doing this without any safety concerns for over a decade.
Bear in mind there are NO SMOKERS in my house/office - EVER, but truly - using a UPS to process battery power into 120VAC is a great idea as is boosting the run-time of your UPSs.

I add only one warning which has been said here before repeatedly:


The bigger battery is for stand-alone power or added runtime - NOT for supporting a load beyond about 60-70% of the UPS's rated capacity.

If you can find them:
My all time favorites are the really old, metal-cased APC units connected with solid house wire to lawn tractor batteries.

The hydrogen gas released by any lead/acid battery has a very obvious odor and is unlike anything else -and I have never, never, never had any problem doing this indoors.

One important point IMO:
Since there is always the possibility for leakage I recommend ALWAYS placing any such battery or batteries into a plastic tub like a dollar store dishpan to catch any dirt or possible leakage if anything ever needs catching.

Solar enthusiasts:
If you seek out your local PC recycling freak who has connections with places that recycle servers and big UPSs you can use those super-high quality inverters with your batteries and save a bundle instead of branded solar-only inverters.

Last word of my opinion on this matter -> this IS the Instructables site for DIY'ers and if anyone who reads these is too afraid to experiment and learn by doing then this I would recommend ONLY buying ready-made, UL approved devices and leaving the DIY to enthusiastic DIY'ers.

Happy DIY'ing !
Blackice5045 years ago
people new car batts are sealed now days so if they are those type its possible to use a automotive batt but if not you can allways have the batts in say a lundry and just run a cable but the other point those power socket ups are too small even the APC700 ( 700watt UPS ) <<< Big box for people who not know they would have trouble keeping the batts in float but also keeping any car batt with wet cell yes the wet cell will dry and make a nice fire so keep an eye out for this.
bubba0076 years ago
Play it safe:
Remember charging a car battery indoors can release harmful (toxic) gas fumes. Be careful where you put the batteries
it releases hydrogen and oxygen,which is what is already in the air,Along with CO2 of course
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