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

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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JacobM232 minutes ago

Ben, I understand that you have wired two 12V batteries in series to produce 24V, and then drawing the 24V from two UPSs which each have approximately evenly distributed loads. I have two concerns.

First, this still lacks redundancy in the sense that both batteries are in series with each other, so the entire power system fails if either battery fails. Cost permitting, would I be correct in saying a 4 battery setup would be superior? Two parallel sets of two batteries in series would provide this redundancy, as well as provide double the power reservoir capacity under normal operating conditions, correct?

Second, I am concerned about overloading the UPSs with double the expected power draw. If one of them were to fail, could the other take on the full power draw depending on which specific component failed? Or, instead would the failed UPS and all devices connected always power down and effectively not play an active role in the circuit? In the former situation, this could cause a fire as forewarned in the original instructable. In the latter, perhaps this could be a safe solution to keep under the same roof as my family. I appreciate your input, thank you.

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
rhemus bubba00715 days ago

Hydrogen gas is not toxic. But it does displace oxygen and it is highly flammable. This is the gas that is released from chemical reaction of charging a lead acid battery. As long as you have decent ventilation around a lead acid battery this shouldn't be a problem.

it releases hydrogen and oxygen,which is what is already in the air,Along with CO2 of course
ben.mcfadden (author)  ReCreate6 years ago
True, but if the hydrogen builds up, as is possible, it could be an explosive danger.
i don't think it will produce enough hydrogen to ...cause an explosion
ben.mcfadden (author)  ReCreate6 years ago
Theoretically, you're absolutely right. But I'm not going to take the blame if something goes wrong. :-)
go ahead,sue me if you die of an explosion...did i say that?
just as long as you don't have lead acid battery bank of ten batteries or so,then the bet is off!
scook ReCreate5 years ago
Haha, I'm pretty sure they have about 25 Lead Acid batteries for power in the television series "The Colony" .
ReCreate scook5 years ago
if it is outdoors, Then the problem is solved...
Hydrogen is lighter than 'Air' so it will float up and out of the way of the hot things.

Oxygen is only explosive in high concentrations, which is to say it has to be very pure to do so. Chances are, it is not going to have the correct mix for it to be combustible at any level that you could create with just 2 batteries.

Combined with the fact that it only produces these gases when it is charging and not discharging....... You should be pretty safe. Just don't keep it all contained in a super enclosed space and you should be fine.
dejvv18 days ago

Hi, I have 2x deep cycle marine batteries 12v 90ah, will this ups be able to charge them? please click on the link for the datasheet


AllenP519 days ago
ErnnieS8 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.8 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

zardiw1 year 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.

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

zardiw1 year 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.
somoney4 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.
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