Make Your Computer UPS Last for Hours Instead of Minutes




Introduction: 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)

Step 2: Remove Battery From UPS

Unplug the UPS from the wall, and unplug all devices from it.
Remove any screws you fine, and open up the case.
If you are as lucky as I was, the battery will have terminals that you can slide off. If not, just cut the wires as close to the battery as you can.
Once you have removed the battery, you will find something like you see in the picture

NOTE: Pay attention to polarity on the battery, and which wire went to when polarity.

Step 3: Extend Wires on UPS

The wires that are in a UPS are typically not long enough to reach much past where the battery sits. We will need to extend them to reach our car battery.

Cut off the the wire terminals (if any) on the wires from the UPS.
Strip at least 3/8 of an inch of the wire on the UPS
Strip at least 3/8 of an inch of the wire we are extending with.
I used a metal crimp to help me get a great connection, but this is optional.
Solder the wires together. This solder joint needs to be able to handle high current. We will be drawing lots of power through here and if we have a voltage drop, the UPS won't last as long.
After making sure the joint is well soldered, place some heat shrink over it, and seal it up good.

Note: Use colors that make sense to you, and will allow you to remember the polarity

Step 4: Drill Hole for Wires

Next we need to make a place for the wires to leave the UPS and go to the car battery.
I drilled a hole. Use whatever size will fit both of your wires.
Add a strain relief so you can't pull on the joints you made, or on the PC board in the unit. I simply tied a knot in each of the wires.
Next pull the wires through the hole, and carefully put the unit back together.

Note: Remember the polarity!

Step 5: Prepare Inline Fuse Holder

Since this is high current, coming from an extremely high current source (car battery), we need a fuse. and you want it as close to the battery as possible.
First, strip the wire on the fuse holder.
Place heat shrink on the wire.
Take your crimp wire terminal that is sized for the thread on your battery posts, or adapter and crimp it to the wire. Then solder. Nothing is complete until it's soldered. Why solder? It conducts electricity better. The joint won't get hot, and you will have a less drastic voltage drop.
Next shrink the tubing.

On the other side of the fuse holder, strip the wire, place the heat shrink on, strip the hot wire you've recently added to the UPS and solder together. Once completed shrink the tubing.

Step 6: Prepare the Remaining Wire

Next, using the same strategy as connecting to the inline fuse holder, connect the Crimp terminal to the end of your ground wire, Solder, and heat shrink.

Remember: Put the heat shrink tubing on before you put the end on.

When you done you should have something like:

Step 7: Attach to Battery, and Test

Next, attach your battery terminals to the battery, and then your wires to the terminals.
Insert a fuse in the fuse holder.
And turn on your UPS.
It will take a long time to charge the battery, but it will also last for a long time in a power outage. Under this setup mine lasted for around 1.5 hours.

Be sure to put the battery in a plastic case with a lid, as, if something were to go wrong on the battery you would want to contain the acid as much as possible. Also, this will prevent you from dropping something and shorting out the battery.

Step 8: A Word of Caution

I learned this the hard way.. it cost me a UPS, and nearly a fire.

The transformer in these UPS's are cheap. They are not designed to be run at 100% capacity for extended periods of time (such as what you will be capable of using this size battery) When I ran my UPS's at 300 watts for more than 30 minutes, the transformer melted through the case. When I pulled out my infrared thermometer it read nearly 400 degrees F!!

I had to redesign my system. I chose two UPS's that were rated at 600 watts each, but used 24 volts (2 twelve volt batteries in series). Under my new setup, I have over four hours of backup capacity as I have two car batteries.

1 Person Made This Project!


  • Anything Goes Contest

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Question 7 months ago on Step 8

I have a 430W UPS with a dead battery. I want to power a 55" LED TV during load shedding. I think that 430W is roughly 4 times as much wattage as I need, probably closer to 5 times. I also believe I need minimum a 6ah battery to cover 4 hours, in the best case scenario (no losses). I plan on making a box with an intake and exhaust fan for cooling, and will position the components optimally for cooling purposes.

My questions are:

Is an 18ah battery big enough to power a 55" LED TV for 2.5 hours, without damaging the batteries?

Will my UPS be able to charge a 12v 18ah battery? Even if it takes 20 hours?

Who killed Roger Rabbit?

Jason South Africa
Jason South Africa

Answer 7 months ago


Your 430watt UPS is theoretically capable of powering your TV since the TV is probably less than 100Watt. Fairly similar to an office PC & monitor. Mid upper range gaming PCs pull just over 200Watt under load - based on my Core i7, 32GB RAM, 7x 3TB HDDs, GTX2060 Super - it pulls around 212Watt playing 2021/2022 games at 4K. Excluding the monitor...

An 18Ah battery is probably not going to give you much of a lifespan unfortunately. Your UPS is probably equipped with a 9Ah (or maybe 7Ah) battery as standard, so an 18Ah is only double the capacity/ runtime.

UPS internal inverters are sadly rather inefficient and waste a lot of energy compared to dedicated inverters, the reason being that a UPS is designed to run for 15 to 20 minutes for a computer user to save their work and shut down, or for the generator to kick in and power the building.

The UPS would be able to charge an 18Ah battery, time would be the limiting factor mostly since chargers don't have a high output (you've probably seen that they have fairly thin wires).

For a load shedding TV power solution you'd be best off getting a dedicated inverter, e.g. a 650watt camping/caravaning inverter and a car battery/ or a deep cycle battery of say 65Ah or a pair of batteries if your inverter of choice is 24V instead of 12V. Though I suggest a camping unit because many of them are 12V. And get a nice automotive charger like an Optimate.

Asking at the camping shops will get you a lot of information for free based on their experiences so far battery life and so on are concerned.

Also, going with a big'ish battery is better, the less you draw from it, the longer the theoretical life span of the battery will be.

The moment you start using UPS as a solution for loadshedding you'll start to see how quickly those little 9Ah batteries go vrot. Running them flat twice a day for a week or two will already start having a negative impact on how long they can deliver.

So yeah, rather buy a car battery for a grand and a half than spend R450 on an 18Ah that's going to go bad in a month or two and need to be replaced. They aren't covered by warranties for anything other than manufacturing defects.

Alternatively you can go with a real inverter +/- 5kVA and a Lithium Iron-Phosphate battery or two. That sort of setup is in the region of R50K, plus another +/- R18K for 4x 650watt solar panels. That would get a smartly configured home almost totally off grid. You'd literally just power your geyser now and then from mains.

With the price of electricity in South africa, spending 40 grand on just panels and an inverter will break even after maybe 4 years - I paid around R16 000p/a for electricity before the most recent price increase.

TBH I don't actually know what I'm paying right now for electricity because I haven't received an invoice for more than 18 months and my Tshwane metro online account is "inaccessible".

I just go to Pick 'n Pay and drop 3 grand every month. Should be okay since I switched to a gas stove/oven end of last year and every light is now LED... so far so good, they haven't cut off my power yet...

Jason South Africa
Jason South Africa

2 years ago on Step 8

Flash forward a few years...

I live in South Africa, we have power problems because there is only 1 parastatal that has the electrical supply monopoly in South Africa.

Due to gross mismanagement and squandering of taxpayers funds, maintenance on power generation, power distribution etc. has been neglected nationwide.

The unfortunate upshot of this is that we regularly have scheduled power outages to divert power to other areas when repairs need to be affected. Google "South Africa Load Shedding" if you need more info or a chuckle.

I've been meaning to buy an expandable UPS for ages now but haven't ever gotten around to it, we've kind of just gotten used to living with the scheduled power outages.

The UPS I was looking at was the APC Smart RT2000 -- it's a 2kva "48volt" sinewave unit with a jack on the back to daisy chain in more "48volt" battery banks.
The unit features an RS232 like interface for programming, but not quite RS232.

The APC banks are 48v x2 packs per bank (the UPS is 48v x1 battery pack) and according to APC the charge circuit can handle 10 banks inside of 12 hours - the UPS being 9Ah and the banks being 18Ah or the capacity to charge 189 amps worth of batteries (and 22U of rackspace) in 12 hours as the recommended maximum, but as they also state, there is no theoretical limit on battery banks as long as sufficient time is given to charge them...

Not a shabby little UPS if you are looking for a stretchable unit that doesn't need to run a server farm, just a few hundred watt.

It is however now discontinued, and has been replaced with an RT2200 or some such. Not a problem for me since a client hospital recently threw out a perfectly good RT2000 that was running the PC on a Philips screening table, and paid us to replace it with a 3kVA unit, and I was requested to please dispose of the "faulty" unit because they didn't want it any more.

As I'd suspected one of the individual batteries had failed, it's neighbour was also quite sick and the other 2 were still pretty decent

I've since cleaned and serviced the unit and replaced all 4 batteries with 4x new 9Ah Yuasa batteries and it's working like a charm, gives me 4 hours as is running my home networking gear.

Now to fix the front grille that's been kicked to hell, and shop around for another 48v single deep cycle battery or some big 12's - something with a similar charge profile.

TL;DR Don't be afraid to adopt a dead UPS, it's quite possibly just the batteries that need replacement, and the expandable units offer a lot more flexibility provided you access their software and tell them what their battery monitor is looking at.


Reply 7 months ago

If you have time, please check my comment above out.

Basically, do you know if a small ups will charge an 18ah battery?


Question 7 months ago

Planning to do this with 2 100Ah Marine Deep Cycle batteries I have. To overcome the very slow charging of the UPS, it should be fine to connect a 12V battery charger and maintainer, right? External charger will do the bulk of the charging while power is on and will be off when mains power is off.


8 months ago

All the wires used in this project are WAYYY too thin!


9 months ago

The main problem with this project is that lead acid batteries for cars will go bad after about a year if they are not used. This is common knowledge -- and I know this from personal experience from having thrown together a such a UPS from an inverter and an extra car battery I had laying around. If using a car battery (and maybe even a deep charge lead acid, don't know about those so much) it needs to be discharged and recharged every so often. A really nice project would be to design an Arduino based UPS that both functions as a UPS and maintains the car battery. The only problem is if the power happens to go out just as the system has (partially) discharged the battery. But it is only a partial discharge. Another solution is to have two lead acid batteries.


5 years ago

I made it!!!
I also put in two cooling fans (one sucks in and the other blows out air)
I ran it for hours (although only a 120 watt laptop)
It was as cold as in idle
Before it heated up alot even when used for 10 minutes
I am using a seperate charger (20AH) to charge my 100AH flooded battery (I know this is not ideal but I change one each year already so it doesn't matter, the cheaper the better )
And it is working like a charm


Reply 1 year ago

i HOPE THAT BATTERY IS DESIGNED TO SIT ON THE SIDE LIKE THAT!? Lead acid's don't like this type of thing usully.


Reply 5 years ago

how long can you run your pc now? how many hours?


Reply 5 years ago

It's a 90 watt laptop
I run it till the electricity comes back again
4 hours
Didn't try more because didn't have a power outage more than 4 hours

Richard Hole
Richard Hole

10 years ago on Introduction

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 . 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.


Reply 4 years ago

Take the UPS cases off and submerge them in a 66 Qt. tub of oil. That's how large transformers are cooled. Oil removes much more heat than moving air.


Reply 1 year ago

Ah but you'd still need a radiator and fan to cool off the oil :D DOABLE.


1 year ago

I am a Senior System Engineer, since over 10 Years now. But this simple Instruction i have never Seen ! :-) Damn Good Job, my Friend.
I live in Europe, we have 220 (230) Volt. So its a little diffrent.
Also have u to Say, if some UPS have more than ONE Battery Pack, the it will be guess maybe 24Volt or 36Volt (12Volt each Battery, for Bigger UPS).
But doesent matter. This was the Best "How To" ivè ever seen :-)
Thank u so much
Maybe u wanna see some Pics of my Setting i made. Just write me

Greets from Swiss


Tip 2 years ago

Strongly suggest you look at the pure sine wave inverters used for Solar panels. A 2000 watt unit is about the same price as a 1500 sine wave VA UPS. and is being used for what it was designed to do.These will charge your battery from a solar panel and come in sizes from a few hundred watts to several thousand watts. Should be able to "fool" the inverter by using a ten to thirty amp. DC power supply in place of the Solar Panel to keep the battery charged. Keep in mind that an inverter providing 2,000 watts is providing ~18 amps at 110 volts. That means the battery will have a load of ~200 amps. Thus this is rather impractical above about 500 watts service along with two or three 12 volt batteries in parallel. At 200 amps you are talking number 2 or larger cables.(Price over a dollar a foot!) Do not be fooled by auto jumper cables as many of these have no. 8 or 10 wire with a fat insulation to make you think it will handle the current. They will NOT handle a hundred amps for more than a few seconds. Thuss you should look at Solar panel Inverters that are using 48, 60 even 120 VDC input. You will pay as much for #2, #1, or #3/0 copper wire (about $3 per foot) as batteries otherwise. Will probably have to build your own DC Power supply for this approach. Some even have a battery charging/maintenance feature.


2 years ago

Hi all, I know this post is very old but I will add some analysis and advises.
From my understanding you "fused" to UPS able to charge 7A Battery to be able to charge a bigger battery, this case a 40-50A Battery. I hope you explained that in your able from the start, and also provide a schematic since it is a electric project, and it is always easier to have a schematic.
1. Deep cycle battery or marine battery is used here, smart move, dont was your time with car battery, because even if you see a car battery rated 70A or more, it doesnt mean those kind of battery are made to produce 70A during a long period of time, not everybody understand this!! They are rated 70A or high, to be able to give a proper kick to start the engine, that all, after all that the car is running on gas, not on voltage either on Amps... I had to point this because some think because I got a high rated car battery so I may use it! You may be you should first know what kind of car battery it is!!
2. if you want do this type of project, with only 2 backup UPS 7A, do not go with to high rated battery!! 50A max!! Why, because the 2 UPS (2 in the case) are gona struggle constantly to be able to provide this big amount of current asked for such high rated battery (50A in our situation), to be able to fully charge it!! Even the autor say that it take a lot of time to charge it completly. Perso for me time is not an issue, I prefere slow and safe charging.
3- Dont attempt this project with Backup UPS that autoShunt down after some time, why?? because those Backup UPS are made just to give you anough time to save your work, and to shutdown your PC when there is a Power Outage!! Even if you increase battery capacity, you wont be able to keep the UPS running because the running time is set by the chip inside the UPS! Not necessarly batt the battery capacity...
4. it is a good idea to add a PC fan to cool the transformers, cause they gona get pretty hot!! not necessarly running them at 12v, even 5v or 9v is a good start, and giving an acceptable noise, or drill some addtionnal cooling hole into the UPS case, for additional passive cooling...

Thx for everybody that contributed to this ible, there are a lot to leaning about UPS and battery from the comments


2 years ago on Step 8

Hi All, Before I start a little history about myself; I am an electrical engineer who work his way up from electrician into the office; since becoming an engineer I have only ever worked on high profile projects such as Hospitals, Rail and the Uk Olympic Pool; My involvement ranged from covering security, fire, and yes some rather large UPS’s units 😊 ok enough said about me.
It is not my intention to get technical; just to make some observations and offer some thoughts for would be builders.
First I would like to applaud the originator as I think the idea and concept of extending UPS time and or repurposing anything for the project is great; however most UPS’s for a computers are designed (they are normally programmable) to allow the computer to shut down within a set time as power is lost; as opposed to systems crashing with possible loss of work when the power fails; if you require a system that will allow you to continue working then this is a totally different scenario. Prior to proceeding you need to ask several questions; what equipment to I want to keep running? For how long do you want the equipment to run?
Once this is established you have your basis for your design. Things to take into account; if you deep discharge any lead acid battery irrespective of sealed or not the battery will lose effectiveness and fail quickly; ventilation not just for dissipation of gases but also for cooling as heat loading could also reek havoc on your UPS’s performance let alone your IT equipment. If you research the manufacturer of batteries you should be able to ascertain battery technical information sheets; which should help battery selection. As mentioned by others there are dangers that can be life threatening; prime examples being; mains power, the release of potentially explosive gases being the obvious ones; not so obvious unless you are electrically minded the DC Voltages and currents can also be life threatening; so Please Do Not underestimate the dangers of DC; anyone attempting this kind of project should do their research. Cable sizing is also critical here! You can research cable sizing; I’m sure that if you look hard enough you may even find online calculators for this you’ll need to have a basic understanding on cable selection though; last thing you need is to melt your cables!
You could of course always consider a small generator (the outgoing mains will need to be fairly stable though) with some form of mains failure detection with auto starting controls (quite often used in hospitals and high end residential properties) time from mains failure to auto start is critical here though; its not as easy as I made this sound though.
For the record I only believe in constructive criticism; I hope that my input has been taken as such.
Lastly I live by several sayings these are:
Always remember your 5 P’s of life! Poor Preparation Produces Poor Performance
Every action has an equal and opposite reaction (while not always 100% correct it makes one think!)
I hope some of you found my input helpful.


4 years ago

This Idea is Dangerous on many levels. First is the danger of NON-Sealed Lead Acid batteries exploding. I have personally seen this happen. Lead acid batteries produce hydrogen gas during operation, that is highly explosive. One on my cousins was hospitalized while trying to jump a car, as the gas ignited and this blew the battery coating him in acid. Car batteries are designed from high amperage and far exceed the capacity of most any retail UPS, so now we have a great chance of electrical fire. The charging circuit of the average UPS is not big enough to charge these large amperage batteries. Don't take my word for it just look at the size of the transformer of a car charger in comparison to the small UPS transformer. If you are lucky maybe all that will burn up are the rectifiers that are too small for such a load and worse wires overheating. I don't know of any UPS's designed to output a steady 60 Amp charge? Most are set for 6 Amps (a 10x multiplier) Many Alternators are between 90 and 140 amps and circuits to support this for a standard Automotive battery. A UPS battery may be a 7 amp/hour battery, or in short made for a 7 amp charge. DO the Math. If you do this maybe you will be lucky and only destroy your floor from the acid leakage, not including what these gases are doing to your Computer as the fan(s) suck them trough your box. One last note, Look at the size of the wires hooked up to you battery in your car and the size used in this description. Not even close in comparison?


Reply 3 years ago

Yes larger or more batteries would cause more damage than smaller or fewer batteries. All lead acid batteries have acid in them and can "explode" if shorted. The advantage is sealed batteries can sit in any orientation without leaking.

Batteries are optionally vented to release pressure of gases formed are hydrogen and oxygen -- specifically during charging. In the chemical reactions, sulphur only bonds to the lead and back to H2SO4. Sulphur is never vented because no other reactions are available with the chemicals Pb, H and O.

James, what would be helpful is details of ROOT CAUSES of these fires and stories you posted. Jump starting a car should not cause the battery to explode, so why does this happen? Some component failed and shorted and turned part of the circuit into a heater -- and the system supported huge currents and had NO FUSE. A battery in normal condition should also not produce enough O2 and H2 during charging, nor have enough volume inside the battery to support an explosion that blows the top off. Was the battery low on water or have other issues like being old and dead and thus having a very high internal resistance?

Was the cause only ignition of O2 and H2, or was there a short or thermal run away?

Short any car battery or sealed lead acid battery -- or almost any battery -- and you have a guaranteed fire.

Cars and equipment with large wires are so dangerous because it allows a large current and lots of power. Any system with smaller wires would (failing a fuse), quickly melt wires -- instead of heating giant wires into molten copper and boiling the battery until it explodes.

The most common fire during jump starting is from crossing the polarity. Be sure to hook plus to plus and minus to minus! A safe way to jump start is to hold on to one of the clips, then put another hand on the insulation of the wires to sense temperature. If something gets too hot, disconnect. Otherwise the insulation will melt and the bare wires will touch the car frame(s), shorting the battery. However if you cross polarity, you may have no time to disconnect -- it will start melting immediately!

Lastly, the large wires you talk about on cars and charging equipment is to support large current; large wires are NOT required and in-fact make a system SAFER. The reason car wires are large is because the voltage is so low, and the nature of electricity. If the voltage is doubled, the current is halved; likewise the gauge or thickness of the wires can be halved too. The efficiency of the transmission also increases and less energy is lost to heat. Wires should support the load. You don't have to charge a large battery at a large rate; the charger simply supplies any amount of electrons at 13 volts, and a thirsty battery will take it. It just takes longer, but no safety issues. Lead acid batteries last longer when charged slowly too. So charging slower will produce less potential oxygen and hydrogen.

When I was a child my dad worked at a large telcom office and data center (8 stories), where they had 6 backup generators in a separate building. However in the center of the main first floor (of the offices and data center) was a giant room full of large tractor-size lead acid batteries, hundreds of batteries. They were setting in shallow water trays with 1.5 inches of water. The batteries cover short power outages and gives time for diesel generators to startup. This has been common for data centers; with proper use, monitoring, fuses, disconnects and ability to dilute acid it has been safe.

We could freak out about all the lithium ion batteries in our pockets, in our headphones, laptops, etc. Any puncture is likely to start a fire that has its own oxidizer and literally cannot be put out with anything, including unlimited water. The only thing that can stop such a fire is a giant cryogenic dump or shower of liquid nitrogen.

You know lithium ion batteries can autocombust if their voltage is too high or too low, or if they are charged or discharged too quickly?

This project is on-target in the sense that using large lead acid batteries is traditionally the most efficient commercial option. Nowadays lithium ion batteries may be effective. However the logical option for consumers is to hack an existing UPS as done here.

We need more support, education and devices to support this type of hack. It might be that we can apply BMS or use a microcontroller to both control charging and discharging and safety, while also shutting the connected computers down. This way we can design our own safe and ideal system from the ground up and leave the UPS systems out of the loop.

All systems should have:
1. fuses
2. temperature sensors on key components: batteries, power supplies, hot / weak points
3. current and voltage sensors on key components
4. ability to shutdown or disconnect components if system shows thermal run away
5. placed away from people and nice things
6. tested before put into operation