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.



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121 Discussions


1 year 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?

4 replies

Reply 18 days 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


Reply 17 days ago

Perhaps you missed the fact that the UPS uses SEALED lead acid batteries. Good SLS's have a AGM adaptation to recirculate these explosive gases. it is not only a gel acid silicate. From wikipedia "More importantly, gas recombination was used to make batteries that were not "watered" and could be called maintenance-free. The one-way valves were set at 2 psi, and this was high enough for full recombination to take place. At the end of charge when oxygen was evolved from overcharge on the positive plate, it traveled through the shrinkage cracks in the gel directly to the negative plate (made from high surface area pure sponge lead) and "burned" up as fast as it was made. This oxygen gas and the hydrogen adsorbed on the surface of the sponge lead metal negative plate combined to make water that was retained in the cell.". Your car battery only has a open vent for the gases and are meant to keep down liquid bubbling, and to what you described in your message the generators and or batteries were in a separate room. This was done for a reason, perhaps you did not investigate the construction of the room? Ever notice the forced air vents, they where not just for heat! When I got my degree in electronic I studied batteries as part of electronics and as a Biomed/imaging engineer working in hospital all my life I am well aware of these backup power systems as they had to be certified to the government and JCAHO (Joint Commission). You might get the point I come from a position of authority, meaning I have knowledge from experience. You are correct that the common lead acid battery will do better a a low charge rate and this is why your automotive battery charger has a 2 AMP setting, also known as trickle charge, and yes this would reduce the amount of gas released, but if you read the link to wikipedia, it clearly states lead acid batteries produce hydrogen and oxygen gas. Not something I would want in a closed environment with me in it. -James

Zach LaffinJamesN158

Reply 1 year ago

Okay, first yes these are acid batteries but the danger is there regardless of the use. That should be something someone should take into consideration regardless of the project or use. Second, the ampere output of an alternator is majority used up by the vehicle that is currently driving that alternator let alone there are systems in place that limit the amount of power out of the alternator when not as much is needed. Three, yes these are high output and a deep cycle car or gel battery would probably be better, but regardless the charging does not require high amperage. These types of batteries can handle high amperage charging for fast charging but even then you don't get an efficient charge. Ever heard of a trickle charger or battery tender? Those charge batteries like these at even a less rate than the on inside this UPS. Lastly the wire size is fine. the battery may be large but the load is not as large and will only pull what it needs let alone the fact he has a fuse and recommends one. That wire size is larger than what is inside the UPS. The wire size is based on the load using the power. Please if you are going to help give warnings, give the right information.


1 year ago

seeing this is an old entry but basics were good but i see some problem the ups will trickle charge larger batteries i use one with dead inverter section APC has a good charging but less so in the inverter section which seems to break first confirmed by my friend electrical engineer who worked for them when they were made in USA. but they would strain or fail if the larger battery my one is 77ah was drained low so if depleted I charge with a larger charger and soon solar.with controller. the second problem using a car or similar battery they vent hydrogen gas very flammable so use caution inside home ups batteries are designed for indoor use most car batteries still outgas even so called sealled ones. i had a car battery on charge in attached 2 car garage and it set my CO detector in the laundry room off even though i smelled no rotten eggs smell. so if used inside in box that's mostly air tight avoid anything to spark when charging and ventilate when open before switch or using tool that might spark especially connecting or removing the wire. I never blew a battery but seen it mechanic got acid burn in face and battery destroyed

1 reply

Reply 18 days ago

In a lead acid battery there only a few reactions possible with the four elements: O, H, S and Pb. The S only bonds with Pb and cannot escape, only H2O, O2 and H2 can come and go. There is nothing in a battery capable of producing a compound with carbon.


Question 4 months ago on Step 2

i tried changing my ups 24v battery (2x7Ah) 1200va with 24v (2x50Ah) but the UPS shutsdown every hour, exactly 60minutes it seems that it has a timer how do i disable this? my UPS is PROLINK 1200VA(SFC) non U version so it is not smart Ups.

James M.L

1 year ago

Would a smart battery charger + auto or marine battery + dc/ac inverter function as a UPS?

2 replies
HaroldG29James M.L

Reply 10 months ago

You would want some type of way to feed wall power to your stuff until the power goes out as many inverters output crap 60HZ that is absolutely horrible for your electronics. It would make no sense to convert 120vAC down to 12vDC, then back up to 120vAC, as this wastes a tremendous amount of energy. I would recommend 3 2-way relays (The kind that has a common and a normally open and closed.) and have them switch on the Inverter and connect it to the output and isolate the inverter from mains.

darryl.kinslowJames M.L

Reply 1 year ago

Yes, that's essentially what a UPS is, except deep cycle batteries would be better.

schoong yein

Question 10 months ago


I have A?? UPS with the live nad neutrel wire reversed, will it damage the charging circuit?

all of those kept on beeping even though it is plug into the wall socket.

what is the fuse rating for UPS?


2 years ago

I would suggest an additional step in the procedure due the wide variety of batteries and UPS- measure the open circuit voltage to the cables/wires to the batteries (with batteries disconnected). Lead Acid (flooded) batteries (auto/marine) charged for a prolonged period (weeks/months) with a voltage above 13.2 for a 12 volt battery/system (26.4 volts for two 12 Volt batteries in series) typically at the will shorten the life of the flooded lead acid batteries and can cause out gassing. .

I have a discontinued Cyberpower UPS 950 Watt (8 minutes) (it does not have a LED display it is about ten years old), OpenCircuit voltage is 27.6 volts with two qty OEM gel 12 Volt 24?Hr batteries. Two newer Cyberpower UPS models with LED displays have a 28 Volt open circuit voltage. The charging circuitry for the batteries seem to be trickle chargers for all three UPS --- they NEVER turn off. This is not good for a flooded lead acid battery. (a) never turning off, and b) the voltage being above 26.4 volts (or 13.2 volts for each 12 Volt battery) This will shorten the life of a lead acid battery. Gel Batteries require a slightly higher float voltage. Auto (lead acid batteries) should be using float charger, the charging can turn off as needed.

Basically, three or cells in negative battery (connected in series) has a history of low water level - almost with out a doubt caused by the difference in voltage difference required between lead acid and gel batteries.

If one has this type of UPS (designed for gel batteries, trickle charging above 26.4 volts and one puts two lead acid auto in series) and desires a long trouble free long life of the auto/marine batteries. The UPS battery trickle charging circuit needs to be disabled, if float charger, the voltage of charging needs to be adjusted, if not adjustable, disabled. Then a float charger designed and installed, if the charging circuit of the UPS was disabled, if one desires a long-life trouble free design.

In so doing, note - chassis ground(S) are connected to battery ground.

1 reply
Zach Laffinplankieee

Reply 1 year ago

This may also be based on the specific UPS though. I obtained a heavier duty one from work that has a larger charging circuit. And in some cases the ones that come with dual batteries are generally larger circuits. The best ups to get for this is ones that are made for and run servers for companies. that is the one I have.


1 year ago

Car batteries are designed for high current, not deep discharge. If you actually drain the batteries multiple times you'll find they lose capacity. There are batteries designed for deep discharge and they will cope much better with being emptied on a regular basis.

1 reply
Zach LaffinAlanH158

Reply 1 year ago

I was just thinking the same thing. Why not a deep cycle card RV battery, only a couple bucks more for a lot longer life.


1 year ago

I had once done the same with one of my UPS rated 1 KVA, connected a 12V 18 AH Battery to it, instead of the 12V 7AH that came with the UPS. It worked great for one month or two, then the UPS stopped working altogether. The heat must be the issue, which the UPS is designed to handle. Now I have done the same to another UPS, but this time I have connected a 120mm computer Fan inside the UPS chassis. I use it for about 50% -70% load all the time. the system is mostly on all the time. It is working fine for last 7-8 months or more.


Reply 1 year ago

Transformers have winding ratios, like 4:1 or 8,000:1. You can use a bigger transformer with the same characteristics, but you'd probably just burn up something else.

Richard Hole

6 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.