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.



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114 Discussions

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

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.

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

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.

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?

1 reply

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.

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.

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.

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.

1 reply

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.

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.

1 reply

I suggest you study electrical engineering before attempting this. It is not a project for a hobbyist.

The charging issues are a cinch. I need two batteries in series to provide 24VDC. For $25 each you can buy two battery chargers/maintainers at Sears. Hook each up to one of the batteries - while they are connected. Insert two diodes backwards in the positive lines from the UPSs to the batteries to ensure the UPSs do not try to charge the batteries. Use AGM batteries if you don't want to risk a hydrogen explosion.

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

1 reply

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

I wonder if this is really a cost effective solution and if it might be cheaper to just get several batteries the size of the one that comes with the UPS.


10 months 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

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?

1 reply

mc battery will most likely work but purchase 2 5ah tougher from the same batch and run them in parallel. wonder how he made out as 2014 entry