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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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?


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.

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


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?


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

It depends. Motorcycles uses AGM batteries. They are different from what shown on the pictures above. See if your battery has AGM on it. Do not use it if has it. There is something special in terms of charging AGM, my motorcycle battery is charged with special charger. UPS charging circuit is to maintain classic way for sealed led batteries.

depends if you have a decent ups

power shield have ups's that use AGM batteries entirely because they last longer

but your best of not using them because of overcharge caused by the ups not being able to sense the charge of the battery (only really happens in cheap models)

a late response, but you can use a very simple and cheap "UPS" for Router, modem, swtich, cctv, alarm, led light. etc. and last much longer without the lost in conversion DC to 120v AC(router adapter) to 12v DC (router adapter) again,

or another special for router-modem with a 18650 lithium battery inside (interchangable) in a wallwart format :

this units supply 12v direct.

have a nice day!

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.