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Convert your Computer UPS to Home UPS

This article explains an interesting topic, how to convert your computer UPS into home UPS. If you own a desktop computer, you may have a UPS that can power your computer for 10-15 minutes after power failure.

By Girish Radhakrishnan

Using an UPS

The purpose of the UPS is to save your work and shutdown your computer properly to avoid potential data loss and hardware loss such as Hard disk of your computer.

Most of us always underestimate the potential of the computer UPS that sitting beside your computer. The average computer UPS can deliver around 600VA, which is enough to power your low power appliances such as fan, tube light, computer, television, etc.

If you own more powerful UPS such as 1KVA you can power your home appliances more.

So how much can my computer UPS can provide? 600VA means the apparent power, but the real power is 60% of specified value. In other words it provides 60% of the VA rating.

For example:

If you have 600VA UPS then 600VA x 0.6 = 360W maximum output.

If you have a 1KVA UPS then 1000VA x 0.6 = 600W maximum output.

If my computer UPS can provide this much power, then why computer UPS only power 10-15 min?

This is because most of the computer UPS is only powered by 12V 7AH battery which is sitting inside the UPS.

To increase its backup time we need to connect several numbers of batteries with same specifications in parallel. The motto of this article is to make a cost effective home UPS from a computer UPS.

The procedure explained in this article is not suitable for beginners in electronics.

Block Diagram:

Block diagram for converting Computer UPS to Home UPS

Simulation and Working

The whole UPS consists of an internal battery and several external batteries, which are crucial part of the UPS. The internal battery which is charged by internal circuitry of the UPS.

No external battery must be connected to charging circuitry during charging period. This is because the UPS is only capable of charging single SLA battery.

Exceeding more than one may overload the charging circuit and may lead to physical damage to UPS such as fried circuitry or may even cause fire. This is exactly opposite during discharge. All the batteries are connected in parallel including internal battery, injecting the power to UPS.

The external charging circuit consists of voltage regulator LM317 and op-amp comparator circuit for full battery cut-off. The voltage regulator gives out 13.75v for charging which is healthy amount of voltage for charging all kind of 12V SLA batteries.

When the battery reaches full battery voltage, relay cut-off the batteries from charging circuit and gives float charging to the battery via 150 ohm/5 WATT current limiting resistor. The relay triggered by op-amp comparator circuit.

There are two transformer one for charging which is 5A or more, the other transformer (500mA) for sensing the presence or absence of mains power.

If the mains are present relays are activated and connected to charger. If mains are absent relays are deactivated and batteries are connected to UPS. The 5A charging transformer may be replaced with SMPS.

The 100 ohm/5 watt resistor for quick discharge of 1000uf capacitor, so that relay can be deactivated instantly during power failure.

During power failure all the batteries connect automatically in parallel powering the UPS. When the batteries are at low state the UPS automatically disconnect its batteries and shutoff itself.

There is always low battery cut-off circuitry in the UPS. Most of the computer UPS gives out modified sine wave during power failure, and mains sine wave during normal state. This is suitable for most of home appliances.


1) Do not omit the internal battery, this plays an important role in giving uninterrupted power output and UPS circuitry gets unstable without the internal battery.

2) It is not recommended to connect more than 5 external batteries.

3) Do not connect this UPS to mains as what we do when we have genuine manufactured home UPS [IMPORTANT].

4) Use a branded computer UPS to proceed this project.

5) Never overload the UPS during normal/backup state.

6) Make sure that all internal and external batteries are with same capacity (AH) and age.

7) Do not connect any inductive loads other than table fan.

8) Place the whole setup at well ventilated area and don’t allow water to contact the setup.

9) Disconnect the gadgets immediately if you found misbehaving.

Author’s prototype

which shows how he could convert his computer UPS to home UPS:

I used two external batteries and charging circuit is embedded inside old DVD player chassis.This is running since 3 years, and absolutely error free.

Need Help? Please leave a comment, I'll get back soon with a reply!


  1. Nice try by Girish Radhakrishnan but theoretically based only lack of practical knowledge. Internal ups transistor are not capable of running long enough (not even an hour). Microtech and APC Ups can survive for some time but not like Intex brand. I have tried on Intex ups and all 4 transistor got burn just after 30 minutes on load of a air cooler only of 200watt. Now I am using UPS transformer as Stablizer + Charger bcoz ups auto transformer have both winding and can be use both function at same time.
    And this Article is very old and should not be placed again.

    Sorry to say but it mention 'this article is not suitable for beginners in electronics' but the person who submitted himself looks beginners in electronics. Not in the standard of get place here in Swagatam Majumdar's sir blog.

  2. Hi Ram, yes it's a good effort by Girish, and I appreciate your acknowledgement!
    A "beginner" can be classified into different levels, some could be in the very initial stages of learning, while some in the higher stages. In the above article the term "beginner" is for those whose are very new and have hardly any knowledge regarding electronics, and it's mentioned to warn and save them from a possible failure and losses with the project...

    By the way why do think this article is old and should not be posted again?

  3. Yes Sir, you have already provided many similar and more efficient solutions. And in my view this circuit have some un necessary things like internal one 7ah battery can be removed. External charging volt can be adjusted to 13.5v to safe charging and keep things simple rather using extra transformer to sensing and extra circuit for auto cut charging.

  4. Hi, Ram Ryan. Thank you for acknowledging your issues :) .

    You mentioned that the transistors got burnt, actually they are power MOSFETs. In my prototype I used INTEX UPS model: Black Armour 725. The transformer driver stage is utilised by IRF3205 by International Rectifiers which are power MOSFET.

    According to their data sheet, Source voltage is 55 volt and Drain current is 110 Ampere and maximum operating temperature is 175 degree Celsius. This means all the parameter used in this project is well under the safe limit. The same MOSFETs are used by many home UPS manufacturer.

    You did not mentioned about the external battery connection, did you connect a 100AH battery parallel with internal battery? or no battery is connected?
    And one important thing you connected a Air-cooler to the UPS which is clearly an inductive load which has high powered fan.

    I mentioned in this project that you must not connect any inductive load other than table fan,it is not good for your UPS and for the appliances. Because the output of the UPS is modified sine wave which is not optimized for inductive loads.

    One of the possible reason for your fried MOSFETs may be due to failed oscillation stage of your UPS. The gates of MOSFTEs stayed high for long enough and short circuited the Source and Drain. (OR) You used a very old ups say 5 Years, which is not capable of handling high power. My prototype is working fine since 3 years.

    And I mentioned in this project that it is not suitable for beginners because they are playing with high voltage situations. I am an intermediate in electronics and I have some good practical experience :).
    If you have any further question regarding this post , feel free to ask.

  5. Thanks Ram, I am sure Mr. Girish will sort out the issues....

  6. Hi, Ram again.

    The internal battery plays an important role. When the power retains after a failure, it takes about 3 second to switch from battery to AC mains. But those external batteries are already disconnected at the instant where power retained. This causes 3 second black out at the output. If you connected a computer at output or any other gadget which need continuous power, is already shut-off.

    The external charging voltage can go up to 15 volt safely, 13.5 volt is used for tickle charging for a Lead acid battery so, it should be above that.

    Regarding the sensing transformer, it is used to trip the relay on and off during power failure and power retain. if omit this ,the external battery never connect/disconnect to UPS.

    You mentioned about omitting the charger circuit. Without the charging circuit how will you charge the external batteries??? Are you going to connect all the external batteries to UPS directly? if you do so, you are overloading the charging circuitry and ending up with fried circuit board.

  7. Hi Swagatam,
    I am sorry to ask a different subject in here. I am making a variable regulated battery charger using one of your circuits for 1.25 up to 38 volt. i am planning to install a digital volt-amp meter in the charger unit to measure the output voltage and current to the load. my problem is, the digital volt-amp meter requires an isolated power of 5-30 volt. can i just attach a regulator IC 7808 in the charger by sharing the same DC with the same rectifying diodes of the battery charger (it is before the charger regulator IC LM338) to get 8 volt power supply for the digital volt-amp meter ? If, it is not proper, please kindly give me the solution. thanks..


  8. Hi Kanta, yes definitely you can do that.

    since the input is 38V so I won't recommend a 7808 for this, instead you can try a simple emitter follower option as shown in the following example article:

    you can replace the zener diode with a 9V 1watt zener for getting a reasonably regulated 8.5V at the output for your V meter circuit

    and also replace the 220 resistor with a 10k resistor

  9. ...the transistor could be a 2N2222 or any similar.

  10. Hello sir,
    I want to make a battery low detector by using op-amp ic . In circuit I want to set a buzzer and relay , when battery get low the buzzer and relay activate but the buzzer get stop in few seconds and relay do not deactivate .
    Help me sir for making this type of circuit.
    Sorry for my bad english. I hope you understand.

    Thanks in advance

  11. Hello Mayank,

    You can try the second circuit from this link:

    replace the LED with buzzer, and connect a 100uF in parallel with the 10k resistor

    the relay will initially stay activated and deactivate as soon as battery goes low....buzzer will sound for 2/3 seconds and stop

  12. Hi Swagatam,
    Thanks for the prompt response. I just notice that i also have to install a cooling fan which requires DC 12 volt. For this, i think i have to increase the voltage of the zener diode to 12volt/1 watt. If so, the transistor need to be TIP 122' right? Then, what will be the value of the resistor. This will be used to power the fan of 0.16 A and the digital V meter..
    Many thanks..

  13. Hi Kanta, you can still go with a 10k resistor for the TIP122 set up, higher values upto 33K can also be's not so critical

    use a 13V zener to achieve 12V at the emitter of the transistor.


  14. Why there are two transformer one for charging which is 5A or more, the other transformer (500mA) for sensing the presence or absence of mains power? While the same 5A transformer output DC can be used to sense Mains Failure.

  15. Hi Swagatam..
    I am sorry for using this unrelated subject to ask you the matters beneth since your Circuit in which i am using was published about 3 years ago.
    I just constructed the second circuit using 6 volt battery, with 24 Led, single series with R combined 3 diodes. For the emergency light, it is functioning well. However, for the charging and cut off section, i set up the LED with the fully charged battery (6.9volt) by triming the pre set of TIP 127, to get very dim LED. When the battery discharged to 4.4 volt, the LED is still dim, then i detached the Battery and measure the collector voltage of TIP 127, the output is zero. I trimed the VR in such away to get the output of the collector to 8.5 volt, the LED became very brigth. I put on the battery and let it get charge. After the whole day, i still saw the LED bright. Then i detached the battery and measured the voltage of battery, it was 6.9 volt. i put on the battery again and trimmed down the LED until dim. After that, i detached the battery and measure the output of TIP 127, it was zero volt. Could you please advise me, what is my fault. I use 7.5 volt transformer of 1 amp and 6 volt 4.5AH batery.
    Many thanks

  16. Hi Kanta,

    I have already answered to this question in the same article...please check it

  17. Hi, Abhirup

    If we hookup the relay on 5A transformer due to high capacitance 2200uf it takes few seconds to trip the relay. Whereas the sensing transformer has low current and 1000uf with 100 ohm bleed resistor, making instant tripping of relay. Just remember we need to reduce the delay of tripping during the power failure.

  18. Hi Swagatam..
    Thanks for your prompt support..
    After going thru all the comments of the emergency led, i decided to modify the circuit by only replace the charging and cut off portion to the circuit no 1, which is with the 2 transistors.

    Other, I am planning to construct an automatic battery charger for 3, 4.5, 6, 9, 12 and 24volt for max 50AH battery. For this, i will choose the circuit of yours with LM 338 and LM 3915, which is 10 steps charging with led lit indicator. Please kindly advise your recommendation.

    Many thanks

  19. Hi Swagatam,

    I have a question with regard to the cooling fan in the battery charger unit, can i just attach rectifier bridge on the same transformer which is also used by the charger. The charger rectifier is connected to 0-30V, and the fan power rectifier will be to 0-12V, both the charger and Fan share the same transformer. However, this is as long as the resulted positive and ground of the charger and the fan will not be touching each other. I propose this because when i calculated the current for the fan and digital V meter, it will be about 350mA which need a 13 V-5 watt zener diode which i cant find this in the market at the moment. Please kindly advise.
    Many thanks

  20. Hi Kanta,

    Please go ahead with your plans, If you have issues feel free to put them in the relevant articles.

  21. Hello Girish and Swagatam...
    Wonderfull project for emergency Backup at home.

  22. I want to try to this circuit but I am little confused over charging circuitry.
    Consider Mains power is ON, so the external batteries are put to charging mode. LM317 is a adjusted to give 13.75v and this voltage is applied as Opamp input signal via voltage divider network.
    And you have used a 4.7v zener so Dat the reference voltage at inverting input is 4.7V
    In this situation the 13.75V signal will itself trigger the opamp and start float charging irrespective of battery voltage. Actually it should trigger based on the Battery voltage na?
    Please tell me if I am going wrong... I m very much interested in this project..

  23. Salil,

    the potential divider network at pin3 of the opamp is probably calculated to trip the relay at 13.5V (actually it should be 14.4V)....and the 317 output will always sink to the battery if the battery is at a discharged level of 12V, then the 13.5V will also sink to 12V and begin rising as the battery gets charged until 13.5V when the relay will trip and cut the supply to the battery.

    A hysteresis resistor across the output and pin3 is recommended to prevent the battery from randomly switching at the threshold

  24. it.
    Thank u soo much..I'll definitely do it.

    One more question.. I had asked about one capacitor in another post .33uf 275v x2 Box type.
    It was connected in series with Phase at the a Timer switch
    I am not getting this capacitor at the local shop.
    Can it be replaced by .22uf capacitor?

  25. 0.33uF x 2 = 0.66uF

    so you can use a 0.68uF/400V in its place, or any other nearby value will also do.

  26. If I use car battery for ups.
    .will the power be 48hours?

  27. Hello,

    Car battery is not recommended for this project.


  28. Hi sir. I have UPS here 600VA (390watts). the battery SLA 12v 7Ah was already swollen due to aging. Can i replace it into higher amp/hr battery? Example 12v 20Ah SLA or 12v 40Ah SLA or 12v 70Ah SLA or 12v 100Ah SLA replacement? So that i can save more spaces on my home instead of buying more battery. I want to power 2 small electrifan about 30 to 60 watts (depending on the speed of the fan)

    One more sir. My UPS have a built in beeping sound/indicator and it is kinda annoying to the ear. How can i remove it?

    cuz im going to replace it. Instead of a beeping sound. I will replace a small digital voltmeter as an indicator for more accurate voltage measurement..

    Thanks you in advance.

  29. Hi Keiichii, yes you can use a higher AH rated battery as a replacement for increasing back up time and for making it compact.

    you can replace the buzzer with a V meter but the meter needle will keep fluctuating in response to the switching output which was meant for the buzzer.....alternatively you can replace it with an LED

  30. Thank you for the response sir... since im going to replace the battery... the UPS will charge my battery the same way as it charge the original battery? No modification will be done?

    Thank you for suggesting LED replacement instead of V meter. Im ganna find the annoying beeping sound of the UPS and replace it with LED.

  31. no sorry that won't happen, you will have to check the max current capacity of the existing UPS charger, if it satisfies the new battery requirements then it's fine other wise you may have to upgrade it with an external supply or a new more powerful transformer/smps.

  32. Hi Keiichii,

    You can use higher capacity battery to UPS, only during discharge process . The on-board charger in the UPS is not powerful enough to charge higher capacity batteries,there is risk of over-loading the on-board charger. so, this is not recommended.

    if you really want to connect higher capacity battery, charge it separately and disconnect the UPS from AC mains during back-up mode. Check for unusual heating or any abnormalities during first test.


  33. Hi sir. My UPS have 6amp charger.... is this enough? If not. I will charge it seperately. I have LM317 adj. Voltage regulator with MJE2955 for current booster.. is this ok to charge SLA battery sir?

    Ok sir i wont charge it using the UPS if the 6Amps is not enough....

    Thank you for you recommendation.

  34. Hi sir.. current capacity of the existing charger on UPS is 6Amp for 12volts. Is this enough? If not enough, i can manually charge it seperately and plug the SLA into UPS if needed..

    i already have an existing simple LM317 voltage adj. Regulator with MJE2955 for current booster that i keep during my college days on my basic electronic subject projects. Is this useful to charge the SLA sir? Lets say.. 12v 40amp/hr or 12v 70amp/hr SLA battery?

    Thank you for your concern sir.

  35. keiichii, the charger capacity should be ideally rated at BatteryAH/10, so if your battery is 40AH and if the charger is capable of supplying 4 amp then it's fine.

    Moreover your charger must include an overload, short-circuit and over charge protection features.

    your LM37 with boost current might not have the above mentioned features, so it may not be safe.

  36. Hi keiichii,

    you can charge with your LM317 charger, make sure it can deliver enough current to the battery. Also add a full battery auto cut-off to your charger, so that you won't overcharge the battery.

    Can you please elaborate,how your UPS can deliver 6 amps for the battery.(any thing specified on the UPS?).

  37. @girish...Hi sir..the UPS states on its back that it has input of 220v~, 50hz 6.0 amps max (from wall outlet to the UPS). And output of 220~ 3.6amps (from UPS to your Appliances load)

    Does the 6amp means transformer sir?

  38. Hi keiichii,

    If your UPS intakes maximum current of 6 Amps, it does't mean that it charges your battery at 6 amp , as you stated on previous comments. The 6 amps is for your connected appliances load and also for the battery.

    For 12V 7ah battery the charging current must not exceed 2 Amps , so the on-board charger don't supply current more than that.

    The 6 amp is not the transformer's specification.


  39. I see.. it means transformer does. 2 in 1 (for load and battery). Ok sir.. thanks. I will use this UPS for dicharge with bigger battery and charge the battery separately.

  40. Sir, do you know the rating of the computer ups transformer???? My frnd gave one transformer for me its taken from computer ups but they didn't mentioned the rating of a transformer....say some average rating sir

  41. Vijay, the specification can be different for different UPS, it cannot be a universal fixed would be difficult to judge.

    however you can confirm it with a meter by the connecting the primary with AC mains, and by measuring the output V and amp accordingly with a digital meter.

  42. OK sir,voltage can be easily measurable but how can I measure the current without loading transformer. Even if I get a load such battery. How can I estimate the allowable current to the load.suggest some techniques sir...thanks for fast reply

  43. you can connect the DMM directly across the secondary winding for measuring the current....make sure to keep the DMM in the 20amp or 10amp "AC" range, and do not exceed the procedure more than 2, 3 seconds.

  44. Oh that means I need to short my transformer secondary through dmm in ammeter mode right???... The current value what I get from this state is considered as maximum rated current ???

  45. yes that's right, and that's why you shouldn't keep it connected for more than 2, 3 seconds, otherwise you could see your meter smoking.

  46. hi mr.Swagatam
    sorry if this might be out of topic, but yesterday a friend gave me a used 2KVA home UPS(pascal 2000Hf, i already google for the manual but pity i cant found one for this type) with no internal battery, from what my friend recall its using 8 battery for backup but he didn't remember the connection if its in series or parallel, so i measure the UPS wire that goes to the batteries and it gave me about 110v. its a big help if u can tell me how i supposed to connect the batteries(in series or parallel ?) since i already bought them, once more Thanks in advance

  47. Hi Peter,

    If you are sure regarding the 110V spec, then naturally many smaller batteries will need to be connected in series since a single 110V battery is not generally available in the market.

    If the 100V is the charging voltage then probably you could employ 8nos of 12V batts in series with it.

  48. may thanks for fast reply, now its working fine :D

  49. Hi sir your topic was so clear, but i have a doubt that can we improve the SMPS as wells as transformer to replace the smallwr battery with higher capacity one?

  50. Thanks Sikkender, yes that's possible..

  51. Lov dis .i do see dis stuff on d net but i thought ups timing is by some software modifications. But seeing it makes me blv it.i wl a usp but i want to use a 75amp battery can i just increase d charge current with transistors let say 2sc5200.tnx

  52. You can upgrade the charger section to 10amp and then use your preferred battery...the UPS output section will also need to be upgraded suitably for operating higher wattage loads

  53. Tnx for d reply.d author of dis post used 21ah 12volt battery in total. If i may ask how long is d backup time for dat battery amp

  54. you can use the following software to find an approximate figure

  55. Why we should not use inductiv load on computer ups more then table fan , why we can't run silling fan on computer ups and why regular inverter can handle that load ?? Plz explain more about point 7 of warning list ??

  56. because inveters are usually more powerful and able to sustain the higher watt load such as a ceiling fan

  57. Sir please let me know how can i strengthen the charging circuit of ups to charge 70Ah batterry with smps inplace of inbuild charging circuit.
    I want to make it like inbuild means it charges battery as in orginal case.

  58. Wasim, I have not yet studied the above article, however I can tell you the universal rule for charging and upgrading any form of battery charging system.

    For charging higher AH batteries you will have to upgrade the current (amp) input to the battery through a current control circuit designed to limit current at approximately 1/10th battery AH value (for lead acid batts), for your 70AH lead acid battery the controller must be tuned to produce 7 to 10AH current.

    This battery can be used for a longer backup with the UPS or inverter, but make sure the UPS/inverter is also current controlled so that a short circuit or overload at the output does not burn the internal devices.

    If you have any specific question you can ask, I'll try to answer.

  59. Thanks Girish and Ram for such useful data,
    Please let me know, what is the least rating of battery which can be used with UPS , as I got the idea that UPS charging circuit is not strong enough to charge high rating battery like 150 AH.

  60. hii, can we use more power packed 18650 battery cells in the old computer ups to power up. I surely think modifications as mentioned by you may be required, can u suggest, if we can then we can have lot of backup in the small space of the ups casing itself. kindly suggest


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