I have 700VA UPS which I had bought in 2009 to power my Desktop Computer.It can power desktop(17" CRT monitor,CPU cabinet ) for 5-6 minutes.It has 7Ah battery(only 1), charging and inverting circuit and a transformer inside. Now I want to modify it to power my Laptop Adapter whose specifications are:

100-240V~1.8A 50-60Hz

The problem is, it only last for 15 minutes. Since the load is only 65 watt, I had expected hours instead of minutes.

So, what should I do to make it last for hours??

I searched for this on google and went through several sites and found some things regarding UPS:

UPS transformer is designed to power high power device(generally 400-500Watt) for short time(5-6minutes). At longer use , it may burn.

So, if I change the transformer inside with lower capacity trasformer( 100Watt), will I get backup for long time(1-2hours). Also, I have cooling fan from desktop CPU cabinet which I can use to cool the tranformer.

Please suggest me ways to modify it.( If you are wondering why I need UPS for Laptop, here in Nepal we have Load Shedding for 11 hours each day and Laptop battery last only for 2.30-3hrs and if I am able to modify a UPS(lying around) for 1-2hrs, it would be great!!)

  • \$\begingroup\$ for best results look for a loptop charger that runs from 12V \$\endgroup\$ Feb 21, 2016 at 5:33
  • \$\begingroup\$ How old is the battery? A 7Ah battery (assuming 12V) is 84Wh thus (allowing for efficiency) should give 65W for about 1 hour. However a 90% efficiency 700VA inverter may be only 50% at 70VA load, so practically, expect 0.5 hour. Then as the battery ages, its capacity will decrease... \$\endgroup\$
    – user16324
    Feb 21, 2016 at 12:38
  • \$\begingroup\$ A battery from 2009 is almost dead now. Your UPS should run for hours with a new one and just a lightly loaded laptop. \$\endgroup\$
    – Turbo J
    Feb 21, 2016 at 14:02
  • \$\begingroup\$ @BrianDrummond , can I add 30-40 Ah battery to my UPS for longer backup. It has 12V lead acid battery. \$\endgroup\$ Feb 21, 2016 at 15:37
  • \$\begingroup\$ @TurboJ, should I buy 30-40 Ah battery for longer backup. It has 12V 7Ah lead acid battery. Can it handle 30-40Ah battery?? \$\endgroup\$ Feb 21, 2016 at 15:42

1 Answer 1


Your UPS is from 2009. I assume that the battery is also almost 7 years old as well. This may be the problem.

Lead acid battery 12 volt, 7Ah. Now UPS usually have a cutoff voltage where they stop draining the battery to prevent damage to the battery. For lead acid battery, they can be easily damaged when discharged past 50%. There are more expensive deep cycle lead acid batteries that can be discharged safely to 70-80%, but it is unknown which type your UPS has.

So let's assume that your UPS sets the cutoff point so that after 50% are used, it turns off. 12 volts X 3.5Ah = 42 Wh. Now add to this the fact that a 7 year old battery in a UPS will have suffered significant capacity loss. So now your useable energy will be even less than 42 Wh.

Your laptop is rated for 65 watts. So if it ran at the 65 watts for an hour it would require 65 Wh of energy and you have less than 42 Wh. I'll let you do the rest of the math, but you can see, even if you got 80% discharge, that low capacity battery coupled with its age - you will never get the several hours your looking for.

Adding to your dilemma, is that the inverter portion of your UPS is not perfect and you will lose at least 15% of your power to it. So 42 Wh is now down to 36 Wh.

It is possible that the UPS has a timer that shuts down after 15 minutes regardless of remaining power, and you could easily reset this by measuring the voltage of the battery immediately after it shuts down. If the voltage is around 11-11.5 volts then the battery probably ran out. If voltage is significantly higher, then a timer shut you down. You will need to measure the voltage right after shutdown, don't wait or the voltage is less reliable.

  • \$\begingroup\$ Also can I expect better backup if I buy 30-40Ah battery. \$\endgroup\$ Feb 21, 2016 at 8:33
  • \$\begingroup\$ Or will the timer still shut it down?? \$\endgroup\$ Feb 21, 2016 at 8:33
  • \$\begingroup\$ Why does the timer shutdown after only 15 minutes?? Even if we assume my battery has only 36Wh of Energy, at 65W load , UPS should last for 33 minutes. Also, I refilled the battery with distilled water, shouldn't this improve its capacity?? Is auto shutdown feature enabled to prevent overheating?? \$\endgroup\$ Feb 21, 2016 at 9:03
  • \$\begingroup\$ A laptop will typically only use the full 65W while charging the internal battery. \$\endgroup\$
    – Turbo J
    Feb 21, 2016 at 14:04
  • \$\begingroup\$ The timer shutdown is only a guess on my part because of the fact that your laptop should have lasted longer given the battery that it has. Adding distilled water to an old lead acid battery will NOT add to its capacity. Also, if it requires water to be added, then it is the kind that can only provide 50% DEPTH of discharge so you only have 3.5Ah useable as I suggested. \$\endgroup\$
    – Filek
    Feb 22, 2016 at 3:16

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