Hacking an old UPS for Massive Capacity and Massive Cost Savings



Take a large capacity UPS with dead batteries, which is worth little more than scrap money, and re-use/’Frankenstein’ it using cheap car batteries. If batteries with a larger capacity than the originals are used, a longer run time can be achieved.



For my example, I will use a APC SMART-UPS 1000VA. This UPS comes with two standard size 20Ah sealed lead acid batteries connected in series to produce a voltage of 24V at 20Ah. A replacement set of sealed lead acid batteries can range from £50 to £80 depending on if you want quality. We remove the sealed lead acid batteries and instead replace them with lead acid car batteries. You can get these from a scrapyard or a car breakers yard. These typically will have a capacity of 40-60Ah, so chances are you will use batteries with a larger capacity regardless.

To mitigate any safety worries, the car batteries are placed in battery boxes that will contain any acid events.


Increase Capacity (optional)

If you want to increase your UPS’s run time then when you are looking around for car batteries, make sure to get a larger capacity batteries.

If runtime isn’t as important, then get whatever capacity (or size) suits you. You will probably find that the capacity of car batteries is larger than sealed lead acid batteries regardless.



  • Take an old computer UPS.
  • Remove the batteries, make note of the connections and arrangement. Your UPS will either be 12V, 24V, 36V or 48V.
  • Recycle the old lead acid gel batteries. You may be able to get decent scrap lead prices.
  • Extract the battery wiring loom from the UPS, the larger UPSs will have Anderson connectors which will make this easier.
  • Acquire used car batteries for cheap, make sure they are the same capacity, or as close as possible.
  • Install your batteries in their battery boxes.
  • Extend the wiring loom out of the UPS and connect to the batteries, via fuses.
  • Plug the batteries into the UPS.
  • Plug the UPS into the Mains. Plug your equipment into the UPS
  • If possible, install the UPS software and let it analyse the new batteries.
  • For unexpected thrills unplug your UPS at random time intervals and see how it handles.


Costings & Supply

Old UPS                         £15 Boot Sale

Old Car Battery x2           £30 Car Breakers Yard

Battery Boxes x2             £23  eBay

Battery Terminals x4       £5    eBay

Grand Total                     £73

New batteries alone can cost £50 at the least. By spending £20 more you can increase your capacity, when your car batteries die, you can cash in on the scrap lead price, and buy more batteries, and then you just have to buy some more batteries.



  • Fuse everything on the low voltage side, and make sure the mains fusing is also okay.
  • Don’t setup without battery boxes.
  • Don’t leave unattended for first few days of operation, make sure they’re safely ‘broken in’. There’s negligible risk of fire or destruction, just measure the voltages, make sure the batteries do not unbalance and make sure the UPS adapts to the new batteries.
  • Install additional cooling within the UPS if you increase the runtime substantially. A virgin UPS will typically only have a runtime of 10-20 minutes under full load.
  • Make sure that your cabling is double the gauge of the original UPS battery cabling.
  • These batteries will need occasional maintenance, topped up with distilled water, etc.
  • As these batteries are not sealed, they will also produce hydrogen gas when in use. Ensure adequate ventilation. Some car batteries come with ‘breather tubes’, these are advantageous as you can pipe the vent to the outdoors or similar.
  • If the batteries do not have the same capacity, they may need to be balanced. This involves charging the battery that has the lower voltage until both have the same voltage.


“Car Batteries aren’t designed for cyclic use”

Correct, they aren’t, and use in a UPS will eventually kill them. But, from my experience, it is better value for money to use car batteries then to use sealed lead acid batteries.

Equally, sealed lead acid batteries are not designed for high current outputs. My UPS sucked at least 100 amps out of its stock batteries when it was new and fully loaded, and their datasheet shows their maximum discharge is only 112 amps, so it’s working them hard. A car battery will be better able to handle large current demands.



If you can mitigate the problems with hydrogen build up, and remember to top up the batteries with distilled water once a year, you should be golden.


Further Reading