(applies to the BR900GI model as well as other in the range, 550GI, 1200GI & 1500GI)
Here is a writeup on how to recondition your UPS with cheaper standalone batteries rather than simply buying the manufacturers replacement pack.
My UPS is the APC Back-UPS Pro 900. I’ve had it about 10 years so well overdue a refresh of the internal batteries. I use the device to protect my Synology NAS as well as the local network and Internet access equipment from short power outages.
Recently, I needed to turn off the downstairs power sockets for 5-10 minutes. Given the age of the batteries they could not carry the load, even for that short period.
Process to replace UPS batteries
With the UPS on its side, open the slide cover at the bottom to reveal the battery compartment.
(Hopefully, your batteries haven’t leaked like mine!)
Remove / unpeel the green and red stickers from each side of the battery pack.
Be careful! You will need these again.
Remove the spade connectors from the old batteries. Be careful if yours have leaked as mine had and wear gloves to protect your skin.
(optionally) Clean up the spade connections as necessary to ensure good contact on the new batteries, be careful not to damage the protective insulation around the connections.
Connect the new batteries cross wiring them in series as colour coded by the cross over wire going from red (positive) on the first battery to black (negative) on the second battery.
(I found it much easier to do this with the batteries stood on their end vertically)
Re-attach the green and red stickers. Note the orientation of the connection to the UPS internally.
(Again, this is easier done first time with the unit stacked vertically as per the photo)
Before replacing the new battery pack it’s a good idea to “reset” the UPS.
Do this with the unit disconnected from the wall and with the internal battery out.
Press and hold the “On” button for 5 seconds to fully discharge the unit.
After the reset, with the stickers on, re-install the batteries into the UPS and replace the cover.
Plug in the unit and let it charge to 100% ready for calibration.
Calibrate new battery runtime
To get a more accurate “time remaining” figure from your UPS, it is best to manually calibrate the UPS once new batteries are fitted.
Manual calibration process
Make sure the battery is 100% charged.
Apply a constant load to the UPS of about 30% for the duration of the entire calibration.
(In a modern low power world this is not as easy as you might think. Classic none energy saving lightbulbs in desk lamps are a good bet to make ~100w but almost everything else these days is low power. Heating products like hair dryers are too much. Do not use equipment that will be damaged by sudden power loss as you will run the UPS till depleted.)
With sufficient load applied, disconnect the UPS from any USB / LAN control source and the mains power. Record the start time
Let the UPS fully discharge and shut down. Note the time.
Once complete recharge the UPS while switched off and without attached load.
After plugging in the UPS to mains power simply press the off button and leave for 12 hours
Turn on the UP and connect equipment to be protected, the runtime remaining should now be an accurate reflection of the load.
Sourcing compatible replacement batteries.
First, you need to work out what power rating your UPS has and how many individual batteries you require. Do this by opening the unit up and seeing what you already have.
For the APC Back-Pro series they have two batteries wired together via an interface plate. Power rating goes up with the model number.
Yuasa batteries from Japan have a good reputation and work well in my experience, they even have a UPS battery finder on their UK website for UPS compatibility.
I found HardwareExpress in the UK particularly helpful and super fast delivery. They do a “pack” for the Back-UPS Pro 900 here:
Using their UPS finder I instead went for slightly larger amp-hour capacity (8.5 Ah vs 7 Ah) but also £10 more at the time of this article.