How to Replace a UPS Battery (and Skip Buying a New UPS)

My UPS stopped backing up my PC whenever the power went out. The screen just went black, as if there were no UPS at all. The electronics were fine. Only the batteries had worn out. After replacing them I got 28 minutes of backup again, so there was no need to buy a new UPS.

Here is how to tell whether your batteries are the problem, how to choose the right replacements and how to swap them safely.

Signs your UPS batteries are worn out

• The PC switches off the moment the power cuts, even though the UPS is on.
• The UPS beeps constantly or shows a battery warning light.
• Backup time is much shorter than it used to be.
• The UPS fails its self-test, if it has a test button.

The sealed lead-acid batteries inside most home UPS units usually last a few years, depending on heat and how often the power goes out. If your UPS is a few years old and shows these signs, new batteries are very likely the fix.

Reading a battery label, in plain words

A UPS battery label shows two numbers that matter:

• V (volts): the battery's voltage, usually 12 V. This must match exactly.
• Ah (amp-hours): how much energy it stores. Mine were 9 Ah. Matching the old rating is the safe choice, because the UPS charger is designed for it.

Also check the physical size and the terminal type, the metal tabs where the wires connect. The new battery has to fit in the same space and use the same connectors.

Many UPS units use two batteries connected together. In mine, a short link wire joins them so the UPS gets 24 V from two 12 V batteries. You replace both at the same time, never just one, because a new battery paired with an old one wears out quickly.

Safety tip: Unplug the UPS from the wall and switch it off before opening it. The batteries can still deliver a strong current, so do not let the terminals touch metal tools, and never connect positive and negative directly.

What you need

• A long Phillips screwdriver, at least 5 inches (13 cm), to reach the deep screws.
• A flathead screwdriver to open the front panel.
• Replacement batteries with exactly the same rating as the old ones. Mine were two 12 V 9 Ah sealed lead-acid batteries.
• Your phone, to take a photo of the wiring.

Steps

1. Remove the small screw that holds the front panel, then the larger case screws.
2. Gently pry off the front panel with a flathead screwdriver. Do not force the clips.
3. Photograph how the batteries and wires are connected before you touch anything.
4. Disconnect the link between the batteries, then the main wires, and lift out the old batteries.
5. Place the new batteries in the same position.
6. Reconnect exactly as before: red to positive (+), black to negative (−), and the link between the two batteries.
7. If a connector feels loose, gently squeeze it with pliers or a flathead until it grips firmly.
8. Reassemble the case and plug the UPS in.

Test it

Let the new batteries charge for a few hours, ideally overnight. Then save your work, keep the PC running and unplug the UPS from the wall. Time how long it lasts. Mine ran my PC for about 28 minutes.

Backup time depends on how much power your PC uses. A gaming or editing PC under load drains the battery much faster than an idle PC. The goal of a home UPS is usually a few minutes to save your work and shut down safely, not hours of use.

Make the new batteries last

• Keep the UPS somewhere cool and ventilated. Heat shortens battery life more than anything else.
• Do not plug printers or heaters into the battery outlets. They draw a lot of power.
• Run the self-test every few months, if your UPS has one.

Recycle the old batteries at an electronics or battery collection point. Lead-acid batteries contain lead and acid, so never throw them in household waste.

The short version

If your UPS no longer backs up your PC, the batteries are the usual cause. Buy replacements with the same voltage, amp-hour rating, size and terminals, replace them all at once, connect + to + and − to − exactly as before, charge them and test the backup time.