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Save your work and shut down connected equipment. Turn the UPS off, then unplug it from the wall outlet. If the battery is hot, hissing, leaking, or the room smells of rotten eggs, leave the room, open windows, and keep flames and sparks away. If there is smoke or fire, leave and call 911.
The plastic case of a sealed lead-acid battery is meant to stay flat. When the sides bulge, the battery door will not close, or the battery is wedged so tightly that it cannot slide out, something inside has generated more heat and gas than the battery is built to handle. The rest of this page explains why, how worried to be, and what to do once the immediate risk is under control.
What swelling means inside a VRLA battery
A valve-regulated battery is built to recombine the oxygen produced during charging back into water, so very little gas escapes. A one-way valve on each cell opens at a modest pressure as a safety relief. Swelling means gas or heat has been produced faster than recombination and venting can deal with, and the softened, pressurized plastic has deformed.
Three mechanisms usually drive it:
- Overcharging. Excess charge voltage drives gas production. If the UPS charger holds the battery well above its float voltage (about 13.5 to 13.8 V per 12 V block at 77 °F, or 25 °C), the battery heats and gasses continuously.
- Heat. High ambient temperature raises the current a battery draws on float, which raises its temperature further. Battery makers' guidance is that life roughly halves for every 8 to 10 °C (15 to 18 °F) above 25 °C, and swelling is the extreme end of the same process.
- Thermal runaway. A positive feedback loop: heat raises float current, more current makes more heat. Aging cells with dried-out separators or shorted plates are particularly prone. It can take a battery from warm to dangerously hot over hours or days.
How dangerous is it?
Honestly: usually not an emergency once the UPS is unplugged, and potentially serious if it is not. The hazards scale with what the battery is still doing.
| Condition | Hazard | Response |
|---|---|---|
| Slight bulge, cool to the touch, UPS unplugged | Low | Remove when convenient with gloves and glasses; recycle |
| Clear bulge, warm, UPS still charging | Moderate: ongoing gassing and heat | Turn off and unplug now; ventilate; let it cool for several hours |
| Hot to the touch, sulfur smell, hissing | High: venting, possible thermal runaway | Unplug if safe to reach; leave the room; ventilate; do not handle until cool |
| Cracked case, liquid or white crust | Acid exposure | Gloves and eye protection; neutralize spills with baking soda; recycle |
| Smoke, flame, melting plastic | Fire | Leave and call 911 |
Immediate steps
- Power down and unplug. Turning the UPS off with its button may not stop charging on every model, because many units keep charging while plugged in. Unplugging from the wall is what removes the charger from the equation.
- Do not keep using it, even "just until the replacement arrives." The battery may still be on its way to failure, and runtime is likely poor anyway.
- Ventilate. Open a window or run a fan in the room. Hydrogen disperses quickly in moving air.
- Let it cool. Leave a hot battery alone for several hours before handling it.
- Do not puncture, drill or squeeze the case to "let the pressure out." That releases acid and can create sparks.
- Remove it only if it slides out freely. Wear gloves and glasses, disconnect the terminals (negative first), and keep the battery upright.
- If it is stuck, do not force it. Prying a swollen battery can crack the case or bend the bay into the terminals. Leave it in the UPS and recycle the whole unit, or take it to a service center.
- Photograph the battery, label and UPS serial number for any warranty claim.
Stuck battery: why forcing it is a bad idea
A swollen battery wedged in a metal bay is under stress already. Levering it with a screwdriver can short the screwdriver across the terminals or pierce the softened case. The UPS chassis is not worth the risk; a whole unit can go to an e-waste recycler with the battery inside. See UPS end of life and disposal.
Finding the cause
The point of a diagnosis is simple: deciding whether the UPS can safely charge a new battery. Use the age of the battery and the environment as your main clues.
| Likely cause | Diagnostic hints | Is the UPS reusable? |
|---|---|---|
| End-of-life failure | Battery 4+ years old; runtime had been declining; normal room temperature | Usually yes, with a new battery |
| Hot location | UPS in a closet, cabinet, attic, or next to a heat source; above about 86 °F (30 °C) for long periods | Usually yes, after moving it somewhere cooler |
| Poor ventilation around the UPS | Vents blocked by carpet, wall or other gear; case hot to touch | Usually yes, after fixing airflow |
| Charger fault (overvoltage) | Battery under 2 to 3 years old; normal room; previous battery also died early; UPS case warm on float | Not until serviced |
| Wrong battery | Recently replaced with different chemistry (gel, lithium) or a wrong configuration | Yes, with the correct battery |
| One shorted cell in a string | One block bulged much more than its neighbors; that block reads about 2 V low | Usually yes, with all blocks replaced |
Our rule of thumb: age tells you whose fault it was
A battery that swells after 4 or more years in a normal room most likely failed on its own. One that swells in under 2 years at room temperature points at the UPS, and fitting a fresh battery to a faulty charger just repeats the problem, sometimes faster. Two swollen batteries in a row from the same unit is a strong signal to retire the UPS.
Checking the charger voltage (only if you are comfortable)
If you have a multimeter and the battery terminals or connector are reachable without opening the main UPS case, you can check float voltage with a new, healthy battery installed and the UPS plugged in for several hours. Do not test with the swollen battery.
| String | Typical float range (V) | Suspicious if sustained above (V) |
|---|---|---|
| 1 block, 12 V | 13.5 to 13.8 | about 14.4 |
| 2 blocks, 24 V | 27.0 to 27.6 | about 28.8 |
| 4 blocks, 48 V | 54.0 to 55.2 | about 57.6 |
The "suspicious" column is 2.4 V per cell, the region where many makers place cyclic or boost charging, not continuous float. Some UPS chargers briefly run higher during bulk charging after an outage, so measure after several hours of stable mains. A reading that stays well above the float range on a fully charged battery suggests a charger fault.
Do not open the main UPS case to reach test points. The inverter, capacitors and mains wiring inside can hold dangerous voltages even when unplugged. If you cannot measure from the battery bay, have the unit checked by the manufacturer or a service shop, or replace it.
Preventing a repeat
- Keep it cool. Aim for a room near 68 to 77 °F (20 to 25 °C), with clearance around vents. Placement advice is in where to place a UPS.
- Replace on schedule. Aged batteries are the ones that run away. See when to replace a UPS battery.
- Use the correct battery. Same voltage, same chemistry, all blocks in a string replaced together.
- Touch-check once in a while. On float, a healthy battery and the UPS case should be roughly room temperature. A warm battery bay is an early warning.
If you noticed heat or a smell before seeing the swelling, the troubleshooting page on UPS hot or burning smell covers other sources such as overloaded outlets and failing components.
Disposing of a swollen battery
A swollen lead-acid battery is still a lead-acid battery for recycling purposes. Once it is cool, tape both terminals, put it upright in a plastic bin or a heavy plastic bag inside a box (in case it leaks), and take it to a lead-acid collection point. Mention that it is swollen if you hand it to staff. Never put it in household trash or a curbside bin. Options and details are in UPS battery recycling.
Frequently asked questions
Can a swollen UPS battery explode?
It is uncommon, but the risk is real. An overcharged lead-acid battery can release hydrogen, which is flammable, and a spark nearby could ignite it. A battery in thermal runaway can also crack and leak hot acid. The practical risk is reduced greatly by unplugging the UPS, which stops the charging that drives the problem, and ventilating the room.
Is it safe to keep using a UPS with a slightly swollen battery?
No. Even mild swelling shows the battery has been through abnormal heat or pressure, and the condition that caused it, whether a charger fault, heat or a failing cell, is usually still present. A swollen battery also delivers poor runtime. Replace it, and find out why it happened before installing the new one.
Will the UPS warranty cover a swollen battery?
Sometimes. Batteries are often covered for a shorter period than the electronics, commonly one to two years, and coverage depends on the maker and on how the UPS was installed and used. Photograph the battery and the UPS label before disposal, then contact the manufacturer with the serial number and purchase date.
Why is my lithium UPS battery swelling?
Lithium pouch and prismatic cells swell from gas generated by electrolyte breakdown, usually due to overcharge, physical damage, heat, or age. A swollen lithium pack is a higher fire risk than swollen lead-acid. Power down, move it away from flammables if it can be done safely, do not charge it again, and contact the maker or a hazardous waste facility.
Sources and further reading
- IEEE Std 1188, Recommended Practice for Maintenance, Testing, and Replacement of VRLA Batteries for Stationary Applications (thermal runaway guidance)
- IEEE Std 1184, Guide for Batteries for Uninterruptible Power Supply Systems
- Battery manufacturer safety data sheets (SDS) for sealed lead-acid VRLA batteries
- US EPA: Universal Waste