Lithium battery swelling: causes, warning signs, and what to do
A swollen lithium battery is a pack that has built up gas pressure inside its own case. That pressure pushes the shell out, so the housing bulges, the label wrinkles, or the pack stops dropping cleanly into the tool.
The short answer is simple. Stop using it and stop charging it. Swelling means the cells inside have started breaking down, and no charger, reset trick, or cooling method will reverse that.
I've pulled a lot of these out of guys' truck boxes over the years, and the story is almost always the same. A pack that cooked in a hot van all summer, or one that lived on a charger for weeks after it was already full.
What is actually happening inside the pack
A lithium-ion cell holds a stack of layers pressed tightly together. When that stack stays intact, the cell keeps its shape. When it breaks down, the electrolyte starts to decompose and gives off gas. There's nowhere for that gas to go, so the cell case puffs.
A pack built from cylindrical cells, the 18650 type most people have seen, tends to show swelling as a bulged case and a pack that goes out of square. A pack built from flat pouch cells puffs more visibly, and the whole wrapper can look like an inflated bag. Either way, the gas is the message.
A cell that has vented is a cell you can't trust, even if it still reads 20 volts on a meter. Voltage tells you there's charge in there. It tells you nothing about whether the layers inside are still where they should be.
The causes I see most often
Heat
Heat is the top cause by a wide margin.
Leave a pack on a dashboard, in a closed van, or in direct sun on a roof and the internal temperature climbs well past the range the cells were designed for. Industry specs generally put the safe charge window for most 18650 and pouch cells at roughly 0°C to 45°C, with a wider window on discharge. Pushed past that, the separator layers and seals take damage that doesn't heal. Repeated heat stress stacks up.
How you store packs matters as much as how you charge them. If your tools live in a truck year round, read up on how temperature swings affect a lithium pack before the next heat wave.
Overcharging and sitting at full charge
A healthy charger cuts the current once the pack hits its target voltage. Cheap or failing chargers don't always catch that cutoff. When current keeps flowing into a full pack, the extra energy drives side reactions inside the cell, and those reactions produce gas.
The same thing happens with a pack that lives on the charger for months. Full charge plus heat is the worst mix you can leave in a van.
Over-discharge and deep drain
Running a pack until the tool dies, then running it again, then leaving it dead in the truck is harder on the cells than most people realize. Deep drain damages the anode structure and the current collectors. Charging it back up afterward is when the damage often shows as gas.
If a pack has been sitting dead for months, charge it somewhere you can watch it, and pull the charger if the case gets warm or starts to puff.
Physical damage
Dropping a pack off a ladder, backing over it, or cracking the case all count. An impact can crush a separator and create an internal short. The short makes heat, the heat makes gas, and the pack puffs. If a pack took a hard hit, retire it even if it looks fine.
Age and cells that drift apart
Cells age at different rates, even inside the same pack. In a pack with parallel cells, the weak one gets dragged along while the stronger ones push current through it. That imbalance builds heat over time. Most name-brand cells are rated for roughly 300 to 500 full cycles before capacity drops off hard, and that's a published industry figure, not something I measured on your pack. Past that range, swelling risk climbs.
Counterfeit packs and overstated capacity ratings
One more I run into a lot. A cheap pack that claims capacity the case can't physically hold gets worked harder than it should on every charge. If a replacement pack claims more than the housing could ever contain, treat that as a red flag, not a bargain. Third-party packs are fine when the numbers are honest, so compare the stated amp-hours against the physical size before you buy.
Telling swelling apart from normal case flex
Use your eyes and your hands.
A healthy pack has a flat, straight case. Set it on a bench and it sits flush. If you can see daylight under the middle, or the label is stretched and wrinkled, that's swelling. If a second pack of the same model feels different in your hand, that's a sign too.
Some flex is normal. A light click as a pack drops into the tool is normal. A pack that needs a shove, or that scrapes going in, is not.
Do not squeeze a swollen pack to test it. That's how you turn a bad cell into a venting cell.
How fast does a bulge turn into a real problem?
Honestly, the spec sheets don't say, and I'm not going to guess at a number. What I can tell you is that swelling doesn't cure itself. A pack that's slightly bulged today will not be flat in six months.
I'd treat any visible bulge as a deadline, not a warning light. You have time to dispose of it properly. You don't have time to keep running it through a season.
What to do when you find one
Pull it out of service. Right away.
- Stop charging it, and stop running it in tools.
- Move it away from anything that burns. A bare concrete floor or an empty metal pan outside works.
- Keep it out of the regular trash and out of an indoor recycling bin.
- Take it to a household hazardous waste site, or a retailer running a battery take-back program.
If you need to park it somewhere for a few days, keep it away from the house and out of the sun.
On shipping, one thing to know. A swollen pack that has visibly changed shape isn't something you can legally mail as an ordinary lithium battery. The UN38.3 transport testing standard and the rules built on top of it exist for exactly this situation, and an inspector who sees a puffed case will reject the package.
Can a swollen battery be fixed?
No. Nothing puts a puffed cell back into a safe state.
Some videos show guys poking a hole to release the gas. That's a good way to start a fire. Others show freezing a pack to shrink it back down. It might look better for a day. The cell is still damaged, and it will puff again. Freezing just hides a problem you need to see.
What you can do is replace the pack. For older or odd platforms, the specialty battery lineup covers packs the big-box stores quietly stopped stocking, and that's usually a lot cheaper than replacing the tool.
One more angle worth checking. If you run one brand's pack in another brand's tool through an adapter, a poorly seated adapter adds resistance, and resistance makes heat. Cross-brand adapters that don't lock up tight are a known source of hot packs, and a hot pack is a swelling pack in waiting.
Keeping the next pack out of trouble
- Charge on a hard, open surface. Not on a pile of rags, not inside a closed case.
- Pull the pack off the charger once it's full. Stop treating overnight charging as the default.
- Store at partial charge, somewhere around 40 to 60 percent, when a pack is going to sit for a while.
- Keep packs out of the sun and out of a parked van in July.
- Retire packs that took a hard hit, even if they still work.
- Rotate your packs so one doesn't carry the whole load.
- Buy replacements with honest capacity ratings and a real return policy.
Buying third-party replacement packs changes none of this. A well-built replacement runs the same chemistry as the original, so the same rules apply to it.
Frequently asked questions
Is a swollen battery still safe to use?
No. Once a pack has puffed, the cells inside have already changed. Running it raises the chance of a vent or a fire. Pull it out of service the day you notice it.
Can I still charge a battery that's starting to swell?
Don't. Charging pushes more energy into cells that are already breaking down, and heat is what makes it worse. A charger that keeps feeding current into a damaged pack can turn a small bulge into a vent.
What causes a lithium battery to swell?
The usual causes are heat, charging past full, deep discharge left to sit, and physical impact. Age plays a part too, since cells drift out of balance as they cycle.
How do I get rid of a swollen battery?
Keep it out of the house and away from anything flammable. Take it to a household hazardous waste facility or a retailer with a battery take-back program. Don't put it in the trash, and don't mail it as a normal package.
Will a swollen battery shrink if I freeze it?
It might look flatter for a short while. The cell damage is still there, and the pack will puff again. Freezing isn't a fix, and it can hide a problem you need to see.