Swollen, hot, smelling or wet e-bike battery: what to do right now
BatteryAny e-bike
A lithium pack that is bulging, hot at rest, hissing, smelling sweet or chemical, or has been under water is a fire risk, not a repair job. This flow is in the order you must act: isolate, move outdoors, watch, then decide where it goes.
Warning: Do not charge, ride, open, puncture, or put in water a battery that shows any of these signs; thermal runaway can start hours after the trigger and cannot be stopped with a household extinguisher.
What you need
- a non-flammable outdoor spot at least 3 metres from buildings, cars and anything that burns (concrete, paving, bare earth)
- gloves and, if the pack is hot, a metal bucket or a metal tray with sand
- a phone to call the local emergency number if it starts smoking
- the bike shop's or the manufacturer's phone number for the follow-up
Step by step
1. Is it actively smoking, hissing or on fire?
Look and listen from a distance. White or grey smoke, a hiss like a punctured tyre, popping sounds or a sweet solvent smell mean a cell has vented.
If OK: No smoke, no hiss, no flames: go to step 2.
If not: Smoking or hissing: get everyone out, leave the battery where it is if it is indoors and you cannot carry it out in seconds, close the door, call the fire service. Do not try to put it out with water or a home extinguisher; only a very large amount of water cools a lithium fire and the gases are toxic. Tell the fire service it is a lithium e-bike battery.
Look and listen from a distance. White or grey smoke, a hiss like a punctured tyre, popping sounds or a sweet solvent smell mean a cell has vented.
If OK: No smoke, no hiss, no flames: go to step 2.
If not: Smoking or hissing: get everyone out, leave the battery where it is if it is indoors and you cannot carry it out in seconds, close the door, call the fire service. Do not try to put it out with water or a home extinguisher; only a very large amount of water cools a lithium fire and the gases are toxic. Tell the fire service it is a lithium e-bike battery.
2. Isolate it
Unplug the charger from the wall first, then from the battery. Turn the key off. If the pack is removable and cool enough to touch with gloves, remove it from the bike. If it is integrated and the frame is hot, leave the bike alone and move the bike as a whole if you can do so safely.
If OK: Pack isolated and unplugged: go to step 3.
If not: Pack too hot to handle, or stuck: move people away, open windows, and call the non-emergency fire service number for advice; do not wait indoors with it.
Unplug the charger from the wall first, then from the battery. Turn the key off. If the pack is removable and cool enough to touch with gloves, remove it from the bike. If it is integrated and the frame is hot, leave the bike alone and move the bike as a whole if you can do so safely.
If OK: Pack isolated and unplugged: go to step 3.
If not: Pack too hot to handle, or stuck: move people away, open windows, and call the non-emergency fire service number for advice; do not wait indoors with it.
3. Move it outdoors
Carry it, in gloves, holding it away from your face, to the outdoor spot. Put it on concrete or in a metal bucket with sand, away from walls, cars, bins and dry plants. Not in a shed, not in a garage, not in a car boot.
If OK: It is outside on a non-flammable surface: go to step 4.
If not: You have no safe outdoor spot: a balcony with a metal tray is second best; a bath or sink is not, because a venting pack does not need air to burn and water makes the electrolyte reaction worse.
Carry it, in gloves, holding it away from your face, to the outdoor spot. Put it on concrete or in a metal bucket with sand, away from walls, cars, bins and dry plants. Not in a shed, not in a garage, not in a car boot.
If OK: It is outside on a non-flammable surface: go to step 4.
If not: You have no safe outdoor spot: a balcony with a metal tray is second best; a bath or sink is not, because a venting pack does not need air to burn and water makes the electrolyte reaction worse.
4. Watch it for 24 hours
Check it every hour for the first 6 hours, then a few times over the next day. Look for heat, bulging that grows, discolouration, smell. Photograph it each time; the photos support a warranty or insurance claim.
If OK: Cool and unchanged after 24 hours: the pack is stable enough to transport. Go to step 5.
If not: Heating up, growing, or venting: stay away and call the fire service; do not try to move it again.
Check it every hour for the first 6 hours, then a few times over the next day. Look for heat, bulging that grows, discolouration, smell. Photograph it each time; the photos support a warranty or insurance claim.
If OK: Cool and unchanged after 24 hours: the pack is stable enough to transport. Go to step 5.
If not: Heating up, growing, or venting: stay away and call the fire service; do not try to move it again.
5. Classify what you saw
Swollen case, seams opened, a pack that rocks on a flat surface: cells have gassed, the pack is scrap. Hot at rest (warm is normal for an hour after a fast charge or a hard climb): BMS or a shorting cell, scrap. Sweet, chemical or fishy smell: electrolyte leak, scrap. Wet inside (dropped in water, flooded cellar, pressure-washed): may look fine, but corrosion on the cell strips and the BMS starts within days, treat as scrap. Wet outside only, from rain, with a dry charge port and no other signs: not a hazard, dry it and continue with the normal battery flow.
If OK: Only external rain water, no heat, no swelling, no smell: dry it for 24 hours, then use the generic battery-dead flow.
If not: Any of the four scrap signs: the pack is not to be used or charged again, regardless of how many LEDs it shows. Go to step 6.
Swollen case, seams opened, a pack that rocks on a flat surface: cells have gassed, the pack is scrap. Hot at rest (warm is normal for an hour after a fast charge or a hard climb): BMS or a shorting cell, scrap. Sweet, chemical or fishy smell: electrolyte leak, scrap. Wet inside (dropped in water, flooded cellar, pressure-washed): may look fine, but corrosion on the cell strips and the BMS starts within days, treat as scrap. Wet outside only, from rain, with a dry charge port and no other signs: not a hazard, dry it and continue with the normal battery flow.
If OK: Only external rain water, no heat, no swelling, no smell: dry it for 24 hours, then use the generic battery-dead flow.
If not: Any of the four scrap signs: the pack is not to be used or charged again, regardless of how many LEDs it shows. Go to step 6.
6. Where it goes
Call the bike shop or the brand's service line before you bring it: most shops will only accept a damaged pack with notice, in a fire-proof bag or a sand-filled metal bin. Alternatives: the municipal hazardous waste point (tell them it is a damaged lithium battery), or a battery take-back scheme (in the EU the seller must take it back). Never post it, never put it in household or normal battery recycling, never leave it in a car.
If OK: A shop or waste point has agreed to take it: transport it outdoors-stored, in a metal container with sand, in an open vehicle bed or a boot with the lid cracked, on the day.
If not: Nobody will take it today: keep it outdoors on the non-flammable spot, covered against rain with something that does not trap heat (a metal tray, not a plastic bag), until you have a taker.
Call the bike shop or the brand's service line before you bring it: most shops will only accept a damaged pack with notice, in a fire-proof bag or a sand-filled metal bin. Alternatives: the municipal hazardous waste point (tell them it is a damaged lithium battery), or a battery take-back scheme (in the EU the seller must take it back). Never post it, never put it in household or normal battery recycling, never leave it in a car.
If OK: A shop or waste point has agreed to take it: transport it outdoors-stored, in a metal container with sand, in an open vehicle bed or a boot with the lid cracked, on the day.
If not: Nobody will take it today: keep it outdoors on the non-flammable spot, covered against rain with something that does not trap heat (a metal tray, not a plastic bag), until you have a taker.
7. Warranty and cause
A pack that swelled or overheated within the warranty period without abuse is a manufacturer claim; the photos, the charger used and the purchase proof matter. If it happened after a crash, a drop, a non-original charger, storage in a hot car or charging a frozen pack, the cause was external and the claim is unlikely to succeed. Write down what happened while it is fresh.
If OK: Within warranty and no abuse: contact the seller in writing, attach the photos, ask for a replacement pack and for collection of the damaged one.
If not: Out of warranty or an external cause: replacement at your cost. Do not buy a used pack of unknown history to replace it.
A pack that swelled or overheated within the warranty period without abuse is a manufacturer claim; the photos, the charger used and the purchase proof matter. If it happened after a crash, a drop, a non-original charger, storage in a hot car or charging a frozen pack, the cause was external and the claim is unlikely to succeed. Write down what happened while it is fresh.
If OK: Within warranty and no abuse: contact the seller in writing, attach the photos, ask for a replacement pack and for collection of the damaged one.
If not: Out of warranty or an external cause: replacement at your cost. Do not buy a used pack of unknown history to replace it.
8. Check the bike and the charger
Before fitting a new pack: inspect the bike's contact plate for melt marks, the harness for scorching, and the charger for a burnt smell or a case that has warped. A short on the bike or an out-of-spec charger can destroy a new pack the same way.
If OK: Bike contacts and charger look and smell normal: fit the new pack and charge it the first time while you are present and awake.
If not: Scorch marks or a charger that smells: the bike or charger caused it. Shop inspection before any new battery goes on.
Before fitting a new pack: inspect the bike's contact plate for melt marks, the harness for scorching, and the charger for a burnt smell or a case that has warped. A short on the bike or an out-of-spec charger can destroy a new pack the same way.
If OK: Bike contacts and charger look and smell normal: fit the new pack and charge it the first time while you are present and awake.
If not: Scorch marks or a charger that smells: the bike or charger caused it. Shop inspection before any new battery goes on.
What you now know for sure
- Pack scrap: cell venting or swelling: Bulging, opened seams, a sweet or chemical smell, or heat at rest, whether or not the LEDs still work. → Outdoor storage, then hazardous waste or shop take-back in a metal container with sand. Warranty claim with photos if within the period and no abuse.
- Pack scrap: water inside: Submerged, flooded, or pressure-washed into the case; may look and work fine for days. → Same disposal route. Do not keep it as a spare; corrosion makes it a delayed fire risk.
- Not a hazard: rain water only: Wet on the outside, dry charge port and connector, no heat, no swelling, no smell. → Dry 24 hours at room temperature and continue with the normal battery flow.
- Bike or charger caused it: Scorch marks on the contact plate or harness, or a charger that has warped or smells. → Shop inspection of the bike; replace the charger. Do not connect a new pack until both are cleared.
- Warm but normal: Warm, not hot, for up to an hour after fast charging or a long climb, no swelling or smell, cools down by itself. → Nothing to do. If it is hot enough that you cannot keep your hand on it, treat it as the first verdict.
Manuals and video
Videos found on YouTube and checked automatically; they are other people's work, not ours.
- How to check your E-bike battery and signs that your battery is damaged. Bicycling
- Part1 | Don't Risk It! Why Fixing That Dead E-bike Battery Might Be a Bad Idea#shorts Febatt -Oscar
Frequently asked
Can I put a burning e-bike battery out with water?
Only with a very large, continuous amount of water, which is the fire service's job. A bucket or a garden hose does not stop thermal runaway and the steam and gases are toxic. Get out, close the door, call the fire service, tell them it is a lithium battery.
My battery is slightly bulged but works fine. Can I keep using it until it dies?
No. Bulging means at least one cell has gassed. The pack can ignite while charging, while riding or while sitting on a shelf, weeks later. Take it out of service today and follow the disposal steps.
The battery fell in a canal. It dried and shows all LEDs. Is it fine?
No. Water inside the case starts corrosion on the nickel strips and the BMS, which later causes a short. The LEDs tell you nothing about that. Treat it as scrap and claim on insurance if you have it.
Still stuck? Take a photo of the display and show it to a local e-bike workshop. Bring the bike's system name (e.g. Bosch Performance Line CX) and the exact code; that saves diagnostic time and money.