The overwhelming majority of batteries are safe when used as intended. The risks come from damage, cheap chargers, short circuits and charging in the wrong place. All of them are avoidable.
Warning signs to take seriously
Stop using a battery and isolate it if you notice any of the following. A healthy battery does not change shape.
- A swollen or bulging case, including a phone or laptop whose screen or lid no longer sits flat.
- Heat that continues after charging has finished.
- Hissing, popping or a crackling sound.
- A sweet chemical smell, or visible white crust on the terminals.
- Discolouration, scorching or melted plastic around the battery.
Do not puncture, crush, bend or open a swollen battery. Do not put it in the household bin. Place it away from anything flammable, ideally outside and away from your home, and take it to a household waste recycling centre. If it is hot, smoking or alight, evacuate and call 999.
Short circuits are the quiet danger
A loose 9V battery touching a paperclip, or a coin cell rattling against keys, becomes a heater element. Keep terminals covered with the original packaging or tape, and store batteries in a case rather than a drawer of metal objects.
This applies to spare lithium cells, power tool packs and power bank units too.
Storing batteries sensibly
Store in a cool, dry place at room temperature, away from radiators, direct sun and damp sheds. The fridge is not a good idea: condensation on the terminals causes corrosion and does nothing for capacity.
Keep them out of reach of children, and separate from food. If a battery has leaked, clean the contacts with a cotton bud and vinegar or lemon juice for alkaline residue, wearing gloves, then dispose of the cell. Never put a leaking cell back into a device.
Charging safely, especially bigger packs
Use the charger the device came with, or a reputable equivalent with the correct voltage and current. Unplug when charging finishes rather than leaving a cheap charger running overnight on the kitchen table.
For e-bikes, scooters and power stations, charge on a hard non-flammable surface away from doors, hallways and escape routes, and never while you are asleep or out. That advice exists because those packs contain a lot of energy in one place.
Travelling with batteries
Spare lithium batteries must travel in cabin baggage, not the hold, with terminals protected. Airlines generally allow cells up to 100Wh without asking, and 100 to 160Wh with approval, but limits and rules vary by airline and country.
Check your airline's current rules before flying, and never check a spare power bank or loose cell into the hold.
Smoke alarms and critical devices
Fit the battery type the manufacturer specifies and test monthly. Do not fit a rechargeable cell in an alarm that expects alkaline, do not fit a higher-capacity cell to get longer life, and replace the whole alarm at the end of its stated service life, usually ten years.
The short version
- A swollen battery is a disposal item, never a repair project.
- Tape terminals on loose batteries to prevent short circuits.
- Charge large packs on a hard surface, awake and present.
- Spare lithium batteries fly in the cabin, never the hold.