Otto Explains — Battery
Why Is My Car Battery Leaking, and Is It Dangerous?
Short Answer
A leaking car battery is usually losing its electrolyte, which is sulfuric acid, because it is being overcharged, its case is cracked, its seals have aged or it has been damaged by heat. It is dangerous: the acid burns skin and eyes, and a battery can give off flammable gas. Don't touch it with bare hands, keep flames, sparks and cigarettes away, and have it assessed and replaced. If the battery is swollen, very hot, hissing or smoking, stop somewhere safe, switch off the engine and keep everyone away from it.
What is leaking from a car battery?
In a conventional car battery the liquid is an electrolyte of sulfuric acid diluted in water: one battery maker's safety data sheet gives about 10–30% acid by weight. Sulfuric acid is corrosive. It can burn the skin and eyes and irritates the eyes, nose and throat, and a leak can also corrode parts around the battery and give off harmful fumes. Signs of a leak include liquid or dampness on or around the battery, corrosion such as a white, powdery deposit around the terminals, and a swollen or bulging case.
Why does a car battery leak or swell?
Overcharging is a common cause: a faulty alternator or charging system, or an unsuitable charger, can push the battery too hard. Other causes include a cracked case after an impact, poor installation or a manufacturing defect, a battery that is simply old, seals that have degraded, and terminals loosened by vibration. Extreme heat can warp the case and raise the pressure inside, which is worth keeping in mind in an Omani summer. Even a sealed AGM battery, which normally recombines its gases, can vent and swell if it is overcharged or gets too hot. Charging faults that also show up on the dashboard are covered in the article on the battery warning light.
Is a leaking battery dangerous?
Yes, and it should be treated with care. Battery acid can burn, and a lead-acid battery can give off hydrogen, a gas that burns in air across a wide range of concentrations. So don't touch the battery or the residue around it with bare hands, don't lean over it, keep children away, and keep flames, sparks and cigarettes well away from it. Don't try to seal a cracked case with tape or glue: a leaking or cracked battery needs to be replaced.
What should you do if battery acid touches your skin or eyes?
Rinse the skin or eye straight away with plenty of clean running water and keep rinsing, take off any clothing the acid has touched if you can, and get medical help, always if acid has reached the eyes. In an emergency, call 9999.
When should you stop the car and keep away from the battery?
If the battery is swollen, very hot, hissing or smoking, or there is a strong, sharp smell from the engine bay, stop somewhere safe as soon as practicable and switch off the engine. Keep everyone away from the battery and keep ignition sources such as flames, sparks and cigarettes away, because a battery can give off flammable gas. If there is smoke or fire, get everyone out of the car, move well away, and call 9999. Don't touch the battery or try to deal with it yourself: have the car assessed where it is or transported. A severely swollen battery has to be disconnected and replaced by a technician.
What does a technician check?
A battery that is leaking, cracked or swollen is replaced, and the charging system should be tested too, because overcharging is a common cause and a new battery can suffer the same way if the fault remains. AutoTronix Express handles battery replacement, and AutoTronix Lab diagnoses charging and electrical faults. A failing battery can also be the reason a car won't start.
Common Questions
- Is white powder on the battery terminals a sign of a leak? It can be. Corrosion around the terminals is one of the signs of a leaking battery. If it keeps coming back, or you can see liquid, the battery needs expert attention or replacement.
- Can a cracked battery case be repaired with tape or glue? No. A cracked case shouldn't be patched. The battery needs to be replaced.
- Can a sealed or AGM battery leak or swell? Yes. Sealed AGM batteries normally recombine their gases, but overcharging or high temperatures can still make them vent and swell.