Car battery smells of rotten eggs, is leaking or swollen: what it means

A car battery that smells of rotten eggs, is swollen, leaking or hot to the touch is damaged and should be replaced. The usual cause is overcharging or heat, which makes the acid inside boil and give off hydrogen and sulphur gases. Do not jump-start or charge it, keep sparks and flames away, and do not touch any liquid or white crust around it.

Safety first

Before anything else:

• Switch the engine off if it is safe to do so, and switch off the lights and accessories.
• Do not smoke, light anything or create sparks near the battery. Hydrogen gas from a battery can explode.
• Do not touch the battery, the acid or any crust on it with bare hands. Battery acid is sulphuric acid and burns skin and eyes.
• Keep the bonnet open to let gas escape, and keep people, especially children, away from the engine bay.
• Do not try to remove a hot or swollen battery yourself. Wait for it to cool and get help.

If acid gets on your skin, rinse it with plenty of clean running water for at least 15 minutes and remove contaminated clothing. If it gets in your eyes, rinse with water continuously and get medical help straight away. If someone feels dizzy or unwell from the fumes, move them to fresh air.

Do not jump-start or charge a damaged battery

Do not jump-start, connect jump leads to, or charge a battery that is swollen, leaking, smoking, boiling, very hot, or smells of rotten eggs.

A damaged battery can be giving off hydrogen gas. The spark when jump leads are connected or removed can ignite it, and the battery can burst and spray acid. Charging it pushes more current into a battery that is already overheating. Have it replaced instead.

What each symptom means

Symptom	What it means	Danger level	What to do now
Rotten egg (sulphur) smell	Acid is boiling and releasing hydrogen sulphide, usually from overcharging or an internal short	High	Engine off, bonnet open, no sparks. Do not drive far. Get it checked.
Swollen or bulging case	Internal heat and gas have built up pressure. The plates may be damaged.	High	Do not jump-start or charge. Have it replaced.
Leaking liquid or wet battery top	Acid escaping through a crack, a vent or a damaged seal	High	Do not touch. Keep it off paint and clothing. Have it replaced.
Battery hot to the touch, hissing or bubbling	The battery is overheating, often from overcharging or a short circuit	Very high	Switch off and move away. Let it cool. Do not touch it.
Smoke from the battery	Severe overheating or a short circuit	Very high	Move away from the car. Call for help.
White, blue or green crust on the terminals	Corrosion from small amounts of gas or acid at the terminals	Low	Not urgent. Clean with gloves on. Check the battery is not leaking.

Rotten egg smell

A smell of rotten eggs or sulphur from under the bonnet usually means the battery is being overcharged or has an internal fault. When too much current goes into a lead-acid battery, the electrolyte heats up and boils, and the gases escape through the vents. The smell itself is a warning that hydrogen, which has no smell, is being released too.

Some drivers notice the smell only after a long drive or with the AC running hard. That still needs checking. A brief smell from a new battery being charged hard after a deep discharge can happen, but a smell that comes back is a fault.

Swollen battery

A battery case that bulges at the sides or top has been overheated. The heat makes the plates warp and the gas builds pressure inside. A swollen battery has lost capacity and can split. It cannot be repaired.

In the UAE, heat alone can contribute: engine bays in summer get far hotter than the air. But swelling usually means heat plus overcharging, or an internal short.

Leaking acid

Liquid on top of the battery, a wet tray, or corrosion on nearby metal means acid is escaping. It can come from a cracked case, a damaged vent, a battery that has been overfilled, or one that has boiled. Acid damages paint, wiring and metal parts around it, so it should not be left.

Do not wipe it up with bare hands or a cloth you then touch. A professional will neutralise the area and dispose of the battery safely.

Hot or boiling battery

A battery that is hot to touch, hissing, or making a bubbling sound is in the most dangerous state. Switch the engine off, move away, and let it cool. Do not open the caps or try to disconnect it while it is hot.

Why the alternator or regulator may be the cause

The alternator charges the battery while the engine runs. A part called the voltage regulator, usually built into the alternator, keeps the charging voltage at a safe level, about 13.7V to 14.7V on most cars. If the regulator fails, the alternator can push 15V or more into the battery. The battery overheats, the acid boils, and you get the smell, swelling or leaking.

That matters because a new battery will not fix it. If the charging system is overcharging, the new battery will be damaged in the same way. So whenever a battery has swollen, boiled or smelled of sulphur, the alternator and regulator should be tested at the same time.

Other causes include:

• An internal short in the battery. One cell fails and the others are overcharged.
• The wrong battery for the car. A conventional battery fitted to a car that needs AGM or EFB, or a battery that has not been registered with the car, can be charged at the wrong voltage.
• An old, heat-damaged battery. Years of UAE summers weaken the plates.
• An incorrect external charger, or a charger left on too long.

Our guide on battery or alternator problems shows how to check the charging voltage.

Can I drive with a swollen or leaking battery?

It is not safe to drive far. If the battery is overcharging, it will keep getting hotter while the engine runs. If you are on the road and notice the smell, find a safe place to stop, switch off and open the bonnet. If the car is at home, leave it parked until the battery is replaced.

Disposal

A damaged battery is hazardous waste. Do not put it in a household bin or leave it in a car park. When we replace a battery, we take the old one away for recycling.

Getting a damaged battery replaced

If your battery is swollen, leaking, hot or smells of rotten eggs, contact us for emergency battery replacement. We come to your car anywhere in Dubai, remove the damaged battery, and test the alternator and starter before fitting a new one, so the cause is found as well as the symptom.