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Rotten Egg Or Acid Smell From The Battery

A rotten egg or sharp acid smell near the battery can mean it is being overcharged or has an internal fault, which releases gas and can make it swell.

High risk

Have it checked promptly. An overcharged or damaged battery can leak acid and, rarely, burst.

Short answer · Why does my car battery smell?

Lead-acid batteries release hydrogen sulphide when they overheat or are overcharged, which smells of rotten eggs. A faulty alternator regulator can push too much voltage. An old battery with a shorted cell can also overheat. A swollen, hot or leaking battery should be replaced and the charging voltage checked.

On this page

Safe checks you can do

  1. Look at the battery caseCheck for swelling, cracks or wet areas.Do not touch a leaking battery with bare hands; battery acid burns.
  2. Note any warning lightsA battery light or high voltage message points to the charging system.
  3. Ventilate the areaOpen the bonnet and let the gas escape; keep sparks and flames away.

Questions that narrow it down

  1. Is the battery hot?It may be overcharging.
  2. Is the battery old?Internal faults become more likely with age.
  3. Is the exhaust also smelling of eggs?That points to the catalytic converter instead.

Possible causes

Listed without ranking. Use the questions above to see which ones fit your car.

  1. Overcharging alternatorRegulator fault.
  2. Shorted battery cellBattery overheats.
  3. Old or damaged batteryGas and acid leak.
  4. Wrong charger usedExternal overcharging.

Questions about Rotten Egg Or Acid Smell From The Battery

Is the gas dangerous?

Battery gas is flammable. Keep sparks and flames away and ventilate the area.

Should I jump start a swollen battery?

No. Have it replaced.