What a leaking car battery looks like
Most car batteries contain sulfuric acid, which is corrosive. Signs of a leak or related damage include:
- Wet spots or residue on top of or around the battery, or in the tray beneath it.
- A strong rotten-egg (sulfur) smell near the battery.
- A swollen, bulging, or cracked case.
- Heavy corrosion that seems to be spreading to nearby cables, the tray, or metal parts.
If you notice any of these, treat the battery with care and have it inspected.
Terminal corrosion vs. a case leak
Powdery white, blue, or green buildup around the posts is common and is not always a sign that the battery is leaking from the case. It often comes from small amounts of gas or acid vapor escaping near the terminals, or from a loose connection. It can usually be cleaned, as described in our car battery maintenance tips.
Liquid coming from the case, cracks, or bulges is more serious. That is generally a reason to replace the battery.
Common causes of a leaking battery
- Overcharging. A faulty charger or alternator can cause the battery to overheat and vent.
- Age. Older batteries are more prone to internal damage and case problems.
- Physical damage. A crack from impact, a loose battery that vibrates, or a damaged case.
- Extreme heat. High temperatures can stress the battery and cause venting.
- Freezing. A badly discharged battery can freeze in severe cold, which can crack the case.
- Overfilling of serviceable batteries, or a manufacturing defect.
Heat and cold both play a part in battery wear; see why car batteries die in cold weather and how long car batteries last.
Risks
Battery acid can burn skin and eyes, damage clothing, and corrode metal parts under the hood. Batteries can also release flammable hydrogen gas, so avoid sparks and open flames near them. A battery that is swollen or very hot can be hazardous.
What to do if your battery is leaking
- Stay cautious. Wear eye protection and chemical-resistant gloves if you need to handle anything. Avoid touching the acid or breathing the fumes.
- Do not jump-start or charge a battery that is visibly cracked, swollen, leaking, or frozen.
- Keep flames and sparks away.
- Have it replaced. A battery with a leaking case should generally be replaced. If you are not comfortable removing it, a shop or roadside service can do it.
- Clean up carefully. Baking soda mixed with water is commonly used to neutralize acid residue on metal parts. Wear protection and do not let the mixture get into the battery. If acid gets on skin or in your eyes, flush with plenty of water and seek medical advice.
- Dispose of it properly. Lead-acid batteries are recyclable, and most retailers that sell batteries will accept the old one.
Also have the charging system checked, so a faulty alternator does not damage the new battery.
Is a battery ruined if it gets wet?
A sealed car battery is generally designed to cope with some rain or splashing, so being wet on the outside does not automatically ruin it. Moisture on the terminals can speed corrosion, though, so dry and clean it. If you cannot tell whether the wetness is water or electrolyte, assume it could be acid and handle it carefully.
Preventing leaks
Keep the battery properly secured, replace it as it ages, avoid overcharging, and inspect it periodically. If other warning signs appear, such as slow starts or dimming lights, see the signs your car battery is dying.