Car batteries leak acid primarily due to physical damage to the plastic case, overcharging which causes excessive heat and pressure buildup, or simply from old age where internal degradation and corrosion compromise the battery's seals. The "acid" is actually a mixture of sulfuric acid and water, and a leak is a serious issue that requires immediate, safe handling to prevent damage to your vehicle and harm to yourself.
Overcharging is a common culprit. When a car's voltage regulator fails, it can send a continuous high charge to the battery. This leads to overheating, causing the electrolyte solution to boil. The resulting gas and pressure can warp the battery case and force acidic liquid out through the vents or weak spots. Using an incompatible or faulty battery charger can have the same effect.
Physical damage from a loose hold-down clamp, a minor impact, or even a sharp object can crack the polypropylene case. Even a hairline crack is enough for the highly corrosive electrolyte to seep out. This often leads to visible corrosion on the battery tray and nearby cables.
As a battery ages, internal factors contribute to leaks. The constant charge/discharge cycles lead to sulfation (the buildup of lead sulfate crystals) and plate growth, which can stress the internal structure. Simultaneously, the lead posts and seals can corrode from the inside out, creating pathways for acid to escape.
| Common Cause | How it Leads to a Leak | Typical Signs |
|---|
| Overcharging | Excessive heat boils electrolyte, creating gas pressure that vents acid. | Battery feels hot to touch, swollen case, rotten egg smell. |
| Physical Impact | Directly cracks or punctures the battery case. | Visible crack or bulge, acid pooling under the battery. |
| Internal Corrosion | Degrades seals around terminal posts, allowing acid to wick up. | Heavy blue/green corrosion on terminals and cables. |
| Extreme Temperature Swings | Causes plastic case to repeatedly expand/contract, weakening it. | Leak may be seasonal, occurring after very hot or cold spells. |
| Old Age / Wear & Tear | General internal degradation and plate growth stress the case seals. | Battery is over 3-5 years old, has difficulty holding a charge. |
If you suspect a leak, put on gloves and safety glasses. Neutralize any spilled acid with a paste of baking soda and water, then have the battery professionally tested and replaced. A leaking battery is a safety hazard and cannot be reliably repaired.
