Battery terminal corrosion: what causes it and how to clean it safely

Terminal corrosion is usually a mix of lead sulfate and copper compounds that form when acid vapor or moisture meets the posts and clamps. It adds resistance, which can cause slow cranking and poor charging even when the battery itself is fine. You can clean light corrosion with a baking soda and water mix and a stiff brush, with the negative cable off first. Heavy or fast-returning corrosion can mean the battery is venting, leaking or being overcharged.

What the colors mean

• White or gray on the negative side or both sides is common. Often sulfate.
• Blue or green usually means copper in the clamps or cables is corroding.
• Wet, fresh-looking crust or acid on the case means something is leaking. Treat it with care.

Corrosion on the positive post can point to overcharging. On the negative post it can point to undercharging. That's a loose rule, not a diagnosis, but it's a reason to check the charging system too.

Why it's worse near the coast

Salt air carries moisture and chloride. In St. Pete Beach, Clearwater Beach, Apollo Beach, Tarpon Springs and anywhere along the water, salt settles on everything under the hood. It pulls moisture onto the terminals and helps corrosion along. Humidity everywhere in Tampa Bay doesn't help.

Heat adds to it. Hot batteries vent more, and that vapor ends up on the posts.

What corrosion does

It's a poor conductor. A layer of crust between the post and clamp means:

• Harder starts. The starter gets less current.
• Poor charging. The alternator can't put full charge back.
• Electrical glitches. Voltage drops and fluctuates.
• A dead car that's actually fine. We've seen cars that wouldn't start with a perfectly good battery, because of a bad connection.

How to clean it

You'll need gloves, eye protection, baking soda, water, a stiff brush (a terminal brush is ideal), a wrench for the clamps, and a rag.

1. Turn the car off and take the key out.
2. Look first. If the battery is swollen, cracked, leaking or smells like rotten eggs, stop. Don't clean it. Replace it.
3. Disconnect the negative clamp first, then the positive. On some cars, disconnecting the battery resets radio and settings or needs special steps. Check the manual.
4. Mix about a tablespoon of baking soda in a cup of water. Brush it on the posts and clamps. It'll fizz as it neutralizes acid.
5. Scrub until the metal is bright.
6. Rinse lightly with a little clean water, keeping it away from vent holes. Dry everything.
7. Reconnect positive first, then negative. Tighten firmly, but don't crush the posts.
8. Protect the terminals with a thin coat of terminal grease, dielectric grease or anti-corrosion spray.

Wash your hands after. Battery corrosion has lead in it.

When cleaning isn't enough

• Corrosion comes back within weeks. The battery may be leaking or venting from overcharging. Test the charging system.
• Clamps are eaten away or cables are green inside the insulation. The cable or clamp needs replacing.
• The battery still cranks slowly after cleaning. The battery itself may be weak. Signs your battery is dying.

Should you do it yourself?

If the corrosion is light and you're comfortable with a wrench, it's a simple job. If the battery is in a tight spot, if you have a car that needs special steps when disconnected, or if anything looks wet or swollen, let someone else do it.

What we do

On every battery we replace, we clean the clamps, check the cables, and protect the terminals before we close the hood. If a clamp or cable is too far gone, we tell you. If the corrosion looks like a charging problem, we test the alternator too. See battery testing and what happens during a mobile replacement.

Coastal tips

If you live near the water in Tampa Bay, check your battery terminals a few times a year. A light coat of terminal protectant after cleaning helps. If your car has a plastic battery cover, keep it on, because it helps keep salt spray and debris off the terminals.