What the crust is
Batteries vent a little acid vapor and hydrogen gas around the posts, especially when they are hot or charging. That vapor reacts with the metal of the terminals and clamps and leaves a crust. White or gray powder is mostly from the lead and acid. Blue-green points to the copper in the cable end corroding as well. Heat speeds all of it up, which is one reason it is common in Texas.
A little crust at the posts after a few years is ordinary wear. A lot of it, or fresh crust on a battery that was clean a month ago, can mean the battery is venting harder than it should, from age or from overcharging.
Why it matters
Corrosion conducts poorly. A heavy crust can leave a car clicking or cranking slowly even with a good battery, or make the lights flicker. Corrosion that returns quickly right at the base of a post can also mean the seal is leaking.
It is worth cleaning even while the car still starts. Corrosion adds resistance, and a charging system that has to push through it can leave the battery undercharged without anyone noticing.
How to clean it
Take your time, and treat the crust as acid residue:
- Put on safety glasses and gloves. Turn the car off and keep sparks and flames away from the battery.
- Loosen and remove the black negative cable first, then the red positive one. Never let a wrench touch the positive terminal and the car's body at the same time.
- Stir about a tablespoon of baking soda into a cup of water. Brush it onto the crust with an old toothbrush or a terminal brush. It fizzes as it neutralizes the acid.
- Scrub the posts and the insides of the clamps until bare metal shows. Wipe them with a damp rag, keep water from running around the engine bay, and dry everything well.
- Reconnect the positive cable first, then the negative. Snug is enough. Lead posts are soft, and overtightening damages them.
- Finish with a thin coat of dielectric grease or a terminal protector to slow it coming back.
Before you disconnect anything
Some cars lose radio presets, window settings or learned idle settings when the battery is disconnected, and a few need to relearn things afterward. Check the manual first, especially on a newer car with start-stop or one that needs its battery registered.
Keep a rag handy, too. If any crust or baking soda mixture lands on the paint, wipe it off right away, since acid dulls the finish.
When cleaning isn't enough
Don't expect clean terminals to save a worn-out battery. Replace the battery if the case is swollen, cracked or wet, if there is a rotten-egg smell, if the cable ends are green under the insulation, or if the crust is back within weeks. A leaking battery shouldn't be jumped.
If the car won't start and you aren't sure whether it is the battery, a jump start is $49–$75, plus the $19 dispatch fee, charged once. A battery replacement is $239–$255 for eligible vehicles and batteries, including the installation.
Common questions
Treat it as acid residue. Don't touch it with bare hands or rub your eyes afterward, and rinse any skin it touches with plenty of water.
Not by itself. Cleaning can bring a car back if the corrosion was the whole problem. If the car is still slow to start, or the battery is old, the battery is the next suspect.
Don't. Disconnect the negative cable first. A metal tool that bridges a battery terminal and the car's body makes sparks, and the battery gives off a flammable gas.