The white, blue, or green powder you see on battery terminals is corrosion — a chemical reaction between battery acid vapor, moisture, and the metal terminals. It builds up over time and creates electrical resistance.
Even a small amount of corrosion reduces the battery's effective output. A battery that tests fine on the bench can crank slowly because corrosion is blocking the connection. The fix is simple: clean the terminals.
To clean: disconnect the negative terminal first, then the positive. Wear gloves and eye protection. Mix baking soda with water into a paste, scrub the terminals with a brush, rinse with water, dry thoroughly. Reconnect positive first, then negative. Apply anti-corrosion compound to prevent buildup.
Severe corrosion can indicate a battery problem. If terminals get heavily crusted within a few months of cleaning, the battery may be venting acid due to overcharging or internal damage. Time to test it.
Salt-belt states (Northeast, Midwest) see the most corrosion because road salt mist accelerates the reaction.