1. What corrosion looks like
A white, green or blue crust on the metal contacts, often left by a battery that leaked or sat in a device too long. It blocks the current from flowing.
2. Start with a dry cloth
Turn the device off and take the batteries out. Wipe the contacts with a dry cloth or a clean pencil eraser to lift light grime.
3. For crusty build-up
Dip a cotton bud in isopropyl (rubbing) alcohol, wipe away the crust, and let the contacts dry fully before you fit new batteries.
4. What to avoid
Don’t use sandpaper or a metal blade, which can scratch and wear the contacts. Don’t soak the compartment in water. If the contacts are bent or pitted, the device may need repair.