You may have opened the battery cover on your cell phone to find the terminals inside are corroded. All battery terminals can become corroded over time, and the small battery terminals inside your cell phone are no exception. If the corrosion is very severe, the metal may have degraded to the point it no longer functions, but most corrosion can be cleaned with the proper materials and have your cell phone working properly again.
Turn off the phone and remove the battery cover and battery.
Wipe out the inside of the battery compartment with a paper towel, removing any loose corrosion.
Mix some baking soda and water in a bowl. Use a minimal amount of water, just enough to create a paste.
Dip the toothbrush in the mixture and rub the corroded areas with the toothbrush until the corrosion is gone. Rub the contacts on the battery with the toothbrush if necessary.
Rub the corroded areas with a small piece of sandpaper or a small wire brush if the toothbrush did not completely remove the corrosion.
Wipe the battery compartment and battery terminals with several paper towels to absorb any baking soda paste remaining in the compartment, as well as any loose corrosion.
Allow several hours for the battery compartment to dry before replacing the battery and turning on the phone. Use a blow dryer on the low heat setting to dry out the battery compartment more rapidly.
If the battery is very corroded, it may be best to replace it rather than clean it.