Isopropyl alcohol will remove any residue left from the previous step.

Lightly dip a cotton swab in highly-concentrated isopropyl alcohol (over 90%).

Don't soak the cotton swab. Only use a small amount of liquid at a time.

Gently rub the battery terminals with the cotton swap to remove any remaining residue from the previous step.

Wipe the residue away with a microfiber or lint-free cloth.

Repeat this process until the terminals are completely clean.

Wait two minutes to allow any residual alcohol to evaporate.