If a disposable alkaline battery leaks inside a piece of personal electronics, the leakage can ruin the device. Most cylindrical batteries, including AAA, AA, C and D cells, are alkaline, as are many button-type batteries that fit in watches and hearing aids. The alkaline chemical in these batteries is potassium hydroxide. This chemical is toxic and corrosive, but you can neutralize it with an acid and clean it without much trouble. Be warned: You use a different agent on alkaline battery leaks than you use on acid battery leaks. There is a chance your leaking battery contains an acid electrolyte. If so, the casing will indicate that, and you shouldn't use an acid to neutralize it.
Remove the leaking battery from the device. Potassium hydroxide won't burn your skin, but it can irritate it, and it can damage your eyes, so wear rubber gloves and goggles.
Put some vinegar or lemon juice in a spray bottle. Both are weak acids that will neutralize the corrosive chemical in the leaking battery, and neither is dangerous to use.
Spray the deposits that have leaked out of the battery and scrub them with a toothbrush. Use a paper towel to remove loose deposits, once you've scrubbed them off, and to soak up excess vinegar or lemon juice.
Repeat the procedure as many times as necessary to remove all the battery deposits. Clean up with a slightly dampened paper towel, then set the device you're cleaning aside to dry for several hours.
Use a similar procedure to clean leakage off the floor or a table top. Neutralize the potassium hydroxide with vinegar or lemon juice, then scrub it off the surface with a toothbrush. Wipe the area with a moistened paper towel when you're done.