Many of the AA and other sized batteries used in flashlights and other devices are alkaline batteries. An alkaline battery contains a liquid that serves as the electrolyte. If an alkaline battery is damaged and begins leaking, it produces a corrosive deposit. This can ruin an electronic device. Cleaning up after an alkaline battery leak is not difficult but involves taking proper safety precautions.
The potassium hydroxide in alkaline batteries is toxic. It can harm your eyes, damage skin and cause respiratory problems. When you clean up an alkaline battery leak, work only in a well-ventilated area. Wear eye protection and rubber gloves.
Before you start, make sure you are dealing with an alkaline battery. Some batteries use an acid electrolyte rather than alkaline. Acid battery leaks require a different clean-up procedure.
If the batteries are still in the device, remove and discard them. You can be faced with two situations. The leak may be recent, and the alkaline fluid, still liquid. Alternatively, a leak may have gone undetected for a time. In this case the alkaline will have dried. The procedures for both situations are similar. You need to make sure you remove all dried deposits during the cleanup process.
To clean up an alkaline battery leak, use an acid to neutralise the alkaline material. Neutralising the alkaline will halt further corrosion of the electronic components. The best choices are vinegar (dilute acetic acid) or lemon juice (dilute citric acid). Both work and are nontoxic. Do not use water. Water will not neutralise the alkaline leak and may corrode the electronic circuits or cause a short circuit. Also avoid any alkaline cleaner such as baking soda. An alkaline chemical cannot neutralise another alkaline. Worse, an alkaline cleaner may intensify the corrosive effects of the leak in some electronic components.
Take an old toothbrush or other small brush and use it to apply vinegar or lemon juice to all surfaces where the alkaline has leaked, especially electrical contacts. If the alkaline has dried, you may need to scrub gently to loosen the deposited material. Scrubbing will also remove any corrosion present. Use a dry paper towel to remove loose material and excess vinegar/lemon juice.
You may need to repeat this procedure several times, depending on how severe the leak is. Continue until you can't see any remaining alkaline material, either liquid or solid. Use a barely damp cloth to wipe away any remaining residue and then set the device aside for several hours to dry completely. Finally, install fresh batteries (don't try to reuse the ones that leaked since they will probably start leaking again even if they still work). If the leak has not caused too much damage, your gadget should start working normally.
Although battery manufacturers use standard sizes (AA or C size, for example), each brand of battery is a little different. You'll get fewer leaks if you use the same brand in any given device, rather than mixing them up. If you won't be using your flashlight, camera or other device for a long period, remove the batteries. Keep the device and especially the batteries dry.