Batteries, in their simplest form, can be thought of as tiny fuel tanks that house a chemical reaction. This reaction generates power through use or self-discharge, during which the liquid electrolyte inside the battery breaks down and releases hydrogen gas. This gas creates pressure inside the battery, similar to how water expands. If the pressure becomes too great, it can rupture the seals of the battery and cause a leak.

The substance that leaks out is usually the liquid electrolyte, which in the case of alkaline batteries, is potassium hydroxide. This substance is corrosive and can cause eye, skin, and respiratory irritation. Once it reaches the open air, it reacts with carbon dioxide to form potassium carbonate, a white powder that you might see caking on the battery’s shell. This is the same white, crusty substance you often see when a battery has leaked inside a device.

So, in essence, batteries leak when the chemical reactions inside them generate enough gas to break the seals. This is more likely to happen if a battery is left unused in a device for a long time, or if it’s exposed to excessive heat. It’s always a good idea to store unused batteries in a cool, dry place and to remove them from devices if you know you won’t be using them for a while.

Here is a picture of the inside of a finger light after it was accidentally turned on for several hours. It has a lot of potassium carbonate powder on the outside. The darker spot on the battery is damage to the metal caused by the corrosive potassium hydroxide.