Why do batteries lose charge over time even when not in use?

All batteries are based on chemical reactions between their electrodes and electrolyte. Even when a device is not drawing current, side reactions at the electrodes and within the electrolyte gradually consume stored energy and convert it to heat. The rate depends on chemistry: alkaline batteries lose about 2 to 3 percent of charge per year, while older nickel-cadmium cells can lose 10 to 20 percent per month. Lithium-ion batteries typically lose 1 to 5 percent per month. Higher temperatures speed up self-discharge. Storing batteries in a cool, dry place slows the process.