A battery, like the every-day type used in household appliances, is a set of electrochemical cells that convert chemical energy into electrical energy. There are a number of different disposable battery types, ranging from zinc-chloride, alkaline, lithium-ion to nickel cadmium. Irrespective of the type, however, all batteries run flat for the same reasons. Knowing how batteries works is necessary to understand how they run flat.
All types of batteries have three main components: a positive electrode, a negative electrode and the electrolyte. The positive and negative electrodes are the parts that are connected to a circuit. On a AA battery, these are found on the opposite ends, where the flat side is the negative electrode and the pointed side is the positive electrode. The electrolyte is the liquid or powder interior, and the composition of this liquid depends on the battery type.
Ions and Electrons
When a battery is connected to an electrical appliance, with both the positive and negative electrodes touching a metal element, a circuit is formed. Within the electrolyte, ions, which are atoms with a positive charge, start flowing in one direction. Electrons, which are negatively charged particles, flow in the opposite direction. All electrical appliances are fueled by electrons.
When a circuit is formed, a series of chemical reactions takes place. The exact type of reactions depends on the battery type. Both the ions and electrons flow inside the battery as the chemicals inside the battery are converted into different chemicals. In the common alkaline battery, manganese oxide and hydroxyl ions are converted from manganese dioxide. The hydroxyl ions in turn react with zinc oxide and water, releasing electrons.
A battery's ability to generate electrons will eventually be depleted when the chemicals within the electrolyte are used up. In the case of alkaline batteries, this is when all of the manganese dioxide has been converted. At this stage the battery is flat.