There are two main types of batteries: primary and secondary. Primary batteries, such as zinc-carbon and alkaline batteries, are not rechargeable and must be discarded after they are discharged because the chemical reactions in them are irreversible. Once they are depleted of electrical energy, there is no way to recharge them. In secondary batteries, however, the chemical reactions are reversible, meaning that when the battery is being discharged, the chemical reactions go in one direction and the battery produces electricity, and when the battery is being charged, the reactions go in the opposite direction and the battery stores energy. These chemical reactions can occur hundreds of times in both directions, so a rechargeable battery will typically last anywhere from three to 10 years, depending on how much it is used and cared for, and the number of charge and discharge cycles it undergoes.