Batteries are electrochemical device that convert chemical energy into electrical energy. Batteries are of two types - primary and secondary batteries. Primary batteries loose their charge with time while the secondary batteries can be recharged for repeated use. Battery cells loose power when the chemical that powers the reaction is finished up. This leads to an increase in internal resistance of the battery and a time comes when there are no reactants are left for chemical reaction and only products of chemical reaction are left in electrolyte in  the battery.