A battery charger is a device designed to push electrical energy through a secondary cell or a battery to recharge it. Different types of rechargeable batteries respond best to different types of charging, therefore it is important to know which charger or chargers are applicable for the different batteries that you need to recharge. For example, a mobile phone should not be charged with a car battery charger, neither should a NiCd charger be used to charge NiMH batteries.
A typical rechargeable battery can be charged many times. Firstly, it is important to be sure that the battery in question is indeed the type that allows for recharging. If it is rechargeable, it can be connected to the charger unit. Once a current starts to flow, the electrodes that have depleted out of the batteries are recharged and the battery's energy is restored. It is important to cut off charging once maximum capacity is reached, or the batteries will have to expend the excess energy and will overheat, leading to risk of rupture, chemical leaks, or sometimes even the explosion of the battery.
As there are different chemical contents in different batteries, there are different methods of battery charging that are the most suitable for each chemistry and battery type. There are battery chargers available for standard sizes, including AA, AAA, C, D, and 9V, and there are also battery chargers available for larger lead-acid batteries. Some battery chargers have slots for the smaller cells to be placed into for charging and these often fit straight into a plug socket for power. Other battery chargers come with crocodile clip connectors to connect to larger, lead-acid batteries.