A battery charger is a device designed to push electrical energy through a secondary cell or a battery to recharge it.
A typical rechargeable battery can be charged many times. First you have to make sure you have the right type of battery that allows for recharging. From here, you connect the battery 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.
Different types of rechargeable batteries respond best to different types of charging, therefore being able to pick a charger suitable for one type of battery may not work well with another. In other words, we should not charge a mobile phone with a car battery charger, but neither should we charge NiMH batteries with a nicad charger.