The battery charger is used to put energy in a cell or rechargeable battery by coercing an electric current through it. The technology and capacity of the battery being charged decides the charge current. There are four types of battery chargers.

First, the simple charger. It works by connecting a constant DC power source to the 18650 battery being charged. The simple charger does not change the output based on time or the charge of the battery. This simplicity indicates that a simple charger is inexpensive. However there is tradeoff in quality. A simple charger takes long time to charge a battery to avoid extreme over-charging. A battery when left in a simple charger for a long time will be annihilated because of over-charging.

Second, the timer charger. The output of a timer charger is stopped after a decided time. Timer chargers were the most usual type for high-capacity Ni-Cd rechargeable batteries. Frequently, it happens that a timer charger and a set of batteries could be purchased as a bundle and the charger time was set to suit the batteries. If the batteries of lower capacity were charged then these would be overcharged. If batteries of high capacity were charged, then these would be partly charged. If batteries of lower capacity were charged, then these would be overcharged. In case batteries of high capacity, they are only partly charged. If the trend of battery technology increases capacity year after year, an old timer charger partly charges the new batteries.

Third, the intelligent charger. Output current is dependent on the battery’s state. An intelligent charger may control the battery’s voltage, temperature and/or time under charge so that the optimum charges current at that instant. Charging is terminated when voltage, temperature and/or time shows that the battery is fully charged. For Ni-Cd and NiMH batteries, the voltage over the battery rises slowly in the charging process till the battery is fully charged. After this, the voltage decreases and this indicates to an intelligent charger that the battery is fully charged.

Finally, the USB-based charger. It is possible to use a USB cable as a power source for recharging batteries. Products include chargers devised to charge standard NiMH cells.