Battery chargers are a must for rechargable batteries. Make sure to use the appropriate charger for the chemistry and design of the battery. Some chargers can charge multiple chemistries of battery, make sure the charger is able to handle the chemistry before attempting to charge. Charging a battery in the wrong type of charger can result in damage to the battery and the charger. Do not attempt to charge any non-rechargable chemistries as it will potentialy damage the charger and anything nearby.
An external battery charger is not required if the device that uses the battery has a built in charger.