A method of charging a battery pack is provided. The method includes: providing a charger comprising a battery interface, a set of terminals, and a power supply circuit for providing a charging current scheme; providing a set of battery packs, each battery of the set of battery packs comprising a charger interface having a physical configuration to mate with the battery interface and a set of terminals having a physical configuration to mate with the set of charger terminals; and providing a charging scheme defined by a relationship between the set of battery terminals and the set of charger terminals.