A cordless power tool system including (a) a multi-voltage battery pack for use with tools of different operating voltages including a housing, a first string of battery cells arranged in electrical series and disposed inside the housing, a second string of battery cells arranged in electrical series and disposed inside the housing, and a power tool interface shaped and configured to interchangeably (1) mechanically and electrically couple with a first battery pack interface of a first power tool that is configured to operate at a first operating voltage, and (2) mechanically and electrically couple with a second battery pack interface of a second power tool that is configured to operate at a second operating voltage, and (b) a battery pack charger configured to charge the multi-voltage battery pack.