Electric energy storage device, power tool system and battery pack

A battery pack, comprising:

• a housing;
• four strings of battery cells disposed within the housing, each string having a positive terminal and a negative terminal, the four strings having substantially the same rated voltage;
• and
• a battery interface disposed on the housing, the battery interface including eight output terminals electrically connected to the four strings of battery cells, the eight output terminals including four positive output terminals respectively connected to the positive terminals of the four strings and four negative output terminals respectively connected to the negative terminals of the four strings;
• wherein the battery interface is configured to be interchangeably coupled to interfaces of a first power tool, a second power tool, and a third power tool, such that:
• when coupled to the first power tool, the four strings are connected in parallel to output a first rated voltage;
• when coupled to the second power tool, the four strings are connected in a series-parallel configuration where each two of the four strings are connected in parallel first and then the two pairs are connected in series, or each two of the four strings are connected in series first and then the two pairs are connected in parallel, to output a second rated voltage;
• when coupled to the third power tool, the four strings are connected in series to output a third rated voltage;
• and the first, second, and third rated voltages are different.