A reconfigurable battery pack power system with interchangeable accessories has at least one modular, portable, rechargeable battery module having a first side, the first side having a first contour and a mounting port and a power output port, and at least one accessory module configured to receive power from the battery module. The accessory module has a second side with a second contour that mates with the first contour on the battery module, and has a power coupling connector and a fastener. The accessory module is interchangeably mateable with the battery module with the first and second contours nested together, and with the power output port engaged with the power coupling connector and with the fastener engaged with the mounting port, so that the battery module and the accessory module form a single integrated and substantially rigid structure.