Herein disclosed is an upgradable SMART (Self-Monitoring Analysis and Reporting Technology) battery pack which is capable of upgrading a SMART function thereof. The upgradable SMART battery pack comprises a battery array, a host interface for supplying power to a host and performing a SMART communication therewith, a protection circuit for protecting battery cells in the battery array from overcurrent or overvoltage, a microprocessor for monitoring a current state of the battery array, calculating a battery capacity thereof and controlling the SMART communication with the host via the host interface, a nonvolatile memory for storing a program necessary to an operation of the microprocessor and unique battery information, and a signal processor for converting a level of a signal to be sent to the host via the host interface into that desired by the host and processing the level-converted signal. The SMART battery pack further comprises an input/output port for performing an interfacing operation between the host and the microprocessor to download a control program and data to be stored in the nonvolatile memory from the host.