Field expandable battery pack systems and related methods are disclosed that allow for increased battery life while reducing cost and still meeting shipping regulations. In part, the expanded battery system includes a primary battery having a battery management unit (BMU) and additional expansion batteries that couple to the primary battery and are managed together by the shared-BMU within the primary battery. Each battery has its own protection circuitry, and the batteries can be lithium-ion batteries. As such, multiple batteries can be shipped together and then combined by a user to achieve a desired level of battery capacity that is managed as a single battery through the use of the shared-BMU.