Abstract: A modular battery system for a portable device including a charging module with a retractable plug and charging circuit for converting an AC line voltage to a DC battery charging current. The charging module combines with a battery module and optional fuel gauge (if present) to form a single unit for recharging, preconditioning the battery, and connection to the portable device. The modules are releasably connected, preferably involving conductive fasteners, so that a user may connect a new battery module to an old charging module. The releasable connection serves as both the mechanical and the electrical interface between the battery module and the charging module. The conductive fasteners may be located in different positions on the modules to accommodate different connector geometries. The charging module may also combine with a battery mounting module, rather than the battery module, so that the system works with conventional battery packs.