A backup battery charging system for a building management system is disclosed. Components of the charging system include an analog power converter, a voltage feedback loop and a current feedback loop. The feedback loops each include at least one digital resistor. The system panel, in turn, includes at least one microcontroller that controls the building management system and also controls the charging system. The charging system is “software defined,” in that the microcontroller controls the charging system by updating the digital resistors in the feedback loops to control the analog power converter. In one example, the building management system is a fire alarm system controlled by a fire control panel as the system panel.