A battery operating apparatus, an auxiliary battery unit, and a battery control method are provided. The battery operating apparatus includes a master battery manager (BM), a fixed battery unit configured to supply a fixed power to a load, the fixed power remaining unchanged over time, and a removable auxiliary battery unit configured to supply a variable power to the load, the variable power varying over time, wherein the master BM is configured to control the fixed battery unit to supply the fixed power among a power needed by the load, and to control the auxiliary battery unit to supply the variable power among the power needed by the load.