A system including a battery assembly for use in a modular handheld surgical instrument, wherein the battery assembly includes a primary battery pack, a secondary battery pack, and a circuit configured to supply a first output voltage to a first load of the modular handheld surgical instrument via the primary battery pack, supply a second output voltage to a second load of the modular handheld surgical instrument via the secondary battery pack, and upon sensing a sagging voltage from the primary battery pack, supply a supplemental voltage to the first load, via the secondary battery pack, to prevent the battery assembly from supplying an output voltage below a predetermined threshold output voltage under the first load.