A replaceable battery unit for a surgical power tool and a method of operating a surgical power tool including the replaceable battery unit. The replaceable battery unit includes a housing containing one or more battery cells, a motion sensor unit, and a controller. The controller is operable to determine, from an output of the motion sensor unit, a sudden movement of the replaceable battery unit indicative of a kickback event of the surgical power tool. The controller is also operable, in response to the determination, to disconnect power from the replaceable battery unit to the surgical power tool.
