A reserve battery including a dual-purpose fill port member that also operates as a battery activation mechanism. The reserve battery includes a frangible barrier positioned inside a case. The frangible barrier divides the case into a first compartment holding cell electrodes and a second compartment capable of holding an electrolyte in isolation from the first compartment. Positioned in the second compartment proximate to the frangible barrier is a fill port member that is movable in response to an applied force for rupturing the frangible barrier. The fill port member also has a fluid passageway for use in transferring an amount of the electrolyte to the second compartment. The dual purpose of the fill port member is that it efficiently uses the available space and permits the design and production of a more compact reserve battery for use in space-limited applications, such as artillery shells.