A portable electrical energy storage device is provided with a frame that includes a plurality of receptacles for receiving a portion of a portable electrical energy storage cell. A cap is provided over the plurality of receptacles and the portion of the portable electrical energy storage cells received in the frame. In some embodiments, a passageway extends between adjacent receptacles. Disposed within the passageway is a plug which exhibits more resistance to thermal energy migration than other portions of the frame that define the adjacent receptacles.