An electric storage battery according to the invention has a porous explosion attenuating material disposed within a head space of the battery case over the electrode elements. The attenuating material forms a layer or lining on the inner surface of the top wall of the case, and preferably also on the inner surface of the upper ends of each side wall between the electrode elements and the top wall. The interior of the head space remains open. Such an explosion attenuating material is effective to limit pressure build-up resulting from ignition of said gases within the case.