Abstract: A vent cap for use in a recombinant battery which includes a hollow barrel configured for gas-tight insertion into a vent hole in a battery cover. A retaining flange is formed at the closed end of the barrel, and a one-way relief valve formed on the barrel near its closed end. A layer of a polyalkylene explosion attenuating material surrounds the outside of the barrel below the flange and surrounding the relief valve. The polyalkylene material provides effective attenuation of explosions that occur in the vicinity of the relief valve. The invention further provides a vent cap assembly wherein the foregoing vent cap is mounted on a plastic vent cover of an otherwise conventional battery container.