a lead - acid battery including a case , a plurality of negative and positive electrode plates arranged in the case , a plurality of separators interposed between the negative and positive electrode plates , and a plenum stabilization material disposed within the case to maintain the positions of the electrode plates relative to one another and to other components within the case . the plenum stabilization material includes a compound that is solid during normal operation of the battery and can undergo reversible physical change in state from solid to liquid when the battery is subjected to or is operated under abnormal conditions .