sentences:  The cells can tolerate overcharging and accidental polarity reversal, and the hydrogen pressure in the cell provides a good indication of the state of charge.However, the gaseous nature of hydrogen means that the volume efficiency is relatively low (60-100 Wh/L for an IPV (individual pressure vessel) cell), and the high pressure required makes for high-cost pressure vessels.The positive electrode is made up of a dry sintered porous nickel plaque, which contains nickel hydroxide.