There have been recent questions posed to plants about possible exposure of the cell plates due to elevated battery temperatures as part of the various conditions and scenarios for the Fukushima type events.  The beyond design basis events typically result in elevated temperatures throughout the plant, including the battery rooms.  The concern is an increased water loss due to increased electrolysis of the water in the electrolyte and evaporation in the cells due to the elevated temperatures.

As a rule of thumb, assuming the battery float voltage remains constant, the charge current into the cell while on float charge will double every 8 °C (15 °F) that the average battery temperature is above 25 °C (77 °F).  Thus the rate of electrolysis will double as well.  The rate of hydrogen generation included in the IEEE standards is conservative in that assumes all of the charge current going into the battery causes the electrolysis of water in the electrolyte.   The charge current can be reduced by adjusting the float voltage for the elevated battery temperature using the guidance in the battery manufactures operating instructions. (Typically, the float voltage is reduced by 5.4 mV for each degree the battery temperature is above 25 °C (3 mV for each degree the battery temperature is above 77 °F.)  Determining if there would be any increase in evaporation is more complex, but the conservatism in the hydrogen evolution rate may envelope this.

Overall, this elevated temperature should not increase the water loss of the cells in the battery so dramatically that one would expect the plates to become exposed.