Battery Thermal Runaway
One possible cause of thermal runaway is a build up of deposits on the plates. The ni-cad battery uses nickel hydroxide as its cathode, a cadmium compound as its anode, and potassium hydroxide solution as its electrolyte. The chemical reaction during charging and discharging leaves crystalline deposts on the plates. Periodic deep cycling of the battery can help remove these deposits.
When the deposits form, the internal resistance increases and creates heat when the battery is charging. The problem comes when the battery gets too hot and leads to melting, catching fire, exploding, etc.
Either way, follow your RFM and follow your emergency procedures. Disconnecting the battery does not mean the battery will cool down by itself. Once the temp gets too high, disconnecting the charge does not guarantee the battery will cool down.
During the late 1960s and early '70s, occurrences of thermal runaway or battery overheat in ni-cads caused by internal resistance, prompted the FAA to issue Advisory Circular 00-33 to alert maintainers of why periodic maintenance is so important on a nicad. Keep the electrolyte levels in the green, connectors tight and treat corrosion to prevent high internal resistance another cause of thermal runaway.
Improper voltage can lead to thermal runaway, disconnecting the battery may help, but if the temp is too high, flipping the switch may not make the problem go away. Follow your RFM.