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NW Washington | Thermal runaway is a problem with NiCad batteries if the conditions are right. I remember problems with NiCad batteries in Navy airplanes and apparently it is a problem with small batteries too. Here is a little info on the subject. More can be found on Google.
"During their recharge cycle, nickel-cadmium batteries react in a manner different from other batteries. Nickel-cadmium batteries will actually absorb heat during the first 25% of the charge cycle (as opposed to most secondary batteries, which generate heat all through their recharge cycle). Beyond that first quarter of the charge cycle, a Ni-Cd battery will generate heat. If constant current is applied past the point when the battery reaches approximately 85% of its fully charged state, the excess heat will cause "thermal runaway" to occur. Under thermal runaway conditions, the excess heat in the battery will cause its voltage to drop. The drop in voltage will cause the charge rate to increase (according to Ohm's Law), generating more heat and accelerating the cycle. The temperature and internal pressure of the battery will continue to rise until permanent damage results.
When using trickle or slow chargers to charge Ni-Cd batteries, the heat build up is minimal and is normally dissipated by atmospheric convection before thermal runaway can occur. Most chargers supplied with, or as a part of, rechargeable devices (sealed flashlights, mini-vacuums, etc.) are slow chargers.
Quick or fast battery chargers, designed especially for Ni-Cd batteries, will usually have a temperature sensor or a voltage sensor that can detect when the battery is nearing thermal-runaway conditions. When near-runaway conditions are indicated, the charger will reduce or shut off the current entering the battery."
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Hand held radio battery fire!
