Nickel-Cadmium batteries tend to put out a constant power right up until they die. The amperage and voltage (particularly important for starts) will remain high for a certain period and then drop like a cliff. Thermal runaway is a condition in which the battery will no longer put out power but continues to recieve a charge from either generators or alternators. Since all this energy is being absorbed and not put out it becomes heat. The hotter a battery is the more difficult it is to put out power, so the condition worsens if it continues to receive a charge. Almost any aircraft with NiCads is equipped with a battery temperature guage and many also have an associated annunciator warning light. The location of a hot battery could be very critical, as many are housed in the wing or wings. Lead-Acid batteries differ in that they take a charge without absorbing as much heat. The voltage and amperage they put out declines very slowly over a long period of time, as the reaction between the Lead (or a substitute metal) and electrolyte (acid) weakens. Voltage and Amperage (specifically cranking amperage) become critical in turboprop engines where airflow is everything and a faster start is a cooler one.