Never failed to amuse at school when guys would argue for Nicads, half a day spent on battery overheat procedures...then when you go to a Lead Acid battery...for a tenth the cost, no inspection and deep cycles... INOP the temp guage, rip out two pages of checklist items. Now it's like car battery...either works or it doesn't.
If you want to do a bit of insightful research you will learn that both Nicads and lead acid have various pros and cons. Which is best for a particular operator is for him to determine, and may be dictated by fleet size, operational environment, economics, whether the airworthiness authority allows a swap (STC perhaps) etc
Previous discussion here
Battery - Lead Acid vs. NiCad preferences. [Archive] - PPRuNe Forums
Despite misconceptions, lead acids may have a thermal runaway. Photo of such in a car.
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All battery systems may be driven into thermal runaway if severely overcharged for a prolonged period at high temperatures. The lead acid battery, when driven into thermal runaway, will fail in such a manner that the aircraft is not endangered.
All battery systems in thermal runaway will produce large amounts of hydrogen and oxygen gas which must be vented outboard of the aircraft.
A nickel cadmium battery in an uncontrolled thermal runaway may get so hot that the battery separator melts causing shorts within the cells and the cell containers melt causing ground shorts to the outside stainless steel container. The result of these shorts is that the battery may catch fire, explode, or the resultant arcing may burn holes in the outer stainless steel box and surrounding aircraft structure. For this reason, the nickel cadmium batteries are equipped with temperature sensors and temperature warning systems.
The lead acid battery in thermal runaway will reach only a relatively moderate internal temperature (approximately 260° F) at which point the water in the electrolyte vaporises and the battery vents steam. As the separator is glass, it is unaffected by this low temperature. The loss of water caused by the venting reduces the conductivity between the battery plates and the battery ceases to accept further charge. The battery slowly cools.
All aircraft I flew professionally only allowed the use of Nicads, no STC being available to allow lead acid, though that has now changed.
http://www.concordebattery.com/otherpdf/finalfaapma.pdf