I was responsible for safety on a large airborne radar system some time ago. For fault reporting memory backup, it utilised a bank of quite large rechargeable lithium batteries, in a series/parallel configuration. At the time, lithium batteries were known to be somewhat risky, and we went to enormous trouble to ensure that the additional risks due to the bank of lithium cells didn't hazard the aircraft - significantly.
At the time commercial airliners were very strict in what could be carried, but as serious camera batteries grew larger, I noticed that the restrictions on them gradually lessened, and kept pace with usage in the real world! A good example of "commercial" engineering...
Malcolm Stewart
Milton Keynes, UK