All batteries are dangerous, not just lithium. Ten years ago I bought a yacht that had two very large 180Ah 12V sealed lead acids made in Germany by Sonnenschein. The dog's balls of batteries. They were only around six months old and one night, motor-sailing I noticed what seemed like a high persistent charge into the batts. 8A or something, I think, but constant and wasn't falling back, indeed, it seemed to be rising. The batteries were in the engine room and when I investigated one of them was HOT - 80C from my infra red thermometer. I disconnected the hot one, and all was well. Easy at sea level; not so easy at FL300. The cause, I am 99% sure, was the (Japanese, Hitachi) alternator that was producing 15.5V constant --- incredibly that was in-spec for the alternator
Now I only use unsealed flooded lead acid batts. Mind you, fine at sea-level (though not all would agree) but certainly not from choice in an aircraft, especially a metal one. Hey, did someone say the Dreamliner is made of plastic?
FLA would be fine unless they try to fly inverted.