englishal, old chap, what are you trying to say?
Sigh....I'm trying to say Lithium batterries are perfectly safe, that is all, and to say that they can't be used in a glider is an excuse.
I've worked with them for years. In the past there were some dodgey old ones, which I only ever came across in two places - inside the fuse of a military bomb, and in offshore equipment deep below the surface of the ocean. But these had big warnings "DO NOT LOAD INTO PASSENGER AIRCRAFT" and you had to treat them with respect and package them up in vermiculite and have graphite fire extringuishers on hand.
Mobile phone type Li batteries are a completely different kettle of fish. Sure bung them on a fire and they will burn, but for them to spontaneously catch fire is a very small chance. Someone will no doubt point to a YouTube video of a laptop battery going up - well these had faults in them. All commercial Li batteries contain short circuit protection and will vent off before catching fire.
As a side note: I was once working up at the Royal Navy Armaments Depot at Beith in Scotland. They took me up to the burning ground to burn off some old (dodgey type) Li batteries...great fun... but you had to literally set fire to the batteries for them to really get going.