From the FAA SAFO
These recommendations are limited to lithium batteries transported in the cargo hold of an aircraft (including cargo holds that are not distinct from the flight deck), and do not apply to lithium batteries carried onboard by passengers and crewmembers, or otherwise stowed in the passenger cabin of the aircraft.
more from the SAFO
FAA testing has shown that encased or enclosed lithium metal batteries may pose a safety risk. Two types of robust, readily available containers were tested at the FAA Tech Center: five gallon steel pails with crimp on gasketed lids, and 30 gallon steel drums with bolt closed ring seals and gasketed metal lids. For both types of container, as few as six loose CR2 lithium metal cells were sufficient to cause failure when induced into thermal runaway by an electric cartridge heater. The confined electrolyte and the molten lithium ignition source formed an explosive condition, forcefully separating the lid from the container. The explosive force in this test was likely high enough to cause physical damage to an aircraft’s Class C cargo compartment.
Probably good advice from Sguppy
Don't underestimate this hazard.