Spare (uninstalled) fuel cell cartridges must be carried in carry-on baggage only.
Quantity limits: Two spare fuel cell cartridges per passenger.
Empty fuel cell cartridges may not be refilled with fuel on the aircraft.
Each fuel cell cartridge (not the electronic device) must be marked with the maximum quantity and type of fuel in the cartridge.
See 49 CFR 175.10(a)(19) for size limits and other requirements.