The FAA has issued an airworthiness directive for the Cessna 525C Citation CJ4. An Oct. 6, 2011 Emergency AD (#2011-21-51) required replacing the lithium-ion main aircraft battery with a Ni-Cad or a lead-acid battery. The current action was prompted by a report of a battery fire that started after an energized ground power unit was connected to a CJ4 equipped with a lithium-ion battery, Cessna P/N 9914788-1, as the main aircraft battery.