TOKYO, Nov 18 (Reuters) - Japan's ANA Holdings Inc, which operates the world's biggest fleet of Boeing Co 787 Dreamliners, said it had to replace a main battery charger on one of the composite jets after maintenance crews detected a possible fault.

The problem was discovered during regular maintenance of the aircraft on Saturday, with a replacement charger installed the next day, a company spokesman said. ANA, which operates 23 Dreamliners, sent the faulty charger to maker Thales SA in France.

Boeing's state-of-the-art jet, has two large lithium-ion batteries that provide backup power to aircraft systems. The meltdown of two of those batteries, one on an ANA flight in Japan and one on a Japan Airlines jet in Boston, prompted aviation authorities to ground the 787 fleet for more than three months.

While minor faults are not uncommon with aircraft, aviation industry watchers nonetheless remain sensitive to any new glitches with the 787, particularly any related to the batteries.

After the earlier battery incidents, Boeing redesigned the power pack and charger system, adding insulation and a steel box to contain any further meltdowns and a specialised vent to eject any smoke outside the aircraft.

Investigators in the United States and Japan have yet to discover the root cause of the overheating.