Japan's largest airline says it experienced repeated problems with the batteries in its fleet of trouble-plagued Boeing 787 Dreamliners, even before an emergency landing this month prompted a worldwide safety review.

All Nippon Airways said Wednesday it had to replace the next generation aircraft's lithium-ion batteries 10 times because they failed to charge properly or showed other problems. The airline, which is the world's biggest Dreamliner customer, says it did not report the problems to safety regulators because they did not compromise the plane's safety.

The entire global fleet of 787s has been grounded after an overheated battery triggered smoke alarms and prompted the emergency landing of a domestic ANA flight on January 16th. Days earlier, a battery caught fire at an airport on a Dreamliner in Boston.

U.S. transport safety regulators say the battery replacements were part of their investigation into the plane's safety. Officials have said they are still not close to completing their investigation.

Boeing has sold or has commitments to build more than 800 of the planes for airlines around the world. Boeing says the 787's revolutionary design will save air carriers money by using less fuel.