TOKYO (REUTERS) - A second lithium-ion battery on the Boeing Co 787 Dreamliner forced to make an emergency landing in Japan last month was found to have irregularities, a Japan Transportation Safety Board official said on Tuesday.

The jet, flown by All Nippon Airways Co, was forced to make the landing after its main battery failed.

The plane's auxiliary power unit (APU) battery, which powers its systems when it is on the ground, was found to have swelling in two of its eight battery cells, the official said.