The Seattle Times, citing sources inside the company, says:
Boeing had numerous reliability issues with the main batteries on its 787 Dreamliner long before the two battery incidents this month grounded the entire fleet.
More than 100 of the lithium ion batteries have failed and had to be returned to the Japanese manufacturer, according to a person inside the 787 program with direct knowledge.
The New York Timesadds that All Nippon Airways, which has more 787s than any other airline, was forced to get rid of 10 faulty batteries in recent months, and "had told Boeing of the replacements as they occurred, but was not required to report them to safety regulators because they were not considered a safety issue and no flights were canceled or delayed."
As a result, the National Transportation Safety Board "only recently learned" that Boeing's battery problems are not new.