Tests on a new kind of battery for Boeing 787 planes were inadequate, and even now, more than a year after the problem kept the planes grounded for months last year, new tests should be devised, the National Transportation Safety Board said Thursday.

Internal flaws can make the lithium-ion batteries catch fire with no warning, the safety board said in a letter to the Federal Aviation Administration, adding that it should develop “abuse tests” that could be carried out aboard a plane to see how well its systems cope with smoke or fire.

In January 2013, batteries on two 787s began smoking and one of them caught fire, a result of a chemical process called thermal runaway that leads to smoke or fire. The safety board, which hopes to finish its investigation by this fall, said Boeing and the FAA had believed that thermal runaway could result only from overcharging the battery.

The board said there was no evidence the battery in the first case, which caught fire while the plane was at an airport gate in Boston, had been overcharged. In the second case, a battery began smoking in flight in Japan. Investigators found no evidence of overcharging in that case either.