Boeing says roughly 100 Boeing 787 lithium-ion batteries have been returned to the company for service since deliveries began last year. Mike Sinnett, Boeing’s 787 chief project engineer, says the most common issue is over-discharge, a condition that can be hazardous for lithium-ion batteries and requires that the eight cells in the battery be scrapped.

Sinnett, speaking at this week’s NTSB hearing on 787 battery problems in Washington, explained that in about two-thirds of the cases requiring service, the battery switch had been left on too long while the aircraft was sitting on the ground unpowered. He said the other one-third are caused by mechanics disconnecting the battery during charging, leading to a fault condition in the battery electronics.

For the cases of the battery switch being left on with the aircraft unpowered, Sinnett says the solution is education. He notes that unlike other Boeing aircraft, the 787 battery powers 21 remote data-concentrators for the avionics system and 17 substations for the electrical system, causing a higher current drain than previous systems.

“When you turn on the battery switch, there’s a 10-fold increase in amount of current flow compared to traditional aircraft,” says Sinnett.

For the disconnection problem, Sinnett says Boeing has added a resistor in the system to keep it from tripping off, preventing the problem that would require the battery to be sent back for service.

He notes that Boeing has seen “very few” returns related to battery mishandling or damage during installation or maintenance.

As a benchmark, Sinnett says that in the past 10 years 23,000 batteries have been replaced in the Boeing fleet of 12,000 aircraft, or an average of one new battery every five years per aircraft.

Boeing designed the 787 battery—built by GS-Yuasa—for a five-year life, though replacement is determined by the results of a capacity test that airlines have to conduct every two years.