Hundreds of mechanics and engineers have been deployed to the nine countries where 787 Dreamliner long-range jets have been parked, waiting for The Boeing Company (NYSE:BA) to resolve concerns over smoldering lithium-ion batteries that grounded all 49 of the planes worldwide.

Work began Monday with major 787 customers All Nippon Airways and Japan Airlines to make a dozen modifications to the large batteries located in the forward and aft electrical equipment bays. The batteries are primarily used to power the plane’s systems while the engines are off. They also power the plane’s navigation lights and brakes.

Changes include placing the cells in a stainless steel box, a job that takes five days per unit, according to William Loftis, vice president of Boeing's 787 program. On Friday, the U.S. Federal Aviation Administration approved the changes, which allowed Boeing to begin implementing the fixes. Last week Boeing resumed production flights, the important testing phase for jets being prepared for delivery. Officials have said deliveries would resume in the coming weeks. Aviation officials still have to grant permission for commercial flights to resume.

Boeing will release its quarterly earnings on Wednesday, and company officials have declined, before the earnings report is released, to say how much the 787 battery problem will cost.