Boeing's batteries used eight large cells; those used by Tesla and SpaceX use thousands of small cells designed into packs with space around each to make it less likely heat from one could build up and set off others.
"Large cells without enough space between them to isolate against the cell-to-cell thermal domino effect means it is simply a matter of time before there are more incidents of this nature," Musk wrote. "It is much harder to maintain an even temperature in a large cell, as the distance from the center of the cell to the edge is much greater, which increases the risk of thermal runaway."
The FAA allowed the 787s to fly again in April after Boeing improved the insulation to keep heat from one battery from spreading to the other seven and enclosed the whole pack in stainless-steel boxes to keep any fire, smoke, or hazardous gases from affecting the rest of the plane, according to The New York Times. Tesla's problem in the Model S car fires was damage from debris, not battery design, the company told the NHTSA.
Its solution was to reinforce shields protecting the batteries, and set a higher minimum clearance level in the software that controls the air-assisted active suspension that is designed to lower Model S Teslas at highway speeds to improve aerodynamics and efficiency.
Two-thirds of 2012-2013 Tesla Model S cars -- including all three burned by battery fires -- came with computer-controlled, air-assisted suspension systems that can be upgraded over the air without a visit to the garage.
The bigger fix does require refitting at a garage or dealership, but it will come free for Tesla customers, Musk wrote in his blog.
Tesla will replace the aluminum plate protecting the battery pack under the passenger compartment with one made of titanium, Musk told the NHTSA and wrote in his blog. It will also add a second plate to cover the vulnerable leading edge of the original plate and a round, hollow aluminum tube mounted forward of both to deflect debris before it reaches either plate.