About 80 Probable Causes Identified in the Investigations
Boeing analyzed the mechanism of the battery failure, and found that the problem was triggered by overheating of one of the 8 cells stored in the battery case, for some reason.
The project team identified about 100 probable causes that would first cause one cell to overheat. They were identified based on theoretical analyses and simulation experiences. Many of the causes were not identified during the investigations of the incidents involving JAL and another Japanese operator.
Next, each and every one of the 100 or so identified causes was analyzed and evaluated. As a result, about 20 of them were confirmed to be "theoretically possible, but realistically impossible" or that "measures had already been implemented", such as overcharge. The cause of the remaining 80 or so causes were classified into 4 categories.
Loosening of electrode joining metal nut of adjacent cells
(Example) If a nut becomes loose, electrical resistance will increase in that specific spot. If an electric current flows, heat will be generated and transmitted to the cell interior through the electrodes.
Load on cell due to short-circuits or changes in voltage (degrades the cell)
(Example) Moisture caused by condensation inside the battery case will spread between cells or between the cell and battery case. If an electric current flows, the electrolytic solution inside the cell will dissolve, creating deposits of lithium metal or moisture. If they react, it will cause internal heating.
(Example) In case of over discharge, the metal used for electrodes will melt inside the electrolytic solution. The next time the battery is charged, the metal will form deposits between the positive and negative electrodes, and short-circuit, generating a large electric current and heat inside the cell.
Problems in the production process (e.g. inclusion of foreign substances in the cell)
(Example) If a piece of metal enters the cell for some reason during production process, the positive and negative electrodes will short-circuit, causing a large electric current and internal heating.