@Kenneth House
Your succinct description of the correct method of charging Li-ion batteries is appreciated.
However, the apparent failure of the Charger/BMU to cease charging the battery at CV (4.025V) once CC/20 (65A/20=3.25A) was reached, must have been occurring often. Deformation of the affected cells must also have been occurring incrementally throughout those periods. So that begs the reasoning behind Boeing's destructive test program, which if followed to the letter, has failed to take account of compounding cell deformation over a longer period.
Page 33 of the NTSB's Interim Factual Report includes the following Boeing test procedure for FAA approval -
Battery testing consisted of full-performance, environmental qualification, and destructive tests. The destructive tests included external short circuit (low and moderate impedance shorts at battery terminals), overcharge (charge battery at 36 volts for 25 hours), high-temperature storage (185º F for 18 hours), and over-discharge (discharge battery to zero volts) tests. Boeing indicated that the tests found no evidence of cell-to-cell propagation failure or fire.
Note that no mention of any cell deformation is made, rather NO "cell-to-cell propagation failure or fire" met the spec..