"...the National Transportation Safety Board said an examination of the data from the plane’s flight recorder indicated that the battery “did not exceed the designed voltage of 32 volts.”
I do hope the NTSB was not trying to say there was no overvoltage problem because the external voltage was below the 8 cell limit.
Many technical reports about the batteries in question note that external charging voltage is no indication of cell to cell voltage. Those reports document cases of individual cells having excessive internal resistance which raised their internal voltage and leads to thermal runaway in that one cell.
The thermal damage then spreads to adjacent cells.
Most technical reports on safety and these batteries point out the need to monitor individual cell temperatures and to control the external voltage and current based on the worst performing cell.