Lyman,
Doubt Mx inspected the actual battery to get the notice...
The Securaplane battery charger carries its own diagnostic tool and records appropriate data. I suspect chargers will be located nearby each of the suspect battery packs, and according to the maker, the earlier BC-1300 series supplies the following:-
Charger faults - Fan speed, CPU Temperature, Voltage, Current, Power, Stored fault history.
Battery faults - Abnormal temperature, Temperature sensor circuit, Cell unbalance sensor unit.
LED display - Eight-character alpha-numeric.
Diagnostics - Self test, Output voltage, Output current, Operating hours, Temperatures, Power.
Time - Date, Software revision.
Charge mode - Bulk charge mode, Topping charge mode, Trickle charge mode.
The likelihood of their flagship B787 charger providing less data than above would IMHO be highly unlikely.
Supposing that the JTSB and NTSB examined the charger records straight up and found nothing untoward except the battery fail event, then it would be reasonable for them to dissect battery cells in order to deduce a reason for that failure.