This may or may not be true for lithium batteries, but it is NOT true for the various lead-acid types! 12, 24, 32, 36, 48, 72, and 120 volt battery banks made from lead-acid cells are charged in series
all the time. I personally used to connect the 24 volt generator to 727s and 737s back in the day and those batteries had no cell-by-cell charger.
AFAIK I never burned up an airplane or if I did it burned up after I went home
