Island Airphoto said
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
The differences are pretty simple.
Lead Acid and NiCd cells (even sealed ones) can tolerate a certain amount of overcharge without damage, as long as the rate and temperature are limited.
NiMH can also tolerate some overcharge, but the end of charge point for fast charging gives only subtle warnings.
As long as individual cells are capable of accepting overcharge, it is possible to balance a series string by a controlled overcharge (equalization) cycle which will assure that even the largest capacity cells in the string can be brought to full charge by overcharging the lower capacity cells.
Then if an over discharge takes place, there will only be major cell damage if a low capacity cell is actually reversed. That can be prevented by a clamp or shunt diode.
Thermal runaway can happen in those types of cells if fixed voltage charging without temperature sensing it used, but it does not happen with a good charger on a series string unless one cell or battery gets much hotter than the rest or the whole string gets out of control.
But there are two characteristics of most lithium chemistry batteries that require more active balancing systems:
1. lithium cells will not tolerate prolonged overcharge the way lead acid or NiCd will.
2. When a lithium cell (any lithium chemistry) is discharged below a certain voltage (dependent on the chemistry, but greater than 2 volts) it is permanently drained and cannot be recharged under any combination of applied voltage. If you try to recharge such a cell, it will enter a hazardous state long before its voltage reaches the normal charging cutoff voltage. That means that some sort of balancing and protective circuitry at the cell level is mandatory for safe series discharge and charging.