Comments by "SeanBZA" (@SeanBZA) on "The History Guy: History Deserves to Be Remembered" channel.

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  21. A well known issue withbatteries, the terminal voltage drops with temperature rise. Thus the simpler charge systems interpret this as being a flat battery, or one with a large load on it, and try to keep terminal voltage constant. This then charges the battery even more leading to heat being generated in the battery, and a loss of electrolyte from both chemical reactions and boiling off. Most modern aircraft will have both thermal control of the charge voltage, and as well a thermal switch on the battery that indicates a hot battery. Being such an old design this was likely not on the Tupelov, and instead they had the old fashioned starter generator on the engine, where it is quite capable of charging the battery at a very high current if needs be, equal to the starting current draw of the engine. One cell would have shorted out, either from flaked off material bridging out the bottom of the cell, or a separator failing. This then self discharged the cell, and the terminal voltage of this one cell dropped. Then the engine voltage regulator simply tried to keep battery voltage constant, overcharging the other cells, and the battery got hot enough that the cell separators melted, and shorted out the battery totally. Not an easy thing to recover from, as the generator can be disconnected in flight and the battery can temporarily can provide the full load, but the battery will discharge very fast. You cannot really disconnect the battery from the DC bus though, at least not in flight, there are isolating breakers and fuses inside the avionics bays, but you would need to open panels outside to get to them. They were very lucky though, old design, mostly mechanical linkages and hydraulics, and no electrical systems on the engines needed to run them once started, and the important EGT gauges are self powered by the engine exhaust heat. More modern engine no electrical power no engines, all is controlled by electric motors and valves.
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