@Taunusflyer
It isn't that simple...
As long there is just one cell (3.7 Volts) it's not rocket science.
If the voltage demand is higher, e.g. 28 Volts, you need 8 cells.
And there is the challenge.
They have to be charged (on 787 serial) AND balanced AND monitored.
If this is not properly done, a thermal runaway may occure.