Battery charging voltage additive?
Since the nominal voltage is additive per cell, is the additional
  voltage as well? As in: do I have to charge at 21 (18.5 + 0.5*5) or at
  19 (18.5+0.5) Volts?


Yes, the 'additional voltage' is additive, but there really isn't additional voltage. The nominal voltage of a lithium ion battery is 4.2V, and that voltage is multiplied by the amount of cells you have in series. 5 cells would be 21V. Lithium ion cells can actually be charged by a lab power supply, you put the supply on constant current, then on constant voltage (which would be the max voltage for the cells in series). (Which is a really fun educational activity when done properly and safely). The curve looks like this:


Source:https://www.electronics-lab.com/using-bench-power-supply-charge-lithium-ion-batteries/