[John S Peterson]

Open circuit voltage of a battery should be 21.6V, as there are 6 cells each at 2.1V. Charging voltages vary based on resistances in the system including the battery itself. Battery temp plays a big roll in charging, as it is harder to charge a cold one rather than a hot one, % of battery charge is also a factor for output voltage, AC ripple is another, but your biggest enemy is voltage drops. This is stuff that I do and it would be fun to do a basic electrical workshop giving the steps necessary to interested parties making it easier to solve electrical issues with more confidence.

John Peterson