Charger AC current Requirement Example:
An efficient charger with a 120v AC input, putting 21 amps into a 75 volt (21 cells x 3.5 volts = 73.5 volts) battery makes about (21 x 75 = 1575 watts) of DC output.
So, the (maybe 94%) efficient (probably "switching" type) charger might draw about 14 amps (120 x 14 = 1680 watts) from the AC lines?
And, perhaps a little less if one actually has 124v AC instead of 120v?
That might explain why a 15-amp breaker does not "pop" when doing "21-amp charging".
Maybe my numbers are not perfect, but the general idea still holds.
Having 16 amps "allowed" gives us an extra 33% over the power we "are allowed" with the 12-volt max limit. So, 20-hour charging would drop to about 15 hours, and a "half" charge of 10 hours would drop to only about 7.5 hours ... possibly suitable for really useful 120v charging while at work.