"If the output voltage is higher, then the power is of course greater...and conversely if the system voltage is lower, the output power is lower, assuming the current remains constant."
I am not clear on the "system voltage". The charger's voltage is set at 14.6v and stays there solid the whole time. 75 is constant the whole time during these two situations.
In "power supply", the battery bank is being drawn down by the inverter pulling 127 amps (MW) and so battery voltage is low. The charger then supplies 75 amps and now the battery draw is 52 amps but its voltage is still low, just not as low.
In "battery charging" the battery voltage is in the 13s at first then climbs. So the charger is facing higher resistance than when doing "supply", but it is still doing its 75 amps.
I have seen it before where a converter will do full amps doing "supply" but not when battery charging. Eg a 7355 in the 5er did 56 amps but only 35 amps battery charging--and on the same wires to the battery.
So I think the charger doing 75 on battery charging could need more input than when doing 75 amps as supply. No?
Brings up an old question I never got clear on--is charger "output watts" using the charger's voltage setting, or is it using the battery voltage in the watts?
