Originally Posted by Dave H

Yeah, its most likely not enough. Some calculations and notes to give a rough idea, using the larger of the 2 units:

20Ah battery, you will be lucky to get 80% of that repeatedly at a useful charger input voltage. That’s being fairly optimistic I believe, and assuming it’s a deep cycle battery (which it must be if you want it to last very many cycles). Which leaves 16Ah.

The charger itself will consume some power. Better to use 12V DC directly from the battery, because going through the inverter section, then through your chargers AC power supply back to DC will just waste more energy. Figure 80% efficiency (+/-10% perhaps). Down to 12.8Ah.

12.8Ah x 12 Volts = 153.6 Watt-hours of available charging output energy.

153.6Wh / 25.2 Volts = ~6.095Ah for your 6s battery. (Somewhat conservative, using full charge Voltage)

6.095Ah / 6 charges = ~1.016Ah = 1016mAh available per charge.