I give up. Believe what you want in regards to battery resistance. You don't understand Kirchoff Voltage Law and there is no such thing as "effective resistance" in a dc circuit. You are making up terminology. The converter does not "see" a load resistance of V/I. V & I are quantities the converter produces. It has nothing to do with resistance. You are applying Ohms law incorrectly. Is there anybody here with electronics knowledge that can explain this to DryCamper?
If you have a PD converter then just run the test. Take the battery down to 12.0V and start charging. Measure current, battery voltage and converter voltage. Take measurements at start, 1, 5, 10, 15, 20, 30, 45, 60 min, 1.5, 2 hrs. One test is worth a thousand assumptions.
Sal