Break-even point between cable length and voltage drop?

It is PWM. All Vdrop does is make the controller hold the high set point at a lower V but by the time the batt gets to a high SOC the amps have tapered

Also amps are lower later in the day when the batt gets full with the 120W only doing about 7 amps at high noon

The 13.8 vs 13.5 is a difference not the Vdrop for each wire at pick a number amps

OP needs an hydrometer. His batt could be sulfated with those 13 .x voltages