sunshine_eggo
Happy Breffast!
Controller says 13.8. Meter says 13.8 at terminals. Battery is 13.3 at terminals and shunt. Both meters agree.
13.8V @ MPPT battery terminals, i.e., where the battery connects?
Yet, the battery measures 13.3V @ the battery terminals?
so, there's a 0.5V drop between the MPPT terminals and the battery terminals.?
Have you confirmed voltage readings with a separate meter?
When controller is at 14.2V, what is voltage at the MPPT battery terminals and what is the voltage AT the battery terminals?