If you can get a hold of a conventional volt meter, it will be more accurate at measuring voltage than the meters that come with those units. A fully charged battery should be higher than 12.9 volts.
Try to measure voltage at the controller input.
As a practical matter, the challenge might simply be that the panel voltage is not high enough to operate the setp. A PWM controller might be better if you only have one panel.
Here is the logic:
- Vmp of the panel - most likely is 18 ish volts.
- Imperfect sunshine / angle on panels - now 17 volts
- Loss in the wires - 0.5 - 1.0 volts
- Voltage drop it takes to run the MPPT Controller ( could be 2 - 4 volts)
This drop is not really a "loss" as much as it is the voltage "differential" required for the electronics to work properly. It is really intended for feeding 12 volt batteries from 2 - 4 panels in series.
EDIT - I looked at their spec and it says that this "mppt voltage" is 2 volts, but it doesn't take much to be off a tad. Very good quality solar charge controllers from other suppliers often list a 4 volt mppt number, so it isn't uncommon.
Add it all up:
(17 at the panel) - (1.0 wire loss) - (3 or 4 volts to operate) = 12 volts or so at the battery.
My guess is that this controller "voltage drop to operate" is what is messing you up.
It is pretty common on mppt controllers - Victron, etc, only difference is that for some brands it is more than for others.
If that is the problem, you can fix it either by adding another panel (in series) or changing to a pwm controller.
If you have a spare panel around, it can be electrically added on the ground for testing purposes. It probably sounds funny for someone to have spare panels sitting around to play with but I work quite a bit with RVs and panels so for me that seems normal.