Warpspeed
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My observation with solar has always been with higher voltages, I have never played around personally with a 12v system.
The aim should always be to keep all the conduction losses as low as possible.
That is especially difficult with a 12v system.
If your MPPT voltage is close to 15v, and your fully charged battery voltage 14.5v, then the whole exercise is pretty pointless if you cannot pull the two voltages almost right together at full maximum duty cycle.
There should be about six amps maximum perhaps.
So all the resistive losses must be brought down to less than 80 milliohms total. The mosfet should be only 17 milliohms, the choke should not be much, and all the external wiring kept as short as possible.
See if you can measure all the voltage drops with the mosfet turned on hard 100%, right around the whole system, and see which is the worst voltage drop offender.
It may need a physically bigger choke with heavier wire, or two mosfets in parallel, or much thicker external wiring to solar panel and battery.
The aim should always be to keep all the conduction losses as low as possible.
That is especially difficult with a 12v system.
If your MPPT voltage is close to 15v, and your fully charged battery voltage 14.5v, then the whole exercise is pretty pointless if you cannot pull the two voltages almost right together at full maximum duty cycle.
There should be about six amps maximum perhaps.
So all the resistive losses must be brought down to less than 80 milliohms total. The mosfet should be only 17 milliohms, the choke should not be much, and all the external wiring kept as short as possible.
See if you can measure all the voltage drops with the mosfet turned on hard 100%, right around the whole system, and see which is the worst voltage drop offender.
It may need a physically bigger choke with heavier wire, or two mosfets in parallel, or much thicker external wiring to solar panel and battery.