DC-DC converter connections
Control of the DC-DC converter is done by pwm input to the voltage control input of the converter. A wire is soldered to this point, the details of how to do it are at https://www.instructables.com/id/Controlling-DC-Converter-Modules/
The converter has a built-in shunt resistor of 4 milliohms which is designed to be used for the current control of the converter. In this project, this built-in shunt resistor is used to measure the solar panel current. This is done by reversing the connections of the negative leads of the solar panel and the battery, compared to the usual setup.
That is, the nectagive lead from the battery is connected to the converter terminall merked "IN-" and the negative lead from the solar panel is connected to converter terminal marked "OUT-". This resulst in the solar panel current flowing through the shunt resistor.
The voltage resulting from the current in the shunt resistor is amplified by a circuit using an AD822A op amp and then connected to an analog input pin of the Arduino Nano.
The MOSFET diode consists of an N-channel MOSFET (type PSMN3R3-80PS which has a very low ON resistance) and a control circuit consisting of 2 N-channel MOSFETs 2N7000 and two resistors.
Details of simulations of a variety of MOSFET diode configurations including this one can be found at https://github.com/farmerkeith/circuits/tree/master/MOSFETdiode