If you have two 175 watt panels each at 42 volts you will need two 8 amp 45 volt diodes.
Installing blocking diode solar panel.
For solar panels we recommend you put one blocking diode on each solar panel inside an abs project box.
In this video you will learn the importance of a bypass diode in solar panel learn how to connect a bypass diode to your own solar cells to improve the eff.
Diode and unidirectional flow of current.
Blocking diode in a solar panel is used to prevent the batteries from draining or discharging back through the pv cells inside the solar panel as they acts as load in night or in case of fully covered sky by clouds etc.
In simplest terms a diode can be understood as a two terminal electronic device which allows electrical current to pass in one direction.
The diode needs to have a voltage and amperage rating above that of the panel.
Learn how and why blocking diodes and bypass diodes are used.