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.
Installing blocking diode solar panel.
Learn how and why blocking diodes and bypass diodes are used.
Diode and unidirectional flow of current.
In short as diode only passes current in one direction so the current from solar panels flows forward biased to the battery and blocks from the battery to the solar panel.
The diode needs to have a voltage and amperage rating above that of the panel.
A blocking diode and bypass diode are commonly used in solar energy systems and solar panels.
In simplest terms a diode can be understood as a two terminal electronic device which allows electrical current to pass in one direction.
If you have two 175 watt panels each at 42 volts you will need two 8 amp 45 volt diodes.