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 (reverse biased). What is a Diode?
A modern solar panel has 3 junction boxes on the back for 3 bypass diodes. Here you can see the diodes inside the junction boxes: Bypass diodes circled. As the name suggests, bypass diodes are used to bypass shaded solar cells. They stop shaded, high-resistance cells from getting ‘hot spots’ and reduce the power loss in the partially shaded panel.
Diodes are widely used in solar panel installations. They are used as blocking devices because they prevent current backflow (unidirectional flow of current). They are also used as bypass devices to maintain the reliability of the entire solar power system in case of solar panel failure.
Third, bypass diodes provide alternative routes around solar cells that aren’t generating current. If a cell is shaded or damaged, its diode will send current around it, preventing losses. Fourth, blocking diodes stop reverse current flow from the battery to the solar panel at night, preventing power drainage.
Solar panels consist of solar cells that convert sunlight into electricity through the photovoltaic effect. Mainly, we use two kinds of diodes for effective solar panels – bypass and blocking diodes. You may be wondering, what is the difference? Well, not much.
Bypass diodes are connected in reverse bias between a solar cells (or panel) positive and negative output terminals and has no effect on its output. Ideally there would be one bypass diode for each solar cell, but this can be rather expensive so generally one diode is used per small group of series cells.