The diodes are electronic elements that allow the current to flow in just one direction. They are generally used in two ways in the photovoltaic systems: as blocking diodes and as bypass diodes.

The blocking diodes prevent the battery from unloading itself through the photovoltaic panels when there is no sunlight. They also prevent the flow of current from inverting inside the panels connected in parallel when a shadow sets on one or more panels.

The bypass diodes protect individually each panel from possible damage produced by partial shadows. They must be used when the modules are connected in series.

Generally, they are not necessary in systems working with 24 V or less. While the blocking diodes prevent a group of panels from absorbing the flow of current from another group connected to it in parallel, the bypass diodes prevent each module from individually absorbing current from another of the modules in the group if a shadow is cast on one or more of its modules.