Why Do I Need a Blocking Diode on My Solar Panel?

Why Do I Need a Blocking Diode on My Solar Panel?

Blocking diodes in photovoltaic systems serve two functions. They can prevent batteries from draining at night, and can isolate damaged or under-performing solar cells. The latter can be solved with either a blocking or a bypassing diode.

Function
A diode is a semiconductor device, a one-way valve for electricity. Diodes are rated in the maximum current they can pass through while on and the maximum voltage they can block when off. When the correct conditions are met, the diode will stop current from flowing.

Shaded Panels
Solar panels contain several solar cells wired together in series. If part of the panel is shaded, all of the cells become less efficient.

Bypass Diodes
A blocking diode in the panel "averages out" any shaded cells. The diode allows the loss of voltage from a shaded cell to be shared among the others, so that the panel functions more normally.

Battery Drain
At night, dusk and dawn, the battery outputs a higher voltage than the solar cells do. Without a blocking diode, the battery will send power to the panels instead of the other way round. Over time, this can drain the batteries.

Blocking Diodes
When the battery voltage is higher than that of the solar panel, a blocking diode shuts off flow. During the day, the blocking diode can also prevent drain from the battery caused by damaged cells. In a function similar to bypass diodes, the bad cells are isolated from the others.