Solar Panel Diodes

We are often asked about blocking diodes and bypass diodes and how or why they are used with solar panels. Unfortunately, there seems to be a little confusion about this subject, so we thought we'd try and clear up the solar panel diode question...

What are diodes?

A diode is a small electronic component that, ideally, has a very very low resistance to electrical current in one direction and a very high resistance to flow in the other direction. There are a couple of different types: a 'vacuum tube' type and the more common 'semiconductor' type. Effectively, both work by allowing electricity to move in one direction, but not the other.

What are solar panel blocking diodes?

Older designs of panels and many cheaper modules can suffer from electrical current flowing back into the solar cells from the batteries, especially at night when they are not producing solar power. To prevent this, a blocking diode is sometimes fitted between the positive terminal on the panel and the battery.

Do you use or recommend blocking diodes?

Our monocrystalline solar cells have an inherently high resistance due to their high quality of manufacture and the general structure of the silicon crystals. This means that we don't have to use a blocking diode to stop current flowing back into the panels. Also, we only use and recommend high quality charge controllers and, 99% of these are fitted with electronic measures to prevent current leaking out of the batteries at night.

What are bypass diodes in solar panels?

All solar panels have bypass diodes fitted in the junction box on the back of the module. Solar panels are actually a collection of solar cells that are connected 'in series' (i.e. positive to negative). Each cell produces about 0.5V, so if we want an 18V panel, we would typically use a total of 36 cells all strung together. The problem with a 'series' connection is that the string is only as good as it's weakest member, so one solar cell being in the shade will result in the whole string of cells not working properly. To overcome this, we split the large string of 36 into smaller strings and use a diode to allow current to flow around a string if it is not performing well. In effect, this means that instead on one shaded cell causing the whole panel to under-perform, only the smaller string containing the shaded cell is affected.

How many diodes do you use?

Most standard solar panels are fitted with one bypass diode every 18 cells, so a Kyocera KD140 panel, for example, has 36 solar cells and 2 bypass diodes; therefore one shaded cell will reduce the output of 50% of the panel. To improve the low light performance of our panels, we often use one diode for every 12 cells, so a shaded cell will only affect 33% of the panel

Why don't you use more diodes?

The bottom line is cost and practicality... If we used one diode for every cell, the junction box would become a lot larger and there would be so many wires that it would turn into a spider's web of cable.