Diodes and Solar Panels

Why are solar panels connected to batteries through a diode?

Diodes and components which allow current to flow in just one direction. It is often recommended that a diode be fitted to prevent reverse current flow back through a solar panel at night time (dark current). However, in a 12 volt system you will actually lose more power from losses due to this blocking diode (drop of voltage of up to 0.7 volts) than you will through the solar panel at night time. With a 24 or 48 volt solar system the losses through the diode would be much lower and so it should be considered. The diode should be fitted into the positive leg of the circuit between the fuse and the battery. If more than one battery is being charged from the same panel(s) then a diode (and a fuse (1)) should be fitted for each battery. Low loss Schottky diodes are best since the least power is loss.

Fitting diodes and fuses between a solar panel and battery bank

One situation where diodes are essential is with bypass diodes. Here a diode is fitted across each panel (in parallel) in a multi panel set up. Most larger solar panels have bypass diodes incorporated into them. When one of the panels in the system is damaged, weak, or (most likely) in shade, then there is the risk that full current from the rest of the panels will pass through the shaded panel and cause overheating damage. Bypass diodes are not essential for 12 and 24 volt systems, but for systems over 36 volts they should always be used.

Web Link References

(1) http://www.reuk.co.uk/Fuse-and-Solar-Panels.htm