A bypass diode is an electronic component typically built into solar panel arrays to protect shaded or weak panels from overheating and potential damage. Solar panel arrays function properly when all the individual panels outputs are balanced. If one or more panels become shaded, weak, or inoperative and no longer produce this average output, an over current condition is caused in them. This condition leads to hot spots in the panels cells which causes a drop in overall array efficiency and will eventually lead to thermal destruction of the non-operational panels. A bypass diode is a simple and cost effective method of avoiding diminished output from causing overheating as it simply cuts the affected panel out of the circuit.
When a solar panel is exposed to sunlight, it becomes what is known as forward biased and transmits its output current in a specific direction. In the dark, the panel becomes reverse biased and current can flow in the opposite direction back into the cells and be dissipated as heat. This situation is addressed by the circuitry of the charge controller which ensures that no energy stored in the system batteries flows back to the panels at night. When a part of an illuminated solar array falls into shadow or becomes defective or weak, exactly the same condition arises. The shaded panels stop producing current and become reverse biased.
The problem with this scenario is that the rest of the array is still pushing output current into the circuit unlike night conditions where the whole array is dormant. This situation sees output current flowing backward into the dormant panel in an uncontrolled manner. This causes the cells to dissipate large amounts of energy as heat which will lead to overheating and eventual thermal destruction of the panel. An additional effect of this sort of unbalanced output is an overall drop in efficiency due to the loss of the shaded cells output and working current dissipated as heat. A cheap and simple way of overcoming this type of situation is the insertion of a bypass diode across each panel.
To address this situation, a bypass diode is placed in parallel with each panel and in a reverse biased direction with its positive terminal attached to the panels negative terminal and vice versa. If the panel is functioning correctly, the diode remains reverse biased and essentially dormant. Should the panel become shaded or otherwise non-functional, it will reverse its polarity and start to draw current in the opposite direction. When this condition arises, the easier flow path offered by the bypass diode will draw the working current around the faulty panel back into the circuit effectively protecting it from a hot spot overload.