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In a residential solar array, bypass diodes are used when panels are in series to prevent a shaded panel from effectively becoming a large resistor. Blocking diodes prevent current from going back into a panel (or series of panels) in parallel with a load or other panel series. From what I understand, these were used in the dark ages (pun intended) of solar energy when people dumped the electricity from the panel straight into a battery bank and didn't want the reverse happening at night.

Do installers typically install blocking diodes for PV panels in parallel, such as with a string inverter?

Parallel Solar Panel Array

The inverter obviously doesn't rectify current so the panels can't draw power from the AC-side, but I am worried about, for example, if Panel 1 is at a slightly higher voltage than Panel 2 and some current produced in Panel 1 being sunk by Panel 2 as a result.

In this case, I am working with "identical" (same manufacturer and model, and likely manufactured around the same date in the same facility) panels.

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  • A photovoltaic solar cell is a diode, and it points the wrong way. I don't know much about grid-tie inverters, but If you connect a solar panel directly to a battery without a blocking diode, then the solar panel will drain the battery when sun isn't shining.
    – Solomon Slow
    Jan 22, 2018 at 15:23
  • @jameslarge Yes, it is a photodiode, meaning there is a reverse leakage current, but grid-tied inverters don't allow backflow. There's no energy storage.
    – Hari
    Jan 22, 2018 at 21:03

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They are required. If the panel becomes shaded it is considered DARK. If the panel is Dirty it is Dark. What happens is your produced voltage has a PATH to the other panel. The Diodes block that action - allowing current to flow in only one direction.

These blocking diodes, also called a series diode or isolation diode, ensure that the electrical current only flows in one direction “OUT” of the series array to the external load, controller or batteries.

The reason for this is to prevent the current generated by the other parallel connected PV panels in the same array flowing back through a weaker (shaded) network and also to prevent the fully charged batteries from discharging or draining back through the array at night. So when multiple solar panels are connected in parallel, blocking diodes should be used in each parallel connected branch.

Excerpt above from here ^

Connecting Solar Panels / Cells

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  • From my understanding, voltage is relatively indifferent to irradiance. Current increases proportionally, but a panel with less incident light still produces the same voltage (which I've also tested experimentally). This is why I want to know what installers actually practice when installing parallel panels (which I'll update my question to reflect).
    – Hari
    Jan 22, 2018 at 21:06
  • @HariGanti, Were you testing the open-circuit voltage? or were you testing the voltage under load?
    – Solomon Slow
    Jan 22, 2018 at 22:10
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    @HariGanti Your picture does not show a microinverter installation but a central inverter. If you used a MicroInverter you would not need the blocking diodes, the microinverter takes care of that. A shorted (even dead) battery will drain a good battery, much like the diode consuming power from the batteries is a Shunt. I used that analogy because it is easy enough to understand rather than discuss breakdown voltages of diodes. In your picture the shaded panel will load the other panel (you will lose performance) the blocking diode prevents that load from being 'seen' by the other panel.
    – Ken
    Jan 24, 2018 at 19:15
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    To answer the question yes blocking diodes are still used in parallel panel setups. The diodes increase the overall effency a panel in direct sunlight compares to one that is shaded will produce different voltages that if blocking or isolation diodes are not used some of the power gets wasted on the shaded panel. The last system I installed used micro inverters at each panel , Over all cost was similar to 1 large inverter but now there are 15 inverters to fail the advantage was smaller wiring and high output voltage so voltage drops were not a problem. With parallel panels diodes are needed.
    – Ed Beal
    Jan 25, 2018 at 15:42
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    @HariGanti in string Inverter installations the answer is yes blocking diodes are required,the purpose of the diodes is not just for battery drainage, but also with partial or dark panel in a parallel setup. Microinverters are different the electronics are actually in that box to handle partial and dark situations as they are at each panel. As commented above by Ed he installed microinverters and they still used them. Microinverters convert to ac at the panel and combine all at the power panel, where as the other methods combine panels [DC] and then convert to ac at a single point.
    – Ken
    Jan 26, 2018 at 9:06

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