Blocking diode with parallel solar panels

[tanglewood]

Originally Posted by s/v Jedi

Someone asked me:


Correct, however most panels don’t have blocking diodes anymore as they count on the solar controller not to discharge the battery during the night.

But I think the underlying question still stands about the electrical behavior of a cell that is:


1) shaded, and



2) Has a reverse voltage applied to it.


This would be the case with two panels (or cells) in parallel, with one branch shaded and the other lit. A blocking diode in the charge controller will keep the battery from draining back through the shaded cell, but it won't stop the lit panel from back feeding the shaded panel.


When forward biased we know that a shaded cell conducts and heats up, hence the use of bypass diodes to divert the current around the cell.


I suspect that a reverse biased cells conducts, but at a lesser rate. So over time it would drain an connected battery of there is no blocking diode. But I don't really know the answer.

[s/v Jedi]

Originally Posted by tanglewood

But I think the underlying question still stands about the electrical behavior of a cell that is:


1) shaded, and



2) Has a reverse voltage applied to it.


This would be the case with two panels (or cells) in parallel, with one branch shaded and the other lit. A blocking diode in the charge controller will keep the battery from draining back through the shaded cell, but it won't stop the lit panel from back feeding the shaded panel.


When forward biased we know that a shaded cell conducts and heats up, hence the use of bypass diodes to divert the current around the cell.


I suspect that a reverse biased cells conducts, but at a lesser rate. So over time it would drain an connected battery of there is no blocking diode. But I don't really know the answer.

Short of doing some tests, I think you’re right: most of the output of the full output panel will go to the controller and battery but some goes into the shaded panel. So if you do parallel and have shading issues, you’re better off to buy a small controller for each panel or when using a single controller, add blocking diodes and take the loss when not shaded (but that may be more than what you loose during shaded conditions).

Fo series connection you can optimize with an extra bypass diode over the whole panel.

Reducing or eliminating shading is the best way. Move the boom over to the rail etc.