Blocking diodes between solar panels

jeepnudger · Senior Member

Blocking diodes between solar panels

dorian · Guru

AIUI, a solar panel consists of several parallel connected strings of solar cells. If one set of series connected strings is in the shade, what is to stop the unshaded cells from feeding into the shaded strings?

As for parallel connected diodes, there was another thread which debunked this arrangement. Unless all the diode temperatures track each other, the result is that one diode ends up carrying most of the current.

Tony Bev · Guru

I can answer in layman terms, (I am not an Electrician) about the reason I fitted diodes

Basically, I was concerned that with two different size solar panels, in two different positions, one would try and charge the other, when the batteries were full, causing damage

I will speak up here and say, credit where credit is due
T1 Terry sent me, free of charge 4 X 15 amp diodes, I used two of them while experimenting, (err playing around), and on-sent two to another forum member

Hopefully the picture below will say a thousand words
It is just a rough sketch I made, to help me remember, what I had done

Hopefully if I had done something wrong, or if the diodes were not required, someone will come along, and put me wise
Solar Setup for Diodes.png

T1 Terry · Guru

The same type of diode can be used for two different functions, as a by pass diode as seen fitted across sections or all larger capacity panels, and blocking diodes, not seen much since BP solar bought out Solarex and then started cost cutting until they finally shut up shop completely.

As most solar panel knowledge relates to grid tie systems these days, the belief is there will always be a path from the generated solar electricity to follow so there is never a likelihood of reverse current flow. In the early days even the off grid controllers diverted the solar to a load that could burn it off if it wasn't required to either power a load or charge the battery.
The problem surfaced when solar controllers simply disconnected from the solar array when the voltage on the load side got too high, something that is even happening with grid tie systems these days, suddenly there are issues with solar panels failing and rather than research why, the problem is put down to poor quality panels or poor quality control etc.

What is happening is the combined output from the array, with panels in either parallel or series connection, will exceed to individual panel output if there is no where else at a lower potential for the current to go. The panel with the lower voltage/current output can not block the reverse current flow so the excess flows back into that panel and turns it into a heater. If it is already suffering heat stroke in harsh mid summer sun, the extra heating is all to much and the solder joints between the modules start to melt, the ones with the poorest joints are the ones most affected because they are getting hot already. Once the solder joint fails, that section of the panel stops producing, now the output is even less so the flow of reverse current increases until all the module strings fail somewhere leaving every string in that panel open circuit. That panel no longer produces so the bypass diode kicks in to reconnect the string and the next weakest joint starts to fail .....
The blocking diode stops this reverse current from being back fed into the panel, ech panel can push its output through the diode but the one way valve effect stops it flowing back in so the panels are protected.
The downside is each diode that has current passing through it will use a small amount of the energy to function and this converts electrical energy into heat energy and is lost to the system's total electrical output.
Normal diodes are 0.7v nominal drop, up to 1.1v when the maximum current threshold is reached or exceeded and if the current flow is high, that can be a lit of heat to dissipate over a small area with no airflow. Schottky diodes have around half the losses so are the preferred choice from this function. There is a circuit called an ideal diode, but if multiple blocking diodes are required it would become rather labour intense building and installing one at each panel. Better to over size the diode capacity to ensure it never sees more than half its rated current flow or upper voltage limits.

T1 Terry