# Choosing the Correct Diode Size for Solar Bypass

#### Luckyned

Joined Aug 16, 2017
2
Bit of back-story is I am moving onta a canal boat so solar is going to be of massive importance to me. Did all my research and chad select panels based on being in a marina with almost zero shading. However, I am now going to be locating my boat on a river and shading is going to be an issue. Loosing even 1 cell on a panel to shading degrade the whole panel massively, this would be compounded by the fact I need to run 2 in series to up the voltage for my charge controller.

I have however come across a solution for this and I’m trying to figure out if it is cost effective but I’ve fallen at the first step. The solution is to place a bypass diode in for every cell in the array. This is a tried and tested solution but what is done traditionally is the bypass is put in for a string of cells. I want to do this for every cell to create the ultimate shade tolerant panel.

The cells are 0.5v and 9amps each. The panels I am looking to create will be 60 cell and I will be wiring 2 panels in series. Therefore, we are looking at a total open circuit voltage of give or take 80v at 10 amps more or less.

So the question is..

Does each diode only need to be the strength of the cell its bypassing say (1v 10Amp) or does its need to be strong enough to take the full open circuit voltage which is more like (80v 10amps)?

My feeling is for the full circuit flow, however I read a something last night suggesting you are only compensating for the imbalance caused by that one cell being in shade.

Any help would be greatly appreciated.

Thanks John

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#### shortbus

Joined Sep 30, 2009
9,449
The cells are 0.5v and 9amps each. The panels I am looking to create will be 60 cell and I will be wiring 2 panels in series. Therefore, we are looking at a total open circuit voltage of give or take 80v at 10 amps more or less.
Just a hobbyist and not an electrical engineer so this may get disputed. If your individual cells only output 0.5V they don't even have enough voltage to overcome the 0.7V forward voltage drop of most diodes. So don't think adding a diode to each individual cell is going to work. But others with more knowledge may give a different answer.

"In a small silicon diode operating at its rated currents, the voltage drop is about 0.6 to 0.7 volts."
From, https://en.wikipedia.org/wiki/Diode#Forward_bias

#### Luckyned

Joined Aug 16, 2017
2
Hi shortbus,

Thanks for the reply.

The diodes that are used in solar panels are Schottky diodes. The suggested forward voltage of around 0.4v. This does mean for every cell in shade you lose one additional cells voltage to overcoming the diode but it is far less of a loss that on an normal series setup.

Did you have any thoughts on the rating size?

Thanks

#### shortbus

Joined Sep 30, 2009
9,449
I don't think that even with schottky diodes on each cell it will help at all, 0.5V - 0.4V is only going to give you 0.1V output. That is the reason they are only used on many cells wired together, so the voltage drop isn't significant.