Bypass Diodes

Imagine you are driving down the highway.  All the sudden, all traffic comes to a stop.  There is an accident ahead.  Luckily, there is a secondary road you can take and keep moving forward.  Without this secondary pathway, you would be stuck on the highway.  Solar modules function in the same way as above.  If there is even a little bit of shade, the flow of electricity is blocked.  By adding bypass diodes, a solar module now has multiple pathways.  This allows for the electricity to flow even if there is a blockage.

Typical solar modules will have at least three bypass diodes.  These three diodes separate the solar module into three sections.  In other words, your solar module has three pathways for electrical production.  If one section of the module is shaded from the Sun, the other two sections will still produce electricity.  This does mean the module will be reduced to 2/3 of its normal production.  However, without the bypass diodes, the production would be zero.

The question now is how these sections are created.  If you look at your solar module, you will notice silver tabs at the bottom.  And typically, there will be three separate silver tabs.  This tells you how the module is divided into sections (See figure 1).  Because of the bypass diodes, each of these sections can function independently of the other two.

What does this mean for your solar array?  If you have a location for your solar array that has some shading issues, you can still optimize that array.  Let us look at two examples:

Figure 1

Example one:  You have morning shade that affects the Eastern edge of your array.

In this example, it would make sense to mount your modules in portrait.  This is just like when you print a piece of paper.  The longer side of the module faces North-South.  By mounting the solar module this way, we are allowing the bypass diode to do its work.  If the Eastern edge of the array is shaded, we may lose a few sections in our modules, but the remaining sections will still produce electricity. 

Example two:  You have morning shade that will cover the bottom edge of the array.

In this example, if we mounted our array in portrait like above, we would have zero production.  This is because we have blocked every bypass diode.  Mounting the modules in landscape would be a better idea.  By doing this, you will lose roughly 1/3 of your electrical production during the shade.  But the bypass diodes still work, and you get 2/3 production.

Remember, when it comes to off-grid arrays, our goal is maximum production.  This allows us to have a more reliable off-grid array.  And that reliability will also translate into a longer lasting system.

Deficit Cycling equals Dead Batteries

In the last article, we mentioned the term deficit cycling.  But what is deficit cycling?  And more important, how does it affect you and your solar array?

Deficit cycling refers to a situation where the consumption of the batteries stored energy exceeds the energy production of the solar module.  This will cause the batteries capacity to slowly drain down towards zero.  Once that happens, you have a dead battery on your hands.

Here is how it works.  When you first get your battery, it is boosted to full capacity.  Let’s say this battery capacity is 200Ah for this example.  At a 50% Depth of Discharge, your available battery capacity for use is 100Ah.  With that in mind, you go ahead and use 100Ah.  Now, lets assume your solar module can only replace 90Ah worth of energy in a day.  This leaves your battery at a 10% deficit for that day (based on Depth of Discharge only).  The next day, you use 100Ah of battery capacity.  Again, the solar module can only replace 90Ah.  At the end of the day, your battery is at a 20% deficit.  In two more cycles, your batteries will be at a full 40% deficit.  A few more beyond that and the battery will begin to drastically reduce in performance and reliability.

An easy way to think about deficit cycling is like your bank account.  If you withdraw more funds than you are putting in, eventually you will have a zero balance.  The problem with batteries is that a zero balance means you finished off the battery and will need a new one.

Technology doesn’t trump a quality design

It bothers me to see how often technology is used to counter a poor system design.  It just doesn't make any sense.  And often times, the technology isn't being used for its actual purpose.

A prime example of this is charge controllers.  A charge controller is a device that takes the input power from the solar module and regulates the output.   By doing this, a charge controller can provide the right type of energy required by your battery.  More advanced charge controllers can actually assist in the maintenance of the battery itself. 

So far so good!  However, the charge controller's efforts are really only as good as the overall system design.  The available energy of a system is not based on the battery capacity.  The true limiting factor in available energy is the solar module production.  The solar module is the fuel for the battery.  A charge controller cannot compensate for a lack of energy production.  You must properly pair your solar module production with your battery.  Otherwise, your result will be deficit cycling.  Or in simple terms, you have a battery that just won’t last.