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.

Fully Charge your Batteries for Max Life

As you may have noticed, several of our articles state to always fully recharge your batteries.  In this article, we are going to explain why this process is critical to battery maintenance.   

During normal discharging, soft lead sulfate crystals form on the lead plates inside a lead-acid battery.  As the battery is recharged, these soft lead sulfate crystals are removed from the lead plates.  If a battery is left in a discharged condition or simply being undercharged, the soft lead sulfate crystallizes into hard lead sulfate.  Unfortunately, hard lead sulfate cannot be removed during recharging.  Thus, the surface area of the lead plates becomes reduced.

The storage capacity of a battery is based on the available surface area of the lead plates.  As that surface area reduces, so does the storage capacity.  Eventually, the surface area can become so reduced, that a battery will no longer accept a charge, rendering it lifeless.

Failing to fully recharge a lead acid battery is estimated to cause approximately 85% of deep cycle lead-acid battery failures.  When it comes to battery maintenance, this is a priority.

As always, take care of your off-grid array and it will serve you well for years.

The Power of Batteries

When it comes to batteries, a common problem is a misunderstanding of the actual power available from the battery.  Unfortunately, clever marketing can perpetuate misleading information.  Before you invest in an off grid array let’s take a minute to discuss the real power of batteries.

All batteries are generally rated by their maximum capacity.  For example, a battery with a voltage of 12 and an ampere-hour (Ah) rating of 100 will be listed as having 1.2kWh (kilowatt-hours) worth of capacity.  This is the accepted practice to standardize the way in which batteries are rated.  However, this rating doesn’t mean that this is the capacity available for use.

Depending on the battery type we choose, our battery will not be able to produce its maximum rated capacity.  Let’s use a lead-acid battery for off-grid solar as an example.  Remember from our earlier article, “Top 5 Battery Mistakes” we do not want to go below 50% depth of discharge. We do this in order to insure our batteries last as long as possible. Therefore, using our battery example above, using a 50% depth of discharge we have roughly 600Wh (watt-hours) worth of capacity available for use.

Here is where most off-grid designs fall short.  Even though we aren’t overtaxing the batteries on paper, we have left out a very important detail.  That detail is how we plan on consuming the power from the battery.  It is true that our battery from above has a rated capacity of 1.2kWh.  But, what we must realize is that the rating is based on a C/20 rate or 20 hour charge rate.  In simple terms, this means that we have 1.2kWh if we consume that capacity over the course of 20 hours. That gives us roughly 60Wh worth of power each hour for 20 hours.

This nuance of batteries generally catches people off guard.  If we consume more than 60Wh each hour, the available capacity of the battery reduces.  We have to remember that batteries use chemical reactions to produce and store energy.  When we speed up the chemical process, we reduce the efficiency of the reactions.  This means our battery’s true capacity will be reduced during higher power draws.  If we consume our power over the course of 10 hours, the battery capacity will reduce down to around 90% of its rated capacity.  In our case, our 1.2kWh battery is now a 1.1kWh battery.

The biggest change we will see in a battery’s capacity is when we consume the bulk of our power in a single hour.  When a battery is required to give the bulk of its energy stores in one hour, its capacity will reduce to around 60% of its stated rating.  This is a significant change in the battery’s capacity.  The same 1.2kWh battery from above is now an 864Wh battery.  Remember, we still do not want to consume more than 50% of the available battery capacity.  Therefore, we have now effectively dropped our consumable power from 600Wh down to 432Wh.

By better understanding how batteries capacity is affected by our energy consumption, we can prolong the life of the battery.  Most early battery failures are the result of over-discharge.  Take care of your batteries and they will reward you with long lasting life.