One of the questions popping up recently is do I need to use blocking diodes for solar panels in parallel? We’re all looking for the best performance possible from our solar arrays. Because keeping those very expensive batteries in tip top condition goes hand in hand with that goal.
I will be looking at how blocking diodes can contribute to that by asking what it’s purpose is? When and if a blocking diode needs to be used with parallel connections. If so, what size would be needed?
Finally taking a little look at bypass diodes and what they are used for within a parallel array. So let’s go and look at what these diodes are all about !!
Do I Need To Use Blocking Diodes For Solar Panels In Parallel?
“You do not need blocking diodes with solar panels connected in parallel. Because if the array is connected to a solar charge controller, the controller has one built-in. If the solar panels connected in parallel are not using a solar charge controller a blocking diode is required.”
What Is The Purpose Of A Blocking Diode?
PV systems use blocking diodes when they are being used to charge a battery. All electrical systems have the same characteristic in the fact that;
Current travels from the highest voltage point in the system to the lowest voltage point.
During the day the voltage flow from the solar panels is usually higher than the battery. This means the voltage and current flow is naturally attracted one way towards the battery.
In very low light conditions and at night when there is no flow of current from the panels. The battery is the highest voltage point and the solar panels the lowest.
Charge from the battery can leak back into the solar panels; a blocking diode prevents this from happening. This is true with all solar panel connections to a battery. Be it a single panel or panels connected in series or parallel.
A Note About Portable Solar Chargers
Just a quick note here if you are a hiker. backpacker, camper or just enjoy the outdoor life. And you are thinking of buying a portable solar charger to keep your smartphone and electronics charged. Make sure it has a built-in blocking diode in the USB junction box.
Because if you leave it connected to the charger by accident at night you might wake up to a flat battery. Plus if you attach it to your pack and it becomes shaded you may lose more charge than you gain.
Before the advent of solar charge controllers a blocking diode had to be fitted between battery and panel. These days apart from needing a solar charge controller to prevent the battery being overcharged. You also need one because as I mentioned earlier it has built-in blocking diodes.
Are There Circumstances When A Blocking Diode Might Be Needed?
For most stand alone solar installations where the panels are charging a battery. A solar charge controller will do the job just fine. But where you might have two panels connected in parallel and shading occurs during different times of the day. You might consider fitting a blocking diode.
For instance if you have an installation to provide power to a shed with a “V” shape sloped roof. You might have fitted solar panels to both sides of the roof. This might mean one side of the roof is shaded in the morning but the other side is in full sunlight. And vice versa in the afternoon.
In this case you might want to prevent the panel in the shade from taking power from the one in sunlight. Fitting a blocking diode between the two panels would cut the shaded one off from the one in sunlight. This would then make the system more efficient by directing all available current to the battery .
Of course you could always choose the more expensive option. By installing a solar charge controller for each individual solar panel.
Hybrid Wind And Solar Systems
You might want to do the same with a hybrid solar panel wind turbine combination. Placing the diode between the turbine and battery to make sure the current isn’t going anywhere else. Of course you will not need one for the solar panel because of the solar charge controller.
If you would like to know more about connecting solar panels together you can do here;
What Size Blocking Diode Will I Need for Solar Panels Connected In Parallel?
You will want to make sure the diode you fit can cope with the amps and voltage flowing through it. You need to make sure that the diode’s capacity is more than you require. Because there are always fluctuations in any electrical system and they need to be taken into account.
The easiest way to calculate for use with solar panels connected in parallel
It is recommended that you find a professional to measure the volts and amps in parallel arrays
So how do you do it without going to the trouble of finding one.
If your panels are all the same wattage, voltage and amperage look at the specs for VOC. This is the open circuit voltage. Let’s use an example of two 12 volt 100w panels with a VOC of 21.2V each.
Because you are using a parallel connection the voltage will remain the same 21.2V VOC. But the amp output for the two panels will be double of that rated on the individual panels.
You don’t want to get a professional in to measure the amp output. And you don’t quite trust what your charge controller is telling you. The easy way is to look at the individual panels maximum operating current or IMP. Which is 5.70A and then multiplied by 2 equals 11.4A.
Just so you know I am using the Renogy 100w Eclipse Solar Panels specifications. So the VOC is 21.2V and the IMP for two panels connected in parallel is 11.4A. But both volts and amps can reach higher levels than that.
If you want to take a look at the Renogy Eclipse you can do so here;
Building In A Safety Margin
So you now need to build in a safety margin because you don’t want fried diodes. The margins should be twice as high as the VOC and 50% higher than the parallel IMP.
So the blocking diode size you would need for this particular parallel array would be 45V 20A. Just so you know lots are described as Schottky Dual Diodes. These have a dual function being both blocking and bypass diodes.
You can buy them at Newark.com. (Just so you know this is not an affiliate link)
Which leads me on nicely to talk briefly about bypass diodes
What Is A Bypass Diode?
Shading can be a problem for either individual solar panels or panels connected in series or parallel. Not so bad for panels connected in parallel. As there are enough connections for the other panels to continue working at full power. Even if one is completely shaded.
But panels connected in series can be put out of action altogether if one of the panels is shaded. Individual panels are also affected by shade which can result in an up to 75% power loss. But how to combat the effects of shade? because it does happen.
This is where the bypass diode comes into its own. Usually incorporated into the solar panel in the junction box. While not stopping the effects of shade it certainly helps combat it so electricity production continues.
How Does A Bypass Diode Work?
Simply put, when parts of the solar panel are in shade. The bypass diode enables other paths for the current within the solar cells to continue flowing. In other words, as the name suggests, bypass the shaded solar cells.
Bypass diodes, also known as free-wheeling diodes, are wired within the PV module and provide an alternate current when a cell or panel becomes shaded or faulty. Diodes themselves are simply devices which enable current to flow in a single direction. Bypass diodes then are exactly as they sound: devices for channeling current by bypassing the solar panel itself. (Source; sunwize.com)
So when buying a less expensive solar panel make sure there is a bypass diode in the junction box. The lack of one could be one of the contributing factors to it’s attractive price but ugly performance.
My Closing Thoughts
95% of the time you will not need to buy and fit blocking diodes for use in a parallel solar array. The solar charger has one built-in so there is no need to worry about it.
There are circumstances where they might need fitting like the one I described or even on a yacht. Where one side of the yacht is in the shade and the other in the sunshine. And it can’t be helped because of the wind direction so might be like that all day.
If you would like to delve deeper into the world of solar panel electrics. You can take a look at my post about fuses and sizing here:
It’s not just about fuse sizes for 50w it covers 30w to 320w solar panels as well
I hope you enjoyed this post and have found it helpful. If you have any questions about blocking diodes and solar panels connected in parallel. Or want to leave your own personal review, please feel free to leave a comment below.