diy solar

diy solar

Solar panel strings: Parallel & Series explained


Works in theory! Practice? That's something else
Sep 20, 2019
Key Largo
When solar panels are hooked up in series you connect the minus of one panel to the plus of the next panel.
The voltages are summed, but the current remains the same:
Putting panels in series is desirable as it keeps the amperage low, and amperage is the key factor in cost of the wire.

Now let's look at panels in parallel. Here all the negatives are connected to each other, and all the positives are connected to each other. So the voltage stays constant and you sum the currents.

With a PWM charge controller you'll want to put the panels in parallel as those devices reduce the voltage to the battery's voltage; and would otherwise waste a lot of power.

When using an MPPT charge controller it will have a maximum voltage and current that you can put on any input. You typically put the most panels you can together in series (called a string); but not so many you exceed the voltage. You repeat that for as many panels as you have and then connect the strings together in parallel.

For example, if you had 6 panels with Vmpp= 22.5, Impp=5.75 and an MPPT with 60 volts and 20 amps max; then you might arrange your panels into three parallel strings of 2 panels in series.

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When a panel in a series is affected by shade, it can reduce the voltage of the entire string, reducing the power output of the string.

One way around this is to use diodes as shown below:

Note that most modern solar charge controllers have blocking diodes built in to prevent draining your batteries at night.

Anyone know a formula to figure out what the diode rating should be based on your panel's characteristics?
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Designing for worst case, the diode should have a reverse blocking voltage greater than the maximum string voltage (not too hard with modern diodes). It also needs to be able to handle the maximum current of the string. This also affects the heat generated in the diode. When we were running tests on the diodes available for that application, (in a compact case with male/female MP4 connectors), they dropped just under a volt across them. That means they will dissipate just under a watt per amp of current they carry. If they're carrying 10+ amps, they're going to dissipate 10+ watts, and they ran EXTREMELY hot, like burn your hand hot, and worry about melting the plastic housing kind of hot. We ended up using power diodes that could be attached to a heatsink to properly handle the heat generated.
The MP4 style ones looked like this:
MP4 diode.jpg
Ideally, a solar bypass diode should have a forward voltage (VF) and a leakage current (IR) as low as possible.
Therefore, the PV junction box manufacturers use Schottky diode for its low forward voltage. The choice of maximum reverse voltage is made as opposed to the number and voltage of the solar cells in the series.
The manufacturers usually determine the breakdown voltage Vc. Its value depends on the solar cell technology (poly-silicon or mono-silicon) and the manufacturing process.

From the same article as above:
The worst possible case with PV panels is when the absence of solar bypass diodes causes a fire. This is possible under certain conditions, such as when a leaf completely covers one solar cell of a series string. Under these shaded conditions, those covered solar photovoltaic cells become consumers of electricity instead of producers.
This means that while the rest of the solar cells are trying to pass current, the shaded solar cell is drawing current. The byproduct, of course, is heat. After many days of operating under these conditions, this additional heat and multiple temperature cycles can cause a fire.
This scenario must be avoided at all costs. Therefore, solar bypass diodes are required in all solar PV panels that are certified for use in the U.S.
This implies it's NOT a good idea to block off portions of a panel to reduce the power unless diodes are being used.

Serials & Parallel: Mixing & Matching different Solar Panels

In general, there are two rules:
  1. Same Amps okay to series connect
  2. Same Voltage okay parallel connect
You can also use multiple SCCs to connect to a single battery, this allows you to overcome any mismatch issues.

Here's a video that demonstrates what is going on:

Here's a video demonstrating how multiple SCCs can work around mismatches.