Use of diodes when connecting solar panels in parallel?

I have a bunch of 24 volt 270 watt solar panels.  Eventually I'll be putting three in series, then combining the strings in parallel, but for a temporary measure, I want to just connect two in parallel.  I've been in tv repair for over 40 years so electrical theory doesn't confuse me, but solar systems are a different story.  I've used four smaller panels to run a car radio display at an old car show swap meet for the last 12 years, and I just ran them in parallel, then into the charge controller.  Now I'm reading about blocking diodes, and I understand what they're for and that some people use 'em and some don't.

My question is, physically, where do you stick them, and related to this, what is the purpose of using a combiner box?  Is the box just to provide a weatherproof place to make the connections?  Are they used to provide the place to stick the diodes?  I see they have circuit breakers inside, and it appears there's one for each panel, or string of panels, but I don't see the point.  If a panel can generate 10 amps, you'd want a breaker bigger than that so it doesn't trip at full output, but what could trip it then?  You can't get more than 10 amps even if there is a short.

This should be common sense, but since I don't have a combiner box yet to look at, can I assume if it's made to combine four panels or strings of panels, you can connect just two or three and save the unused ones for adding on later?

Thanks bunches;

caradiodoc (author) 2 years ago

Thank you all for the dandy information. These are store-bought panels. I never considered that you might think I have the time to solder a pile of cells together. Wouldn't know where to start.

I popped the cover open earlier tonight on the box on the back of one of the panels, and I found four circuits, (wires), coming in from the cells, and three diodes between them. From what I can tell, those diodes are to prevent one section of the panel from feeding into a different section that might be shaded. It also looks like when multiple panels are in series, one with strong output could pass its current through the shaded one with only the voltage loss of those three diodes, and not whatever the loss would be through the cells.

Regardless, I'm going to use diodes in the final installation with an Outback system that can handle up to 150 volts in. Half a volt dropped by a diode I don't think will be noticed! Thank you again. I'll be back with more questions in the near future.

iceng2 years ago

If you are charging a capacitor or a battery you need a one-way valve ( Diode )

Because as light fades the solar panel turns into a resistor and will discharge your battery !

If you are just running something off of the solar panel ( like a motor ) there is no need for diodes....

Many solar applications are low voltage and adding a diode subtracts the diode forward voltage drop Undesirable from the output voltage about 0.32V.

This diode situation usually needs an extra solar cell to overcome the loss !

The last pic demonstrates a circuit that acts like a diode but has a highly desirable magnitude less forward voltage drop of o.04V !!!

petercd2 years ago

The diodes are to prevent the battery discharging into the panel at night. Most folk connect the panels to an inverter which sometimes have their own blocking diode, some simple checks with a current meter will indicate if you need one in your setup or not.

Mine I put in my diy charge controller housing, but they can even be on the panel too if that suits you.

First some terminology. A solar cell is a stand alone silicone wafer with no other wafers soldered to it and it's not in any kind of housing. A solar panel it a grouping of cells in a series/parallel configuration of one form or another typically inside a housing. Store bought panels will typically have a protection diode already in place. If your panels are a grouping of other panels then you won't need a diode. If your panels are made up of individual cells you've soldered together and put in a housing or frame then you will need a diode. As with most electronic devices you'll want to place the diode on the negative lead between the panel and the battery. Really it doesn't matter but the negative lead is the general rule of thumb. So long as you have the polarity of the diode correct.

The combiner box is just a water proof junction box for the panels. A single node to wire all the panels to so you have a single cable going to the charge controller. Some have breakers to protect the system as a whole. If the charge controller fails and has an internal short it will try to pull all the current it possibly can from the panels. Thus becoming a fire hazard. The controller should have a fuse on both the input and output for protection as well. The breakers are there as added protection and you may not want or need them at all. Each solar setup is different and will use different parts. You'll have to work through all the numbers and figure out what you'll need.