Matching solar panel voltages

[kenoli]

I have a small solar system designed for short term outages and potential earthquake emergencies. It consists of a couple of panels, a controler, a "control panel" that I wired for my use, some small 12 volt battery chargers for 18650 batteries, outputs for a ham radio, some 12 volt storage batteries and an inverter. I have a few of 12 volt solar panels wired in parallel. Someone just gave me a solar pane that puts out 38 volts in the sun and seems to work, generating amperage upon demand. Can I hook up three 12 volt panels in series (which would add up to 36 volts) and then hook up those three running in series in parallel with the 38 volt panel? Any particular problems to look out for?

Another question: My controller will accept any input voltage up to 40 volts and puts out 12 volts. When I hook up say 38 volts to it, does it convert the excess voltage into current or does it just burn it off? I realize I am not providing info on what kind of controller it is, but I am interested in how controllers commonly deal with extra voltage. I presume mine is pretty standard.

--Kenoli

 

[JeepDaddy]

You need to keep the sum of the open circuit voltages, Voc, of the panels in series below the controller maximum voltage rating, 40 volts in your case.

does it convert the excess voltage into current or does it just burn it off?

The controller converts the solar voltage (higher) and current (lower) into battery voltage (lower) and current (higher).

 

[Bud Martin]

If it is typical MPPT Charge controller, then it is just a smart Buck converter that step down the PV input Voltage to the lower Voltage and higher current for charging the batteries.

 

[kenoli]

Does anyone have an answer to my first question about matching the voltages of the solar panels. Can I add the 12 volts panels by wiring them in series to 36 volts and matching that in parallel with the 38 volt panel?

Can I deduce from your reply, Bud, that the controller will give me more current from the 38 volt panel by converting the voltage it has to decrease to 12 volts into increased current flow when needed?

Boy the technology has really increased since I was building controllers with 555 timers and individual transistors and zener diodes back iin the 1970s. Back then most inverters were square wave inverters with modified sine wave inverters just coming onto the market. There were really no true sine wave inverters. It was less than a decade back then since IC chips even snowed up. Transister electronics was still somewhat new. I was repairing tube instrument amplifiers using a huge oscilloscope from the jet proulsion laboratory I had bought at a second hand store and working with a ham radio that still had output tubes. I was living in the mountains miles from a telephone. All our electricity came from a pelton wheel generating power from high pressure water. Solar panels were even somewhat new and I had gotten into electronics building controllers for the pelton wheel generator for ourselves and others in the area and then started building solar controllers for solar panels as they showed up. They were a lot simpler.

 

[740GLE]

If the VOC of the controller is limited to 40v, be careful with of its current output limit, you may find it can only output a certain amount so you may not be gaining too much by adding more panels.
Also VOC of that panel new may exceed of the charge under colder conditions.

 

[JeepDaddy]

Does anyone have an answer to my first question about matching the voltages of the solar panels. Can I add the 12 volts panels by wiring them in series to 36 volts and matching that in parallel with the 38 volt panel?

For one, your 12 volt panels are most likely not 12 volts, they are probably closer to 18-19 volts open circuit (Voc)

For two, see post #2.

 

[Sun]

Seems like the OP’s question is: if everything clearly stays below 40V, will I have an inefficient setup with a 36V input and a 38V input connected to the same controller? Will the controller be able to convert both inputs simultaneously to a single output with a lower voltage (12V) and a higher current?

I know the answer would be YES if both inputs were the same voltage but I don’t know how much energy would be lost with different input voltages (of 36V & 38V). Does anyone know?

 

[Bud Martin]

You can learn about mismatched panels and the calculation to get the result of the mismatched panels:

Mixing solar panels – Dos and Don’ts • SOLAR POWER SECRETS

Mixing Solar Panels: Discover Now Dos and Dont's of Mixing the Same Types or Different Types of Solar Panels. Act Now to Save Your Money!

solarpanelsvenue.com

 

[DIYSteve]

I would be wiring your 12v (18v open circuit) panels in series of two then wiring the pairs in parallel including your newer higher voltage panel (parallel into the 12v pairs).
This would keep everything fairly balanced and if you have a good MPPT SCC, (I use the EPEVER Tracer 4215BN) then you can rest assured your system is good.

Also by wiring 2x 12v panels in series then the pairs in parallel, you then effectively have a 24v PV system which means that your SCC can handle double it's rated 12v PV wattage.

 

[Sun]

So in summary from everyone's comments and the excellent article on the Dos and Don'ts of Mixing solar panels:
-It is inefficient to mix panels with different specs because the total wattage you may collect can be limited by the weakest panel's current or voltage (depending how they are connected).
-In cloudy weather, some panels also may produce much less than their specs which even further limits what the better panels may be capable of providing.
-The 3 panels you believe to be providing 12V each (for a total of 36V in series) may actually be providing 18V or so each in reality, so you need to double check that you're using the open circuit voltage value (or measure it), and make sure you're using the correct numbers for your calculations. Your MPPT controller will of course burn up if the three 12V panels are actually providing 18V each, or if your 38V panel is actually putting out more than 40V.
-If your 3 panels in series ARE actually producing 36V, your large panel is producing 38V, and you connect these in parallel it is not possible to calculate exactly how much wattage you are losing (due to their difference of 2V) because you didn't tell us how much current each panel produces. Under full sun you probably won't be losing much though. (WATTS lost = 2V x total current expected from all panels).

 

[DIYSteve]

I am assuming that the 38v panel was it's open circuit voltage so most likely a decent 24v panel. This is why I suggested pairing 2x 12 volt panels in series then parallel connecting everything to the SCC.
If I was wrong about the 38v being it's open circuit voltage then disregard my suggestion.

 

[Sun]

Yes, that’s a great observation. It sounds like the OP may not be using the open circuit V’s for the calculation. This video shows what’s possible but also doesn’t address the efficiency calculation if the different panels open circuit voltages aren’t close to one another.

 

[Hedges]

40V is a low enough input limit that it might be a PWM controller not MPPT. In which case all you get is sum of Imp from parallel panels/strings.
If MPPT, then Vout x Aout = Watts out = Vin x Ain.

Yes, you can parallel one panel having about 36Vmp with two series-connected panels having about 18 Vmp. If one totals 36V and the other totals 38V, it is well enough matched to get near full power.

But if the panel is 36 Vmp, then on a cold day, maybe on a normal day, Voc will exceed controller's 40V max spec.
Even adding up Voc rating from panel label isn't sufficient. You need to also get temperature coefficient of Voc from data sheet (typically between -0.2% and -0.4% per degree C) and adjust for the historical record coldest temperature of your location.

 

[Zwy]

I have a small solar system designed for short term outages and potential earthquake emergencies. It consists of a couple of panels, a controler, a "control panel" that I wired for my use, some small 12 volt battery chargers for 18650 batteries, outputs for a ham radio, some 12 volt storage batteries and an inverter. I have a few of 12 volt solar panels wired in parallel. Someone just gave me a solar pane that puts out 38 volts in the sun and seems to work, generating amperage upon demand. Can I hook up three 12 volt panels in series (which would add up to 36 volts) and then hook up those three running in series in parallel with the 38 volt panel? Any particular problems to look out for?

Another question: My controller will accept any input voltage up to 40 volts and puts out 12 volts. When I hook up say 38 volts to it, does it convert the excess voltage into current or does it just burn it off? I realize I am not providing info on what kind of controller it is, but I am interested in how controllers commonly deal with extra voltage. I presume mine is pretty standard.

--Kenoli

Get another charge controller and wire it to just the one panel. Than put this in parallel to your current CC. Ensure the VOC of the panel is not greater than CC input rated voltage.

 

[kenoli]

Wopw! thanks for all this. I think I can harvest exactly what I wanted from it. I will measure my VOC of all the panels and read the cautions and see what I can do. I do have an MPPT controller.

Thanks all,

--Kenoli

 

[kenoli]

The following queswtions are just to clarify for myself some dynamics of solar panels. Your confirmation or correction would be appreciated.

In establishing the voltage of a penal for purposes of serial or parallel wiring of multiple panels, one should always use the VOC, that is, the voltage each panel puts out at no load. Is that right? The max current through panels in series is always the max current of the panel with the lowest maximum current rating. Should an MPPT type solar controller show the VOC of a panel at its input or does it pull the panel down to a lower operating voltage? Or...will it show the VOC at input iif little or no load is drawn from the conrtroller but pull the coltaget down if there is a heavier load on the controller output?

The 38 VOC panel I described in this question shows the same voltage when attached to my MPPT contoller with a minimum load place on the conroller.

 

[DIYSteve]

The MPPT Controller cannot show the VOC as this is the panels open circuit (or unconnected) voltage. Once the panels are connected to the SCC they are in a circuit so only the voltage reaching the SCC can be measured. When connected in parallel it is the lowest voltage that's matched, the current from each set in series which is then connected in parallel is then accumulated. The resistance of the wire can affect the current reaching the MPPT SCC but it controls the total current reaching it before it converts to the battery voltage. Example 24 volts 10 amps is converted to 12 volts 20 amps for a 12v battery system.

 

[Hedges]

During conditions of no charge current (e.g. batteries floating), there will be no current through PV wires so voltage at MPPT controller will be exactly voltage at PV panels, which will be Voc of the panels.

If two different PV panels are connected in parallel, Voc of the combination will be Voc of the panel with lower Voc (or slightly higher).
For instance if a "12V" panel with 22 Voc was connected in parallel with a "24V" panel with 44 Voc, all the current produced by 12V panel plus most of the current from the 24V panel would go through the (forward biased) PV cell diodes of the 12V panel, driving it slightly higher than its spec Voc.

The MPPT controller display would reflect Voc of the PV array when it isn't delivering current. While charging battery, it would reflect Vmp of the array, minus whatever voltage drop in the wires.

The MPPT controller has to be able to withstand Voc of the array, because that is what it will be exposed to.

 

[Tecnodave]

Short cut to answer the question......most polycrystalline solar panels have the same voltage per cell...count the number of cells in each string...if they match you can connect mismatched strings in parallel.....example most 12 volt panels (really 18 volt) are 36 cell where 24 volt panels (really 36 volt) are 72 cell.....about .5 volts per cell. The ones that are hard to integrate this way are the 60 cell grid tie types...usually about 30 volts. I would guess that the OP has 36 cell and 72 cell panels.

There are exceptions to this..I have several arrays of Panasonic HIT multilayer 96 cell panels that are 53.5 volts. These are monocrystalline cells with an amorphous polycrystalline cell on top of the monocrystalline cell. These will not match up with any other panel due to the higher than normal cell voltages of approx .600 v.p.c.