Formula for 2 voltages in Parallel?

[davedottcom]

I'm not even sure if this is possible...I can't find any formula that solves for it!

Here's the scenario:

• Solar Panel A puts out 27.5 Volts DC

• Solar Panel B puts out 24.5 Volts DC

Can you parallel them together?
If so, what would the combined output voltage be?

Would it be the average?
26 Volts? :-?

 

[drbond24]

Depends on if they are connected in series or parallel and what the orientation of their polarities are.

Edit: I see where you said they are parallel. Missed that the first time. I think I'll wake up some more before I comment further rather than risk the taste of my foot in my mouth this early in the morning.

 

[bcorbin]

Ask yourself this:

Would I put two batteries of different voltage in parallel?

Basic circuit laws say that two items in parallel will always be of the same voltage, or events will occur to make that true.

Assuming your panels are not equipped with some sort of variable output, I would say this is a bad idea.

 

[LarryFine]

We'd really need to know the outputs with a load, but the likelihood is you'd get the higher voltage with the lesser-output panel contributing less.

Are these identical panels with a voltage difference, or different panels?

 

[davedottcom]

Are these identical panels with a voltage difference, or different panels?

Well, at first I was thinking of 2 different panels with 2 different voltages.

But, even if the panels are identical, they will still vary in output voltages I would think.

Lets say we have 2 identical panels.

In full sun they can each produce the following:
24 volts DC
7.1 Amps
170 Watts

Now with (2) of them combined together, in full sun (connected in parallel) we would have:
24 Volts
14.2 Amps
340 Watts

So far so good?

But, now a tree branch blocks the sun from hitting the second panel, while the first panel is still in full sun.

The shaded panel's voltage output will drop drastically.

So, now you have 2 different voltages paralled together for a total of ??? volts?

 

[kingpb]

I would imagine an appropriately placed output voltage regulator is going to take care of this issue.

 

[bcorbin]

I think I understand your question a little better now, Dave. Originally, I thought you meant two different types of PV cells. If that is true, then I still stand by my original comments.

However, if they are the same type, and intended to output the same voltage, then they can be connected in parallel. They will both produce the same voltage, and the fully-illuminated one will produce more of the current than the partially-shaded one.

 

[coulter]

As Larry said, you will need some load/voltage data. To get the two voltage measurements, the two cells had to be separated. So when they were measured, were they open circuit or loaded?

Where am I going? I don't know what the model is.

I would have suspected each panel can be modeled as a voltage source with a series impedance. But if it is, then the open circuit voltage is pretty much the same, partially shaded or not. If so then the parallel cells will both have current output and the voltage will be somewhere in between. I have four cells out at my cabin. As I recall, this model fits - but I don't know that.

However, if the open circuit voltage is lower for the partially shaded one, then the model is pretty screwey. Have to think on that one.

edited to add - which is pretty close to bcorbin's answer

carl

 

[davedottcom]

Maybe my confusion is in the way I'm looking at it. (As 2 seperate panels)
I think as soon as they are connected in parallel, they actually become one panel!?!?

Sort of like seperate cells in the same battery. You can have a "dead" cell in a battery which will bring down the overall voltage right?
I'm not sure if comparing solar panels to batteries is a fair comparison or not.
I obviously don't know enough about either one!

If I understand correctly the PV panels produce a voltage that will vary depending on the exposure to the sun, temp etc...
So, they don't put out 24 volts immediately or constantly. Each panel will experience many different variables that will effect it's output voltage.

Am I completely wrong about that?
Is the voltage ALWAYS 24 volts?

 

[bcorbin]

The voltage will be 24V or very close to 24V over a suprisingly large range of illumination. At some low value of illumination, the voltage will decrease rapidly to zero. It's an logarithmic curve (someone with expertise verify this?), typically with a very sharp "knee". As long as the two panels are both within the very nearly linear portion of the curve, everything would be fine.

 

[davedottcom]

Ok, well if the Voltage does remain a constant 24 volts (For the most part!) That would make much more sense to me!

I think I'm getting it now. The variable would be the amount of current, not the voltage.

So, assuming that is true... the panel will produce zero volts right up until it reaches the point of "Sun saturation" (for lack of better term) in which it was able to produce the 24 volts.

So there is no formula. The voltage is simply constant.
And paralleling 2 different voltages is a no-no! (Noted!)
Someone could have just said that!

I'm getting a couple of different PV panels soon... can't wait to play with them!

Thanks

 

[coulter]

... The variable would be the amount of current, not the voltage.

I don't think so. The variable is the series impedance. Draw it out.
Voltage source (relative constant as bcorbin says) in series with a resistor. Output at the terminals is dependent on the current draw.

... So there is no formula. The voltage is simply constant.
And paralleling 2 different voltages is a no-no! (Noted!) ...

I don't think that is the case. Again as bc says, it just isn't linear. Yes you can parallel two PV, on shaded, one not.

carl

 

[mic]

There is no issue with a small difference like this. Even within the panel there are likely solar cells wired in parallel with different output voltages.

The voltage will probably be pulled down to the lower level similar to charging a 12V bat with a 16V charger. The only risk is that the lower voltage panel could become a load on the higher voltage panel in the absence of an actual load, in effect reversing the current flow in that panel. Many panels have 1 or 2 diodes to protect against this.

 

[rattus]

deja vu:

deja vu:

Reminds me of the time that an Army photographer tried to boost his 4.5V flashgun with a 6V battery in parallel. Couldn't understand why both batteries quickly failed. Doesn't sound like a good idea to me. You could install series rectifiers to prevent reverse currents in the low voltage bank, or maybe this is inherent in the photocells. Anyone know?

 

[winnie]

Don't try to think about it as connecting two devices with different voltages in parallel. Think of each panel as a device with a particular voltage/current curve. The curves will be different for different panel types, and different for the same panel in different conditions (sun, temperature, age, etc). But each panel can be described by its voltage/current curve. At open circuit you will measure one voltage; at short circuit you will measure 0 voltage but some current, and everywhere in between you will have a combination of voltage and current.

bcorbin made the key point in post #3. Two devices electrically in parallel have the same terminal voltage, and if they don't start out with the same terminal voltage, something will have to give until they have the same terminal voltage.

In the case of a stiff voltage source like a lead acid battery, you will see lots of current flow, discharging the higher voltage battery and charging the lower voltage battery.

But in the case of a solar cell, which is not a particularly stiff voltage source, the load current will divide between the two panels such that the output voltage of each is the same.

The idea of thinking of these systems as stiff voltage sources in series with a resistance is a rough approximation to this, however for solar cells the voltage/current curve is not as linear as a series resistance would imply. But to get an idea of what is happening, imagine a stiff 30V source in series with a 5 ohm resistance, and a 25V source in series with a 4ohm resistance. Put these two resistor limited sources in parallel, and then connect this combined source to a 1 ohm load.

-Jon

 

[coulter]

jon -
As you know, it's a mathematical model. In the model, voltage sources are capable of infinite current - limited only by the series impedance. The stiffness is the series impedance.

The only difference between the battery example and the PV example is the series impedance. A 24V truck starting VRLA might be a few milliohms. My 55W, 12V PVvs are likely an ohm at noon in June in Arizona.

However at noon in December at Latitude 65, they are likely several ohms.

carl

Please ignore this posting. I have come to the certain conclusion that I absolutely do not understand the model for PV cells.

carl

 

[tallgirl]

The output of a solar panel is a curve with amps on one axis and volts on the other. Even in full sun, if the load is higher, the voltage output is lower,and if the load is lower, the voltage is higher. There are devices, called Maximum Power Point Tracking charge controllers, that try to find the "maximum power point" by increasing and reducing the load to see if the power output goes up or down.

What does this mean for the subject instance? The higher Voc panel will experience the greater load and move further down the V curve (the value of V decreases from Voc to Isc) and the lower Voc panel will see less load and it will move up the V curve. The result is that the lower Voc panel will have a lower load on it, but the terminal voltages will all be the same.

Rattus asked about bypass diodes. Most have them already. They wouldn't apply here as V quickly falls from Voc at which point the lower voltage panel will get to contribute. This time of day (7:15AM CST) my array starts creeping up to 80VDC, then the charge controller wakes up, applies a load, and drags the array back down to 60VDC trying to charge batteries. It gives up, the array Voc goes back to 80VDC, and the cycle continues until the sun is actually up and the fog burns off.

As regards paralleling the panels in question, once you've added bypass diodes and connected them in parallel, get a 12V charge controller and hook them up to a 12V wet battery, and hook that up to an inverter -- free electricity! The panels will be held at or near 14.4VDC when there is no load on the inverter, and who knows where with a load.