Low voltage panel clamps high voltage connected in parallel

[Kassim]

As a beginner, I wonder where excess voltage is sunk when a higher voltage panel is connected in parallel with a lower voltage panel How does that affect the longevity of service life of the panels. I don't do it but if it happens what will happen, will it cause either of the panels to overheat? I notice it is the lower voltage panel that overrides higher one by forcing the output down. Could the effect be compared to two zener diodes of different ratings say 36v ZD to 33vZD connected in parallel I understand that the high voltage pv will always operate under short-circuit.
Have a wonderful time.

[inetdog]

It depends on just how different the open circuit voltages of the two panels are.

There is a rule of thumb that says that when your panels are feeding an MPPT device, whether GTI or CC, you can put two panels in parallel without losing much potential power if their voltages (Voc compared to Voc or Vmp compared to Vmp) are within 5% of each other.

You can look at the curve of current versus voltage (for a given amount of usable light on the panel) and see that when zero current is being drawn the voltage is at its highest (Voc). As you draw more and more current the power (product of V and I) rises from zero until it reaches a maximum. Beyond that maximum the power starts to go down again until you reach a short circuit condition (V=0, I=Isc) and the output power is again zero.

Unlike a zener diode, PV panels have a gentle curve of voltage versus current, rather than the sharp knee (near right angle in the curve) that a zener diode has.

If you plot the power output on the same graph (with different scale of course) you will see that as you move a small distance (in current and voltage) away from the Maximum Power Point (MPP) the power output changes only slowly. This happens because the MPP is a maximum and the curve is therefore level (but not flat) at that point.

Now take two panels in parallel. Say one of them has Vmp = 32V and the other has Vmp=36V. That is a difference of over 10%, so you will not get the full rated power from both panels at the same time.
Instead the device that the panel array is feeding will look for the MPP of the two panel combination.
If you start at the MPP of the higher voltage panel (call it panel A) you will find that the lower voltage panel (B) is not producing much power at all, and pulling more current from the pair by dropping the load voltage will decrease the power from A slightly but increase the power from panel B by a lot, since the slope of the power curve for B is high at that point.
The end result is that the MPPT device will end up at a set point somewhere midway between MPPa and MPPb.

If you are feeding into a PWM charge controller, on the other hand, the array voltage will drop close to the battery terminal voltage when the CC is running at full charging power, independent of the voltage rating of the two panels. Both panels are likely to be operating below their individual MPPs.

Finally, if the voltage difference between the two panels is extreme, (say a factor of two) you may actually see some current flowing in the reverse direction through panel B, driven by the higher voltage of panel A. This is not a desirable configuration and can actually damage panel B.

[Kassim]

A good explanation indeed.

[Mike90250]

This would be something that happens at installation via ignorance or fraud by a contractor, or some sort of catastrophic damage to a panel, generally, it's just silly to choke a higher voltage panel with a lower voltage panel.

[Kassim]

It's interesting you know this. You must have done it at one time or the other in your solar life at least for experience and experiments to know what to tell to a questioneer with authority.

[Sunking]

OK you could be talking two different characteristics. Either mis matched panels, or using a PWM controller. Sounds like panels.

When panels are in series the Voltage adds, and current is limited to the lesser of the panels. So lets say you have a 100 watt 18 volt battery panel with a Imp = 5.0 amps in series with a 300 watt, 30 volt Imp = 10 amps. What you end up with is 48 volts @ 5 amps = 240 watt panel from 400 watts. Solution, throw the 100 watt panel in the garbage so you can have at least 300 watts to work with

In Parallel current adds, and voltage is limited to the lowest voltage panel. Same two panels in parallel gives you 18 volts @ 15 amps or 270 watts from 400 watts. Solution is throw the 100 watt panel in garbage so you can have at least 300 watts to work with.

Moro to that story is use matched panels. In Parallel Voc needs to be roughly equal. In Series Imp needs to be roughly equal. Otherwise you are screwed.

OK now for PWM controllers. Most important thing to remember is Output Current = Input Current. Once that math soaks in your skull lets connect that 300 watt panel up to a PWM Charger with a 12 volt battery. How much charge current is there from the 300 watt panel? 10 amps, as that is what the Imp of the panel is, 10 amps. So you have 10 amps charging into a 12 volts battery = 120 watts from a 300 watt panel or 60% loss of power. What about that 100 watt low voltage battery panel it puts out 5 amps or 60 watts out of 100 watts. Solution is throw the PWM Charge Controller and the 100 wat panel in the garbage and go get a 25 amp MPPT Charge Controler.

Again once you understand the math is child's play 5th grade math. A MPPT Controller Output Current = Panel Wattage / Battery Voltage What does this equation tell you? It is screaming loud and clear Input Power roughly = Output Power. You put in 30 volts @ 10 Amps (300 watts) input = 25 amps @ 12 volts or 300 watts on the battery output. Doh!

Moro to the whole story. Never use low voltage expensive battery panels or a PWM or inexpensive fake MPPT controller. If you do, you are screwing yourself and like it. Smart money uses inexpensive matched Grid Tied panels with a real MPPT charge controller. Much less expensive than the other way. It takes at least a 300 watt PWM system to equal a 200 watt MPPT system. The 300 watt PWM system takes at least 2 panels, tiwice the wring and racking. Just in panel and controller cost the PWM system will cost you $600 to $700. The 200 Watt MPPT system will cost you $400 to $500. and hell even at 300 watts is less than PWM with 50% more power.

So what did you buy? Are you smiling or frowning?

[Kassim]

😁😁 200 watt 36v 5 amps + 140w 7amps 21v gives me 15 amps to charge a 12V battery, painlessly.. I noticed that the 21v panel clamps the 36v as long as charge is on, the clamping occurs when controller switches off ie when the battery is full. As far as the battery is concerned it is receiving what is ok for its consumption and controller not mppt takes care of the charging. Pls note that I try questions that comes to mind for future reference in places it may be needed.

[Mike90250]

It's interesting you know this. You must have done it at one time or the other in your solar life at least for experience and experiments to know what to tell to a questioneer with authority.

It's called education, Simple math, electricity and electronics. first year stuff. Never had to actually do it to know that it is a physical law.
You are welcome to play with stuff, and even burn your house down, but that is why building codes exist, to protect innocent people from those who want to learn by experience,