Internal panel amperage

KhellSk · December 2018

Given that I have 2 panels, one 16v 3amp and the other one 20v 5amp connected in parallel to a charge controller.

With full sunshine on it, with the smaller volt panel pulling down the 20v to 16v, wouldnt that raise the internal amps inside the ''20v'' panel to 6.25a ?

WaterWheel · December 2018

From what I understand when mixing panels not only is the higher voltage panel de-rated to match the lower voltage panel but output in watts from the higher voltage panel is also decreased so I suspect the internal amps inside your higher voltage panel would remain about 5 amps.

littleharbor2 · December 2018

When you are giving these stats are you referring to Voc. And Isc. or Vmp. and Imp. and the same for both panels?'

The combined amp total is whatever it is at any given time but the current coming out of either panel isn't going to rise being that the current is flowing out of the panel, not back into it.

BB. · December 2018

If you have a PWM charge controller, mixing 16 and 20 Volt (Vmp?) panels is not going to hurt anything.

~16 volt Vmp panels are typically 32 cell panels, not the "normal" ~18 volt Vmp 36 cell panels used with PWM controllers and 12 volt battery banks.

In the "Old'en days", ~15 volt Vmp panels were used with 12 volt battery banks as "self regulating" panels--No charge controller needed (panel would nearly stop charging as the battery when over ~14-15 volts).

Today, for any "reliable" solar/battery power system, much better to use Vmp>~17.5 volts with a solar charge controller.

Solar panels are "current mode" devices... Normally, the output current of a solar panel is proportional to the amount of sun hitting the panel (as long as the output voltage of the panel is Vmp or less) (Isc short circuit is greater than Imp maximum power by 10%-20% or so).

Also, as folks asked above, you could be mixing up Voc/Isc (open circuit/short circuit) voltages vs the Vmp/Imp (maximum power) numbers we generally use to design a system.

And, Voc/Vmp fall as the panels get hot... On a very hot day under full sun, the solar panel voltage can drop to almost 80% of rated voltage... So a 16 volt Vmp panel would be derated to (*.8=) ~12.8 volts Vmp-hot. Solar panel curves are sort of rounded at the top, so you will still get reasonable current at 1 volt higher (I.e., ~13.8 volts)--But you can see that there is a reason that "12 volt" panels are rated Vmp~17.5 to 20 volts (to allow for hot panels and cold batteries).

For MPPT type systems, mixing panel voltage is more of a hit/miss issue... There are now two or three "peak Power" points with two different panels in parallel--And which peak power the MPPT controller will pick--Only the designer (and/or testing) will know.

If you want more details, this thread may help:

http://forum.solar-electric.com/discussion/5458/two-strings-in-parallel-with-unequal-string-voltages

-Bill

KhellSk · December 2018

BB. said:

If you have a PWM charge controller, mixing 16 and 20 Volt (Vmp?) panels is not going to hurt anything.

~16 volt Vmp panels are typically 32 cell panels, not the "normal" ~18 volt Vmp 36 cell panels used with PWM controllers and 12 volt battery banks.

In the "Old'en days", ~15 volt Vmp panels were used with 12 volt battery banks as "self regulating" panels--No charge controller needed (panel would nearly stop charging as the battery when over ~14-15 volts).

Today, for any "reliable" solar/battery power system, much better to use Vmp>~17.5 volts with a solar charge controller.

Solar panels are "current mode" devices... Normally, the output current of a solar panel is proportional to the amount of sun hitting the panel (as long as the output voltage of the panel is Vmp or less) (Isc short circuit is greater than Imp maximum power by 10%-20% or so).

Also, as folks asked above, you could be mixing up Voc/Isc (open circuit/short circuit) voltages vs the Vmp/Imp (maximum power) numbers we generally use to design a system.

And, Voc/Vmp fall as the panels get hot... On a very hot day under full sun, the solar panel voltage can drop to almost 80% of rated voltage... So a 16 volt Vmp panel would be derated to (*.8=) ~12.8 volts Vmp-hot. Solar panel curves are sort of rounded at the top, so you will still get reasonable current at 1 volt higher (I.e., ~13.8 volts)--But you can see that there is a reason that "12 volt" panels are rated Vmp~17.5 to 20 volts (to allow for hot panels and cold batteries).

For MPPT type systems, mixing panel voltage is more of a hit/miss issue... There are now two or three "peak Power" points with two different panels in parallel--And which peak power the MPPT controller will pick--Only the designer (and/or testing) will know.

If you want more details, this thread may help:

http://forum.solar-electric.com/discussion/5458/two-strings-in-parallel-with-unequal-string-voltages

-Bill

Thanks!! They r actually 22yrs old 15.9v

I would have to count the cells

jonr · December 2018

Say you have a 32 cell panel and the diodes that it is made of have a Vf of .6V* each. Greater than 19.2V could cause reverse flow and heating. A 36 cell panel will exceed 19.2V, so I'd use a blocking diode. Or test to verify that Vf is > .6V.

* - number from Bill. But my guess is that you need > .7V to get significant current, so it would be OK.

BB. · December 2018

In general, the solar cells are "diodes" that are reversed biased (for forward current flow in panel) and able to block (without damage) upwards of (roughly) 12 volts. (cells "go high resistance" if in shade/dark).

That is why there are are "bypass" diodes every ~24 cells maximum.

This is to prevent damage if there is, for example, a shaded cell (or panel) in series with one or more other series connected panels (such as two "12 volt" panels in series to charge a 24 volt battery bank).

You can put a series connected "blocking diode" inline with a panel--Generally done if the impressed voltage is over ~12 VDC. Higher voltage can cause excessive leakage current.

However, depending on what type of diode you get, they have voltage drop themselves (0.2 to 0.7 to 1.0+ volt drops are typical). Installing blocking diodes is both a loss of power (4 amps * 1 volt drop = 4 Watts of self heating) needing a heat sink... And you can have issues with electrical insulation issues (many diodes have metal packages/heatsink connections that are "live" electrical connections).

And diodes can fail either shorted (no good as a "diode") or open (like a fuse)... Just more stuff and connections that can go wrong.

Using blocking diodes is something that is best avoided, in my humble opinion.

-Bill

jonr · December 2018

Not clear to me what Bill is saying about how much applied voltage a solar panel can withstand before damaging amounts of current flow. Certainly more than 12V for a 16 or 20V panel. My guess is 22V and 25V respectively.

Below is a good image showing which way solar cell diodes are oriented and why applying a moderately higher than normal voltage to the + terminal can cause reverse panel flow. If Vf (f = forward) is exceeded.

The anode of each cell is normally at a greater voltage than the cathode - hence forward biased.

Reverse bias can occur if part of a series string is shaded. But this has nothing to do with what is being discussed here (paralleling panels of different voltage).

Bypass diodes also allow a series of cells or panels to continue supplying power at a reduced voltage rather than no power at all when there is some shading.

http://www.creative-science.org.uk/projects/solar/solar_panel.GIF

https://qph.fs.quoracdn.net/main-qimg-3ca80789e7b11585cc66df42ce46421f

mike95490 · December 2018

Remember each blocking diode need to have it's own isolated heat sink.
Say the panel is rated for 5 A, and you have a decent schotkky diode with a .5V drop at 5A. That's 2.5w of heat to get rid of, in the summer heat.

Internal panel amperage

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