Series Wiring of Different-Wattage Solar Panels?

[Beausoleil]

Just a quick sanity check:

I've recently acquired used 75W and 110W solar panels (one each). They have similar open circuit and rated voltages, and the rated currents are 4.4A and 6.6A, respectively. Any harm in running them in series (and using an appropriate charge controller for my 12V battery bank). I have some 10 AWG cable on hand, so I'd like to use that if possible and keep the current down, rather than running them in parallel. The round trip run would be about 50' or so.

Am I barking up the wrong tree, or just bite the bullet, paralleling them and buy #8 cable?

[cal40john]

You need an MPPT controlller to do series. Downside for in series is if any of the cells in either panel is shaded you lose the output of both panels. In parallel only the panel with a shaded cell stops working.

John

[hellosailor]

Either way, buy the MPPT controller and you gain about 10% in effective power transferred from the panels into the batteries, no matter how you set them up.

I'd argue opposite John wrt wiring them up in parallel though. If you have one panel in shade and the shaded cells knock the output down, say from 16v to 11v, that panel is not going to charge at all. If you have two panels wired up in series though, you're starting out with 32V instead of 16, and that same 5v drop still leaves you with 27V which is enough to ensure you still are charging the batteries.

You'll still have a net power loss, but my understanding is that with the MPPT magic, you'll have LESS of a loss because you won't be losing one panel entirely.

[roverhi]

The installation wiring diagram for my mppt controller shows wiring different output panels in parallel (nominal 12 volt). They only reccomend wiring matching panels in series (nominal 24 volt). PV panels put out 18+ volts at full output so it's not like you are skimping on voltage by wiring unlike panels in parallel. My controller is a BZ Products 250 running two Kyocera 130 watt panels in series.

[osirissail]

See: How to wire solar panels in series for specific voltage: 12volts to 24volts at The Alternative Energy Store

[cal40john]

Originally Posted by hellosailor

Either way, buy the MPPT controller and you gain about 10% in effective power transferred from the panels into the batteries, no matter how you set them up.

I'd argue opposite John wrt wiring them up in parallel though. If you have one panel in shade and the shaded cells knock the output down, say from 16v to 11v, that panel is not going to charge at all. If you have two panels wired up in series though, you're starting out with 32V instead of 16, and that same 5v drop still leaves you with 27V which is enough to ensure you still are charging the batteries.

You'll still have a net power loss, but my understanding is that with the MPPT magic, you'll have LESS of a loss because you won't be losing one panel entirely.

I don't believe that's how solar panels work. When one cell is shaded it effectively becomes a large resistance. This reduces the ability for current to flow though any of the cells that are in series with that cell, effectively shutting down all panels wired in series. MMPT magic won't help you here.

If they are wired in parallel and a cell on one panel becomes shaded, it just looks like a big resistance so the current from the other panel will go to the path of least resistance and its current will continue to go to the battery.

John

From:
Photovoltaic Panel Efficiency, Inherent and System Constraints

Partial Shading

Solar panels obviously produce less power when they are shaded and should idealy be situated where there wil never be any shadows on them.
There may be situations where this cannot be avoided, and the effects of partial shading should be considered.
A shadow falling on a small part of a panel can have a surprisingly large effect on output. Not only will the cells that are shaded be producing less power, but as the cells within a panel are normally all wired in series, the shaded cells afffect the current flow of the whole panel.
If the affected panel is wired in series (in a string) with other panels, then the output of all those panels will be affected by the partial shading of one panel. Therefore in a situation where partial shading cannot be avoided, there may be a case for not having the panels wired in series to produce the higher voltages that can be used with some inverters.



Another more basic look:
http://www.learnonline.com/pdf/The%2...l%20Output.pdf

[hay4man]

I would recommend connecting these panels in parallel. As you've specified, they are rated for different currents, and if you connect them in series, they will be forced to operate at the same current, and your stronger panel won't be operating close to its peak operating point.

What is the voltage rating? Is it the same number of cells connected in series within each module? If it is close, then you should definitely connect them in parallel.

Just remember that if you connect them in parallel, install blocking diodes in series with each module.

See the bottom graphic in this link:
Mismatch Effects in Arrays

[osirissail]

Yes, go with a parallel installation.
- - But your basic question was seeming to ask if your #10 awg wire could be used at 50 ft of round trip run - - answer: Not quite. Given new panels outputting rated watts - which is highly unlikely especially if the panel are old - - the calculated max round trip for 10ga awg is 40 feet. To get 50 ft you would need to go with #8 awg wire.
- - So measure carefully, I usually use a ball of twine/string and run it down the exact route to measure the actual distance involved since wire is not cheap. Also I can use the string/twine to "pull" the wire when actually doing the installation. You need to pre-mark the string/twine every foot with a magic marker. Masking tape will hold the string in place if there are long runs in open spaces.

[Strygaldwir]

One of the significant advantages, as I understand it, of wiring the panels in series is that you increase the voltage. The increase in voltage means less loss due to resistance in the wiring. So while 10 AWG wire may result in unacceptable voltage loss at 12 or so volt, it may be okay at 24 volt. The other advantage may be that if you have 20 volts available from series wired panels, this can still be used to effectively charge 12 volt batteries or provide current for devices on the other end.

My panel seems to drop voltage when parts of it are shaded. That's the observations I have made by watching the MX60 status panel.