# Solar Panels connecting in series/parallel

#### ronph

Joined Feb 4, 2013
25
So I have just received 4 pieces of solar panels each puts out 10 volts on direct sunlight (7.2 volts on a hazy day). They will be used to charge 12 volt batteries 3 - 7mAh. Have not yet measured the ampere output.

I would like to connect two panels (makes 2 sets) in series making a total of 20 volts (which takes care of the voltage side) then each set connected in parallel to increase the amperes.

Each panel has a small junction box with + & - leads coming out but am unsure as to what is inside* (perhaps a diode circuit only a guess here). So two panels I connect one positive to the negative (series) as one set and doing the same for the other two panels which would be the second set. Then I parallel the two positive and two negative on each set to bump up the amp output.

Question1: Is my line of thought on the connections correct?
Question2: Will setting my multimeter to the amp and connecting the + probe to the postive of the panel doing the same to the - side give me a correct amp reading?

Thanks

* the junction box is sealed so opening it up may just damage the internal circuit.

#### #12

Joined Nov 30, 2010
18,210
The wiring seems right but putting an amp meter directly across the output will give the solar panels less than 1 ohm as a load. Yes, you can find the maximum current at a tenth of a volt, but that's not anything you can compare to the real output you will get while charging a 12 volt battery. There is a curve comparing volts and amps, different for every solar cell arrangement and every different manufacturer. You can discover this by using a variable load. You probably can't get the manufacturer to hand you one.

Other people here know better than I about whether you need any diodes to keep the panels from fighting. I rely on them to speak up.

#### ErnieM

Joined Apr 24, 2011
8,058
Sounds like you have a full panels and not individual cells: are you sure there is no part number or manufacturer on them anywhere? If so, then they would "hand you" a full spec sheet including voltage-current curves.

Using the ammeter as you say is fine, that is the short circuit current. Also keep the meter there and measure the open circuit voltage: there are the 2 end points. They don't tell you the whole story, just which street it lives on.

AFAIK panels can be connected directly for series and parallel but ultimately need a diode when directly charging a battery. I'm unsure if this is due to the battery back driving the solar cell and damaging it, or just dumping current that discharges the battery.

#### ronph

Joined Feb 4, 2013
25
ErnieM, the panels were used and it did have a part number but could not find any information on the internet (made by Qualcomm). The company that sold this are liquidators so they did not have any manual or specs other than a test with the multimeter. Each panel is 6 x 14 inches only and the front face of the solar panel I believe is expoxy coated. Will try to post pictures soon.

#### ErnieM

Joined Apr 24, 2011
8,058
Gotcha, orphaned parts. Well, open circuit voltage and short circuit current are two easy measurements to make. Might as well do each panel individually to see if they are still matched.

It gives you an idea what they can do. Not complete info, but good enough.

#### ronph

Joined Feb 4, 2013
25
So yesterday I went to Lowes to get me sort of a frame to mount the solar panels and settled with the cheapest way which is aluminum L bars. A bit flimsy but its only a project. The panels came with 3M double sided tape to it was easy once I cut the aluminum to size. I should have tested out each set as one produces 12~+ volts and the other around 14~+ volts although when I tested each panel individually gave 10 volts. I could in fact just swap the connections to get close to even voltage (mess of wires at the back). On the rear of the panel you will see the aluminum frame attached and each set connected in series.

Now my question is, when these two sets are connected in parallel obviously one gives 12+ and other 14+ would there be any forseeable issue on the voltage difference if connected in parallel? And if so, would there be any options of equalizing the output? Checked out each panel and one of them produces a little more voltage than the other 3 panels hence one set shows 14+ volts.

Thanks

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#### ErnieM

Joined Apr 24, 2011
8,058
You connect two panels in series that individually give you 10 volts and together you get either 12 or 14 volts? Nah, something is wrong there, you should get 20 out of that.

As is, connecting 12 V with 14 V... sorry, makes my brain hurt to think of that. I have no answer beyond that may not be enough to charge a battery: a lead acid needs some 14.5V, plus that diode drop so call it 16V minimum. (I don't know what kind of battery you use.)

#### ronph

Joined Feb 4, 2013
25
ErnieM, my apologies, I was measuring voltage of the set on hazy overcast skies hence I was getting 12/14 on each set. Theoretically, I would be getting 20 volts on each set in direct sunlight.

Anyway, my question still is.... would there be any issues if one set produces 2~3 volts higher than the other if they were paralleled or am I just over reacting since each set should in fact produce 20 volts (direct sunlight) I did measure each panel on direct sunlight and gives 10 volts?

Thanks

#### ErnieM

Joined Apr 24, 2011
8,058
would there be any issues if one set produces 2~3 volts higher than the other if they were paralleled
As long as each leg gets a reverse blocking diode (fancy words for just one series diode in each parallel leg) (so you need 2) there should not be any worries.

Glad you found the missing voltage.

#### ronph

Joined Feb 4, 2013
25
Well, I finally got the tabs & wire to make the series/parallel connection for the panels and got 20volts on direct sunlight and varies from 14 to 20 depending on how overcast the sky is. My cheap multimeter which only goes up to 250milliamps goes over the scale on direct sunlight when probe is placed on the pos & neg terminal (crude).

ErnieM - have place a diode as suggested.

Have posted a separate thread as I do have a 1 milliamp analog which I wish to extend the range to at least 1A.

Making progress here. Am in the process of making another charge controller for this solar panel project since the first type is now being used on my small 2.5 x 2.5 inches/cell x 6 solar cells connected in series producing around 27 volts in direct sunlight. This small one is used to charge up my 8AA battery pack and still in the testing stage but looks promising.

Appreciate all the insight since this was not taught in Business Management in college . So am still learning more about electronic circuits through the internet.

#### tindel

Joined Sep 16, 2012
734
What kind of batteries are you charging? It's a dangerous thing to just charge batteries directly off of a solar panel. You need something interfacing to the battery typically. If you're using a charger that requires 12V input from a car cigarette lighter, that's another story, as it will already have that interface.

I would suggest a blocking diode just to keep the battery power from going through the solar cell.

#### ronph

Joined Feb 4, 2013
25
Tindel - I have just finished my 2nd home made charge controller using only 6 parts and adjustable output voltage. The circuit itself has a blocking diode so I am not worried. Since I am charging 12 volt battery packs and small SLA batteries (not the large car batteries), I set the voltage to 14V. Am currently in the testing phase since this is my 2nd solar panel assembly. It's around 10x bigger (at 2' x 14") than the first one I made. Am discharging a 4.5ah battery as I write this and will be connecting it to the charge controller/solar panels tomorrow and will be logging the battery voltage to see how well and how long it takes to charge.

Cheers