Two solar panels directly into controller

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Two solar panels directly into controller

Postby Vspec » Mon Apr 29, 2019 7:57 am

I have two solar panels of the same brand and same rating.

I plan on having them correctly wired in parallel but for testing purposes, would there be any ill effect if I connected the outputs (wires) of each panels, to the same terminals on the (PWM) controller?

Basically two wires instead of one, per terminals, respecting polarity of course.
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Re: Tow solar panels directly into contoller

Postby John61CT » Mon Apr 29, 2019 8:26 am

PWM has very limited Voc input range.

Should not connect in serial, unless a weird low voltage panel, in which case need the SC min/max specs.

Link to the datasheets?
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Re: Two solar panels directly into controller

Postby Vspec » Mon Apr 29, 2019 8:33 am

Per panel:
    Voc: 21.85V
    Lsc: 6.03A
    Vmp: 18V
    Imp: 5.5A
Controller:
    Max Input 540W (18V / 30A)
    Max output: 12V/24V, 30A

Unless I am mistaken, my crude diagram shows a parallel setup. Except instead of connecting the wires from each panel into a "bus" and then connecting the "bus" to the charge controller, I connect both panels directly to the input terminals of the charge controller.
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Re: Two solar panels directly into controller

Postby MtnDon » Mon Apr 29, 2019 9:19 am

The panel Voc is higher than the max input rating of the controller. That is a problem under some conditions unless the controller has circuitry to limit the input. A cheap PWM controller probably does not have such a thing. . Voc


Electrically, the controller cannot tell any difference between the wires from the panels being routed through a buss connector of any kind and then ONE + and - wire connected to the controller, OR, TWO wires connected directly to each of the controller terminals. That does assume the terminal connectors can handle two wires at each terminal.

Note that IF you decided to have more than 2 panels in parallel you then need fuses where the panel wires are combined. If only two in parallel you are okay, assuming the panels are built to pass UL testing.
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Re: Two solar panels directly into controller

Postby Vspec » Mon Apr 29, 2019 10:34 am

Thanks for the explanation.

In the best of conditions, I have not seen the panel output more than 17V.
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Re: Two solar panels directly into controller

Postby MtnDon » Mon Apr 29, 2019 2:15 pm

Voc = Voltage, open circuit. If you disconnect the panels from the controller, have a bright sun shining on the cells, the voltage will read much higher, closer to the Voc. Then it will fall to be closer to the Vmp or lower. So if you connect the wires to the controller in bright sun the voltage will peak close to Voc for a brief moment. Whether or not that causes trouble with that controller is an unknown.

Voc also increases as the temperature falls. The opposite is also true. In winter as the first sun rays hit my panels at home or cabin , the voltage overshoots the rated Voc of the panels by about 25%. The controller used there can easily accept that. You could also run into that situation.
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Re: Two solar panels directly into controller

Postby John61CT » Mon Apr 29, 2019 2:51 pm

Yes I've had panels spike **over** VoC on cold mornings in snowy mountains, also at sea level from a "flash" cloud edge effect.

Even quality gear like Victron, I keep VoC at least 10-15% below their max rating. Amps/watts can go over but OV can quickly fry a SC.

With a cheap enough PWM who cares, just carry a spare.

But yes, parallel only there obviously
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Re: Two solar panels directly into controller

Postby Vspec » Fri Jun 21, 2019 6:03 am

A few weekends ago, I was playing around with my setup with the two panels (100w, 5.5a,18v) connected in parallel at the controller (500w, 30a, 18v PWM). I was getting very sun exposure. I was outputting enough amp to even run the highly inefficient thermoelectric cooler off the load terminals. My tiny 35ah AGM battery reached float fairly quickly.

I got curious and stuck the multi-meter on the inputs for the solar panels at the controller and was getting between 21v-22v. The case of the controller was not overly in spite of sitting directly in the sunlight. If I did turn on the load, the input voltage would drop to 17v-18v.

What is going on here? It would be quite suspect that each panel would be able to produce more than their specified amperage. None of the panel were fused. The final install would be fused for the max amp of the controller.
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Re: Two solar panels directly into controller

Postby John61CT » Fri Jun 21, 2019 7:18 am

Power = watts, they don't* go over on that

Voltage varies a lot especially on MPPT, that is the point.

As amps are needed, the SC raises voltage.

No power draw, goes quiescent
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Re: Two solar panels directly into controller

Postby Vspec » Fri Jun 21, 2019 7:58 am

SC ?

Controller is el cheapo PWM type
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