Stephen avatar image
Stephen asked

Understanding: Series Vs Parallel for 2 x150W panels with SmartSolar 75/15

Wow... I've read a LOT of great info on this site. But there are a lot of numbers and my pea-sized brain still has a couple of questions. Here's what I know: (Please correct anything I've misunderstood!)

* Panels in SERIES Voltages add, and current remains the same, while for panels in PARALLEL their Amps add together and voltages remain the same.

* Battery charging begins once the panel Voltage is more than 5V above the voltage of the battery.

* SmartSolar controllers can handle more amps from the panel(s) than indicated by their model numbers - they just don't push more than that charge into the battery above the limit (eg my SmartSolar 75/15 will limit Amps into the battery to 15A

* Voltage indicated by the model number (eg 75V for my SmartSolar75/15) is a hard limit that will damage the controller if exceeded by the total open circuit voltage stated on the panel(s).

I already have a SmartSolar 75/15 and one 12V 150W panel with these specs:

VOC 22.3V and Vmp 17.9V with max current 8.73A

I currently have a 21V 150Ah AGM battery but plan on replacing it with a 200Ah LiFePO4

I have a second panel the same that is not connected, and the possibility of a separate 310W panel also. Here are the questions:

Assuming I don't want to buy another Charge Controller - 1. If I connect the second 150W panel in Series will I start charging earlier in the day or if its cloudy or partially shaded because the voltages are added and therefore higher earlier in the day than if they were in Parallel?

2. In series voltages add. So 2 panels at Voc 22.3 x 2 = 44.6V This is under the 75Vlimit... any problem with this aspect?

3. In Series the current is the same - since the panels put out 8.73A my 15 A controller wont be close to the max it can push to the battery. Is this a waste of the potential of the panels?

3b. Would It be smarter to put the same 2 panels in parallel, accept that they wont start charging as early in the day or as often in partial shade or cloud but get more amps (which might on rare occasions when everything is perfect exceed 15A since 2 x 8.73A = 17.46A, and therefor be limited to 15 A by the controller)?

4. If i were to replace the 150W panel with a singe 310W 12V ( Voc 40.5V, Imp 9.4A) panel would I get similar results to the total 300W of a 2-panel in parallel setup?

I know there is fair bit here. I hope your answers can not only educate me but also help clarify for future readers who, like me, spent a long time trying to understand this stuff by amalgamating knowledge from previous posts...

battery chargingvoltagecharge current limitcharge controller compatibilityamps
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1 Answer
kevgermany avatar image
kevgermany answered ·

Couple of things to add

Panel voltage increases as temperatures drop. This needs to be taken into account when matching panels to controllers.

Input current to controllers is less than output, due to voltage change in the controller to provide correct charge voltage to batteries.

Isc - short circuit currents also need to be taken into account.

Power is Volts * Amps. Ignoring shading, there's no power difference between parallel and series connected panels. If distance between panels and controller is high/significant, series connected panels will have lower losses in the cabling due to the current being lower.

If you intend to use mismatched panels, in series max power currents should match, in parallel max power voltages should match. Small differences are ok, but will reduce overall output. Otherwise use more than one controller.

It's likely the single 310W panel will outperform two poorly matched 150W panels. If the 150W panels have identical specifications/use/age there will be little difference.

Although connecting panels in series gives more charge hours, local conditions will determine how much you gain. And it is not as simple as that. In low light, despite panels in series, low output will occur so the gains are smaller than it may appear.

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Stephen avatar image Stephen commented ·

Thanks for your answer. I can't find info on or about the panels I have regarding the increase in panel voltage as temperature decreases. Could you give some ballpark number to work with? I assume the panel open circuit voltage stated by the manufacturer is at an industry standard temperature, right? I'm at 45degrees south, so as the temperature drops here the sun angle is usually low too so I'm wondering if in my situation now (75V limit on the controller and 22.3Voc on each panel if I need to even be working out the panel V change as temperature drops, or have I got such a buffer that is of no concern? Thank you for your expertise!

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kevgermany avatar image kevgermany ♦♦ Stephen commented ·
There should be a sticker on the underside of the panels indicating the temp compensation, in mV/C or V/C, it will be a negative value. This should in reference to 25C, (but check.... sometimes it's 20C).

Confusingly the value is expressed as the voltage decrease as temperature rises, but for MPPT protection we're interested in the opposite.

Victron monocrystalline panels are -0.35V/C so at 15C voltage increases by 3.5V (per panel if wired in series). It's a useful ballpark and will tell you if you need to check further.

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Stephen avatar image Stephen kevgermany ♦♦ commented ·
That is great clear info. Thanks very much for taking the time to reply :)
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