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Old 15-09-2021, 09:56   #1
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Series or Parallel Panels?

I'm struggling a little with my solar design. I'm planning on installing THREE of these 210W panels on my Hunter 38.

The panels will be over the rear half of the bimini top and extending a little over the davits, aligned longitudinally behind the end of the boom.

I'm struggling with how to wire, series or parallel. I'm concerned about two things; first, if in series, partial shading of one panel taking the whole string down, second, if in parallel, the gauge of the wire from the branch connectors to the MPPT controller (guess, 30 ft).

Thoughts? Comments? Suggestions?
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Old 15-09-2021, 10:09   #2
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Re: Series or Parallel Panels?

I ran my four panels (totally 400 watts) in parallel because of the shading issue.
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Old 15-09-2021, 12:24   #3
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Re: Series or Parallel Panels?

Reading some of the published articles I find on the 'net some of them give what appears to be contradictory reasoning for their choices. One article mentions wiring in series because of the shading issue, but series raised the panel voltage above battery voltage and minimized the issue. Another mentions running in parallel but using a brach connector to join all the grounds, but running separate leads for the positives, which sounds like the ground side is then too small of a gauge.

To be "best" it seems like I would need to run all three pairs of leads separately all the way to the controller, OR, probably use 6 AWG from two branch connectors close to the panels.
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Old 15-09-2021, 13:34   #4
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Re: Series or Parallel Panels?

A small amount of shading can knock out an entire panel. If the panels are in series, it can knock out the entire set. Apparently some panels have bypass diodes which can mitigate this issue, but the tests I've run on my panels indicate small shading does indeed cause a serious degradation of output. So again, this is why I run mine in parallel. If one goes down, it doesn't affect the performance of the others.

I also run all my panel outputs to a single junction box, which I then feed to my single controller. This works fine since all my panels have a similar output voltage. Apparently this is not a good idea with panels of differing voltages.

From what I've read, the truly best practice would be to run each panel to its own dedicated controller. But I should be clear, I'm far from an expert in any of this stuff. I'm just sharing what I think I know, and have learned. Hopefully some of the real electrical experts will chime in here.
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Old 15-09-2021, 13:54   #5
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Re: Series or Parallel Panels?

This is a fairly good primer:

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Old 16-09-2021, 09:35   #6
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Re: Series or Parallel Panels?

Parallel. I have two strings of panels. One with 2 Victron 115W panels in series. One with two 100W panels in parallel. The parallel string gives a consistent better daily output, even though the Victron panels have bypass diodes to minimise shading issues.

The drawback is that the parallel string needs slightly larger cables. For next season both strings will be parallel.
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Old 16-09-2021, 09:42   #7
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Re: Series or Parallel Panels?

Assuming a Victron MPPT that needs battery voltage + 5 to start, battery voltage + 1 to continue charging, I'd put them in series. Max output is at 17 volts on those panels, open circuit voltage likely in the low 20s. So you'd probably give up some usable output in the morning / evening by putting them in parallel. Even more so on a hot, windless day where panel voltages will be lower.
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Old 16-09-2021, 09:49   #8
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Re: Series or Parallel Panels?

My panels combine to similar total watts. (Parallel)I had it run with 10awg wire, roughly 20, feet last year. It worked but did lose efficiency and get hot.
I have upgraded to 8 Guage this year to improve this. Unfortunately most controllers don't handle wire any larger.
I have heard of running leads from each panel to a terminal post, with a short 8awg to the controller from the post. I haven't tried it though.
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Old 16-09-2021, 10:02   #9
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Re: Series or Parallel Panels?

I have 4 panels in parallel. The 3 over the davits go to a common junction and then to the controller using 4 awg wire. The 4th is on the lifelines and is wired separately to meet the other wiriing just before the controller. Three of the panels are identical and the fourth has voltage within 1 volt (33) of the other 3. This has worked fine.
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Old 16-09-2021, 10:05   #10
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Re: Series or Parallel Panels?

Quote:
Originally Posted by rslifkin View Post
Assuming a Victron MPPT that needs battery voltage + 5 to start, battery voltage + 1 to continue charging, I'd put them in series. Max output is at 17 volts on those panels, open circuit voltage likely in the low 20s. So you'd probably give up some usable output in the morning / evening by putting them in parallel. Even more so on a hot, windless day where panel voltages will be lower.
Yes, that is correct, but in my case that makes little difference. The serial panel starts charging a few minutes earlier. But over the day the parallel wins. Consistently. I thought the opposite because Victron recommend to connect them in series. So basically I loose some 20% on the serial panels compared to the parallel. Without shading the serial gives more peak power, but shading is a way of boats.
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Old 16-09-2021, 11:05   #11
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Re: Series or Parallel Panels?

We have 2 panels each with their own controllers feeding the main bus. It is often the case that one panel is producing more than the other due shading or sun angle. This allows each panel to maximize output.

Panel efficiency is also a variable, especially with the Victron controllers. Somewhere on the Victron site there is an explanation for this. Our panels, Solara 150 watt, have a high efficiency rating, they typically start producing power early in the morning (by 0800) and continue until late in the day (until 1700). Granted the power produced at each end is not very much, but every milliamp helps.
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Old 16-09-2021, 11:09   #12
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Re: Series or Parallel Panels?

On a sailboat the total output from parallel connection usually betters series connection.

However, the chosen panels have a low voltage even for nominally 12v panels. The danger is the Vmp may drop below the battery voltage in some circumstances, especially hot climates. The other issue is the popular Victron controllers have a high start up current. Normally for panels like this I would suggest a combination of series or parallel connection, but this is not feasible with an odd number of panels.

My guess is that parallel connection is likely to still be slightly better even with these panels, but given the added difficulties of thicker wiring that is needed with parallel connection, series connection may be the practical choice, as any loss is likely to be slight especially if planning to team them up to a Victron controller.
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Old 16-09-2021, 11:16   #13
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Re: Series or Parallel Panels?

The issue is maintaining panel voltage with Victrons minimum op point. In heat , Vmp can fall to a point where the mppt controller merely getting in the ways

With a single panel , sometimes you can’t even get the Victron to start S you need +5v over Vbatt , sometimes a PWM controller is actually better

IN your case with Vmp of 17 volts a single panel, or a set of parallel panels might never activate the controller at all.

In my opinion the panel operating point of these panels is likely to be near the battery anyway , so a PWM controller will be better then an mppt one.
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Old 16-09-2021, 11:33   #14
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Re: Series or Parallel Panels?

I have both series and parallel from the same set of panels.

The problem arose when I installed a Victron MPPT to charge a LiPo battery in parallel to an existing so called MPPT charger (I say "so called" because it is said to be MPPT but I suspect that in reality is just a series charger and maybe the manufacturers are confusing PWM with MPPT)

Because of minimum 5V overhead on the Victron controller it was not taking a share from the panel when the panels were all wired in parallel and providing a nominal 12V. Consequently I separated 300 watts of panel and wired them in series so that the two groups have a common negative but the 300 watt panels are wired with their negative wired into the remaining panels positive buss and their positive to the Victron controller.

So now I have 600 watts of panels wired in parallel and another 300 watts consisting of 2 each 150 watt also wired in parallel but the 600 watt unit supplying to both a 12V nominal input controller and as 12V nominal to the bottom half of a 24V nominal supply to a second controller.

As strange as it might sound the system is working well with both the LiPo and AGM banks reaching full charge every sunny day.
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Old 16-09-2021, 12:02   #15
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Re: Series or Parallel Panels?

Solar cells act as current sources rather than voltage sources. That is why when they are in series one shaded cell kills the whole string. The best way to make a good solar system for boats where partial shading is a problem is to place all cells in parallel - but that would give a very high current at very low voltage, which would require big wire and a charge controller that would boost the voltage to what is needed to charge batteries. A charge controller of that type would cost more and be less efficient than what is currently available. A good compromise would be to put small strings of cells in series (to give, say 6-10 volts), parallel many of these small strings (diodes are required) and build power conversion circuitry into the panels to keep wire size low. The technology is actually not difficult; if you have any fluorescent lights running from 12 volts, something similar is built in. A system like this would likely have a lower peak efficiency than the current approach, but would more than make up for it in tolerance to shading and less than ideal siting of panels. Unfortunately, the marketplace has not generated this solution, as boats are such a small market. So, my solution is to use a number of small panels in parallel rather than one or two big ones. Of course, the best approach is to give each panel its own controller, but that gets expensive.
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