#165423  by Neddy
 Sun Sep 13, 2020 10:04 am
ianganderton wrote:
Sun Sep 13, 2020 1:43 am
I haven’t seen that graph before and the logic flies in the face of the best practice I see on various solar forums I lurk on.
So what does that tell you!

"I suspect efficiencies will vary between different MPPT controllers".
They do - but not by very much. Regardless of controller model or brand, the same "higher voltage = lower efficiency" pattern remains.
If you want, I could post other examples here, just ask.

"Higher voltage means....... significant improvements in cable efficiency’s."
Transmission losses over any given cable run can be as high or as low as you like. You get to decide what percentage is acceptable. An inadequately cabled Serial system could easily be less efficient than an adequately cabled Parallel system.

"The voltage headroom (of Serial connection) gives the MPPT controller more to work with as voltages drop on cloudy days."
But what has the controller been able to do with this extra voltage "headroom"? Nothing! Look at the left side of the graph. Even on very cloudy days, low voltage parallel connected systems are still plugging away at 90% or more while high voltage serial connected systems have plummeted off the chart.
Voltage "headroom" is clearly something to be avoided!
 #165425  by scubadoo
 Sun Sep 13, 2020 12:30 pm
Is this a fair test or very flawed methodology? The numbers match my multimeter and DC clamp meter(s).
I can alter the 50A Victron MPPT solar controller output in 1A increments.
Measured just now. 24V solar panels. Set at 3.00A output to the 12V nominal battery
My calculations. Fully repeatable with miniscule variations.
Input 41.55W - Output 39.96W = 96.18% conversion efficiency.

Click to enlarge image
Screenshot_20200913-120107.jpg

Again my simple test method and maths may be completely flawed. Comments welcome.
 #165496  by ianganderton
 Mon Sep 14, 2020 7:37 pm
Neddy wrote:
ianganderton wrote:
Sun Sep 13, 2020 1:43 am
I haven’t seen that graph before and the logic flies in the face of the best practice I see on various solar forums I lurk on.
So what does that tell you!

"I suspect efficiencies will vary between different MPPT controllers".
They do - but not by very much. Regardless of controller model or brand, the same "higher voltage = lower efficiency" pattern remains.
If you want, I could post other examples here, just ask.

"Higher voltage means....... significant improvements in cable efficiency’s."
Transmission losses over any given cable run can be as high or as low as you like. You get to decide what percentage is acceptable. An inadequately cabled Serial system could easily be less efficient than an adequately cabled Parallel system.

"The voltage headroom (of Serial connection) gives the MPPT controller more to work with as voltages drop on cloudy days."
But what has the controller been able to do with this extra voltage "headroom"? Nothing! Look at the left side of the graph. Even on very cloudy days, low voltage parallel connected systems are still plugging away at 90% or more while high voltage serial connected systems have plummeted off the chart.
Voltage "headroom" is clearly something to be avoided!
Been looking into this a bit and it’s really interesting. Obviously not as clear cut as I thought!! Image

The difference in voltage efficiencies do seem to vary between SCC’s, in the Epever AN range the graphs show noticeable differences between the smallest and the 40A version. The 40A version has a much smaller difference.

I haven’t had chance to look at differences in wiring efficiencies and cost yet but will do over the next few days when I get chance.

Finally I just want to say thanks for bringing this to my attention. Great to learn something new.


Sent from my iPhone using Tapatalk
 #171881  by Izzle
 Thu Apr 08, 2021 1:35 pm
I note this item is a few months old, however the parallel/series debate came up last weekend at the National Rally in one of the 'Solar' workshop sessions. This intrigued me as there are advantages and disadvantages in both applications, but not being an electrical boffin I wanted to find out more.

Question: To help with the varying light situations (time of day, season, shading etc.) why don't we use a DPDT (double pole, double throw ON-ON) switch which enables the parallel/series change over to be made simply at will. If the hardware and wiring sizes are selected for the worse cases, what are the issues?
 #171883  by scubadoo
 Thu Apr 08, 2021 3:08 pm
Opinion based purely on my experience and testing over the years.
My only interest is total panel energy produced in watt-hours over a day given the same location and time of year.
Time of day, low light, cloud, daylight saving etc. makes no difference.

We have 3 x 270W-ish 24V nominal solar panels. 1x mono, 1x poly and 1x mono perc.
Each unshaded panel outputs virtually the same energy into our battery regardless of being connected individually, paired, in series, parallel and connected to one or two Victron MPPT solar controllers.

Partial shading any panel when series mounted all but kills output to our battery. Panels with bypass diodes should perform better.

Given our panels I can't improve on 3x parallel fed into a single MPPT controller.

Others will have different results. ;)
 #171886  by Nut17
 Thu Apr 08, 2021 5:35 pm
I had to go to Wikipedia to get a handle on that "aphorism" Matt.

Perfect is the enemy of good, or more literally the best is the enemy of the good, is an aphorism which is commonly attributed to Voltaire, who quoted an Italian proverb in his Dictionnaire philosophique in 1770: "Il meglio è l'inimico del bene".[1] It subsequently appeared in his moral poem La Bégueule, which starts[2]

In my case, I have two strings of three 200 watt perc panels - each string wired in series to a 40 Amp SRNE MPPT Controller. I am very pleased with the performance and note than on occasion the satellite dish will cause some shading on one of the panels. Despite being wired in series, the array affected is usually only dropped in output by around 35%. Interesting to watch the controller ramp up to match its mate when this is the case and I push the down button on the sat dish to eliminate the shading. My system will deliver in excess of 80 amps in late December so exceeds the total theoretical potential output.
 #171942  by Neddy
 Fri Apr 09, 2021 10:11 am
Izzle, there are no issues that would affect your switchable solar wiring proposal. The question I have is this. Why would you ever want to switch to Series panel connection? What "time of day/season/shading" situation can you envisage where Series panel connection would ever be superior to Parallel?
Have you been able to view detailed performance data for your MPPT controller? That should tell you all you need to know.
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