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Voltage drop from solar panel Voc to MPPT under load

J

jimcalf

New Member
Joined
Aug 1, 2020
Messages
73
Location
Akron, OH
I have a new solar installation w 4 x 215W panels in 2S2P wiring to a Victron SmartSolar 100/50. Measured Voc is 45.1V, which is expected for these panels. It is April 24th in Akron, OH and with the panels mounted flat on my RV roof. I'm getting about 478W max production at noon on a cloudless day with clean panels. No shading. That's not great considering their theoretical output of 860W. I also noticed that the voltage is dropping from 45.1Voc to around 36V under load. I've double checked all my connections. Would this be considered a normal voltage drop or should I investigate my wiring? I'm using the pre-wired MC4 roof port with 10awg wire that came with my trailer. 2108TB Li Schematic V2_0.jpg
4-24-2022.jpg
 
time2roll said:
What is the panel Vmp rating? This would be the ideal voltage under load.
Click to expand...
Vmp is 21.8V. These are the cheap Home Depot "Nature Power" panels and they don't provide the specs you would get with a better name brand. Here is the only label.
1650828900322.png
 
Agreed, that's good yield for flat mounted panels that far north this time of year. That should improve over the next two months until the summer solstice.

I notice that you have a 50A SCC but you are using only 8AWG wire from the SCC to the bus bars. 6AWG would be a better choice to handle the 50A. However, you'll never get over 35A with only 830W on a 24V system. Probably closer to 30A for the most you'll ever see with flat mounted panels. If you add 2 more panels you'll get closer to 50A.
 
rmaddy said:
Agreed, that's good yield for flat mounted panels that far north this time of year. That should improve over the next two months until the summer solstice.

I notice that you have a 50A SCC but you are using only 8AWG wire from the SCC to the bus bars. 6AWG would be a better choice to handle the 50A. However, you'll never get over 35A with only 830W on a 24V system. Probably closer to 30A for the most you'll ever see with flat mounted panels. If you add 2 more panels you'll get closer to 50A.
Click to expand...
I had considered using 6awg cable from the MPPT to the Lynx, but the run is literally 12 inches and my roof is maxed out. It's only a 23' trailer. 8awg is a little easier to fit into the terminals on the 100/50, which were actually really tight even on the 8awg.
 
jimcalf said:
I also noticed that the voltage is dropping from 45.1Voc to around 36V under load.
Click to expand...
Note that a panel's Voc is what you would measure on an unconnected solar panel. The voltage you actual see under load is not Voc. It should be a voltage near the Vmp.
 
I call normal. Panels are getting warm too. Need a 50* breeze and tilt the panels. Otherwise this is often why people go over panel on the controller.
 
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Vmp for silicon panels is normally 0.81 to 0.85 x Voc. With strings in parallel it will likely be toward low end with matching variance degrading SSC Vmp search determination slightly.

I_mp is typically in range of 0.95 x I_sc.
 
Thanks everyone for your feedback. I now know that this is totally normal behavior. I noticed that as the day waned and the panels were producing less power, the voltage was going up into the low 40s. I guess there is a relationship there where under max current, the Voltage drops a bit. Lots to learn about the ebbs and flows of the solar and batteries now that the system is built and running.
 
I am in a similar region, Rochester NY. Today was cool and full sun. Voc is 49.6 and was getting 39.3 while charging (Vmpp is 40.9).

4x 415w in parallel. 4x 10awg > 1x 6awg > 150/100 MPPT > 1x 2awg > main bus/battery
 
I've found that the higher the power delivered, the lower the voltage shown on the Touch 50. Guess it's some kind of ohms law thing.
 
jsalbre said:
@jimcalf A bit off topic here (sorry), but how did you get the outside temp display on your screen?
Click to expand...
I loaded GUI_mods onto my Cerbo GX. It allows you to choose to show many temp sensors. On that photo, I only had one wired up so far, which was the temp sensor that came with my Multiplus, but plugged into my Cerbo GX and ran under my trailer. I have it on a nylon post to it really just touches the air, nothing that conducts temps. Since then, I bought another temp sensor for the underbed area holding the inverter and other electronics, so now I also have "underbed temp" listed on the Touch 50. I'll use that to track heating of that area under heavy workload, and potentially to trigger relay 1 to turn on a fan that exhausts heat to the outside.
 
jimcalf said:
I loaded GUI_mods onto my Cerbo GX. It allows you to choose to show many temp sensors. On that photo, I only had one wired up so far, which was the temp sensor that came with my Multiplus, but plugged into my Cerbo GX and ran under my trailer. I have it on a nylon post to it really just touches the air, nothing that conducts temps. Since then, I bought another temp sensor for the underbed area holding the inverter and other electronics, so now I also have "underbed temp" listed on the Touch 50. I'll use that to track heating of that area under heavy workload, and potentially to trigger relay 1 to turn on a fan that exhausts heat to the outside.
Click to expand...
Ahh, that makes sense, thank you. GUI_mods wasn’t around last time I had a Cerbo so I hadn’t seen it before. None of the screenshots I’ve seen had a single temp across the bottom like yours, so I was thinking it was some kind of weather feed. When I finally get all my gear installed I’ll have to play with extra temp sensors.

Thanks for the info!
 
jimcalf said:
Vmp is 21.8V. These are the cheap Home Depot "Nature Power" panels and they don't provide the specs you would get with a better name brand. Here is the only label.
View attachment 92448
Click to expand...
These look like exaggerated specs. Typical Vmp is 0.81 to 0.85 x Voc. Typical Imp = 0.95 x Isc.

Higher panel temp reduces output. At noon in Arkon OH in May you still have quite a bit of offset angle between sun angle and flat horizon panel.

Numbers in chart are equivalent hours of full sun per average day. In May for a panel laying flat to ground you average about 2.6 hours of equivalent full sun power a day. That is about 2 kWH's per day in May for warmed up panels you have.
Akron OH NREL solar data.png
 
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jimcalf said:
I loaded GUI_mods onto my Cerbo GX. It allows you to choose to show many temp sensors. On that photo, I only had one wired up so far, which was the temp sensor that came with my Multiplus, but plugged into my Cerbo GX and ran under my trailer. I have it on a nylon post to it really just touches the air, nothing that conducts temps. Since then, I bought another temp sensor for the underbed area holding the inverter and other electronics, so now I also have "underbed temp" listed on the Touch 50. I'll use that to track heating of that area under heavy workload, and potentially to trigger relay 1 to turn on a fan that exhausts heat to the outside.
Click to expand...

I'm going to have to look into the GUI_Mods package. I added wireless temperature sensors that show up in the Touch 50. I know where to find the temperatures, but it would be nice to have it on the front page.

I still have the temperature sensor that came with my Multiplus. I'll have to plug that in to get battery compartment temperatures. I have the sensor, so I might as well use it.
 
RCinFLA said:
That is about 2 kWH's per day in May
Click to expand...
I've seen as high at 2.4kWHs in the last weeks, so your predictions are pretty spot on. As I've used the system more and gone through the historical highs, I have seen over 600W of production from the panels on peak days. I thought that was pretty good for flat panels in NE Ohio in late April. So, I've gone from being concerned to being really happy with the results. The best improvement I saw, cleaning the panels! LOL! It's amazing how much dirt accumulates on flat panels. Thank God I left an easy walkway around the panels to give 'em a spritz of water and a wipe. You can really see the late afternoon shading in the photo due to trees behind the trailer (to the west). I do get full unobstructed sun till about 2pm though, which is a good period for testing.PXL_20220424_215715093.jpg
 
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