VOC, Growatt 3kW off-grid inverter and panel configuration

[h2onet]

System:
3kW Stackable 48V 150VDC 80A Off-Grid Inverter by Growatt
Used Axitec AC-410MH/144V Panels (quantity 4)
48V 100AH LiFePower4 Battery by EG4 (Does not matter for this question, but including it anyway)

I have a small off-grid cabin and am building my first system. I thought all my research was going great, but am having a problem with the configuration of my panels to fit into the VOC limitations of my Growatt inverter. The Growatt inverter requires a minimum of 80 VOC to charge the battery, with a max allowed VOC of 145 VOC. With the panels I purchased, my original plan was to have two runs of 2 panels, since the panels have a stated vmpp of 42.3 volts and a VOC of 50.4 volts. After testing the panels for the last few weeks, I am only averaging about 30-40 volts per panel. This means that my original plan will not provide enough constant voltage to consistently charge the battery, but it I run all 4 in series, the peak voltage will be higher that the max allowed VOC for the Growatt inverter.

The easy fix would be to use 3 panels in series, and not use the 4th, but I would really like to use the power from all 4.

Now here comes my noob question: Can I run a series of 3, and then use a combiner box or Y connector to connect the one remaining panel? Are there any other configurations that would work that I am not seeing? My Growatt only has one solar input, and I really do not want to buy another small inverter for the one panel if I do not have to (though that is an option I guess). I could also buy 2 additional panels to even out the strings, but I bought my current panels used, so not sure I could find more (at least locally).

 

[rmaddy]

If you put 3 panels in series on your Growatt all-in-one you can buy a standalone charge controller for the 4th panel. You don't need another all-in-one just to connect the other panel.

When you say you are averaging 30V-40V per panel, is that Vmpp or Voc? You certainly can't put 3 panels with a Voc of 50.4V in series if the max input voltage is only 145V.

Is it really true that you need a minimum Voc of 80V to charge a 48V battery? Most MPPT charge controllers only need a Voc around 5V higher than battery voltage to begin charging.

 

[h2onet]

If you put 3 panels in series on your Growatt all-in-one you can buy a standalone charge controller for the 4th panel. You don't need another all-in-one just to connect the other panel.

When you say you are averaging 30V-40V per panel, is that Vmpp or Voc? You certainly can't put 3 panels with a Voc of 50.4V in series if the max input voltage is only 145V.

Is it really true that you need a minimum Voc of 80V to charge a 48V battery? Most MPPT charge controllers only need a Voc around 5V higher than battery voltage to begin charging.

rmaddy, thanks for the quick reply. I bought the Growatt Inverter from signature solar, and on the page for the controller, there is a video (

) where they talk about the VOC needed to charge the battery, and specifically mention that 80 VOC is needed.

I am definitely new to solar and am not an electrician, so I am not sure about your Vmpp or VOC question. What I am doing to get the voltage I am referring to is to take my tester and place one lead on the positive mc4 and the other on the negative mc4 and read what is on the screen, I would love to know the difference between the two. I think it is VOC, because it is not under load, but if that is not the correct interpretation, would love to know the difference.

Oh and I bought these panels from a solar installer that was selling them because they didn't meet manufacturer guaranteed specifications, so they did not put them on to the array they were building. I do not expect these to reach the max VOC or Vmpp.

 

[MichaelK]

Oh and I bought these panels from a solar installer that was selling them because they didn't meet manufacturer guaranteed specifications, so they did not put them on to the array they were building. I do not expect these to reach the max VOC or Vmpp.

OK, this is why your numbers are low. The way you describe measuring the voltage, with the MC4 leads disconnected, you are measuring the Voc. Voc means the voltage when zero current is flowing. The Vmp is the highest voltage the panel can produce at it's maximal power point (under load).

You can try hooking up three panels in series and see what their Voc is. Keep in mind that you need to factor in the coldest day of the year because panel voltage goes up as the temperature goes down. For freezing, multiply your voltage by 1.12X. For frigid -40degree weather, multiply by 1.25X.

Let's say you wire all three in series and you get 120Voc. Multiplied by 1.12 you get 134V. Multiplied by 1.25 you get 150V. So, on a frosty morning, it's likely to be OK. If you live in North Idaho, then most likely it's not OK.

Are you sure about your numbers though? Looking at this 48V Growatt, I see a Vmp of 60V as minimum. If that is the case, then wiring them 2S2P would be fine. One thing though this chart does not explain is what's the highest solar input amps the unit can handle. The 80A maximum listed is for the output amps. Since the unit says it's max array power is 4500W, and the max voltage is 145, that sort of implies that the maximal amperage the unit could handle is 4500W/145V = 31A. That's a guestimate though.

 

[Quankl]

OK, this is why your numbers are low. The way you describe measuring the voltage, with the MC4 leads disconnected, you are measuring the Voc. Voc means the voltage when zero current is flowing. The Vmp is the highest voltage the panel can produce at it's maximal power point (under load).

You can try hooking up three panels in series and see what their Voc is. Keep in mind that you need to factor in the coldest day of the year because panel voltage goes up as the temperature goes down. For freezing, multiply your voltage by 1.12X. For frigid -40degree weather, multiply by 1.25X.

Let's say you wire all three in series and you get 120Voc. Multiplied by 1.12 you get 134V. Multiplied by 1.25 you get 150V. So, on a frosty morning, it's likely to be OK. If you live in North Idaho, then most likely it's not OK.

Are you sure about your numbers though? Looking at this 48V Growatt, I see a Vmp of 60V as minimum. If that is the case, then wiring them 2S2P would be fine. One thing though this chart does not explain is what's the highest solar input amps the unit can handle. The 80A maximum listed is for the output amps. Since the unit says it's max array power is 4500W, and the max voltage is 145, that sort of implies that the maximal amperage the unit could handle is 4500W/145V = 31A. That's a guestimate though.
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+1 Jumping in here. Your question about the chart not stating the maximum solar input amp is what is making things dificult for me as a beginner. I realise they dont state max input amp for a LOT of their products. I've seen it on only the 12KWSPF 3000TL LVM-ES 48v model on page 42 of the manual (18Amps). Persons have told me they believe the Unit would only take the maximum it can take. But thats a lot to chance of "belief". If anyone can help with that specific max Amp this can take. I'd appreciate it.