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Solar panels 20V over battery bank voltage

RogerD

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Sep 26, 2019
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I heard that your solar panel voltage should be at least 20v over your battery bank voltage.

I assume that this is because the higher the panel voltage the less sun it will take to get power in and get above the battery bank voltage so your charge controller starts charging. So they start charging earlier in the day instead of later?

Yes/no and is 20v over the true minimum you should strive for?
 
Usually the number bandied about is 5v over battery voltage. Some SCCs have an MPPT range this is a bunch over battery voltage.

I think strive for 80% of max input voltage you SCC will accept.
 
@MisterSandals comment makes perfect sense.

I have a cargo trailer with 2s2p 300w panels and an MPPT at 58.5v. This is actually a bit below the MPP Solar PIP spec of 60~115vdc PV input range for the 3000LV model. I'm barely getting above my float of 54.5v but I'm getting 70% of 1200w max even though the panels are flat. Maybe I'd get 80-85% if my PV voltage was higher.

I also have MidniteClassic 150s (max 150v) on 3s5p 285w arrays where the MPPT runs 87v-90v to a 48v battery bank - and its definitely working as I routinely get in range of 95% of max panel ratings at 25deg slant June/July.
 
Usually the number bandied about is 5v over battery voltage. Some SCCs have an MPPT range this is a bunch over battery voltage.

I think strive for 80% of max input voltage you SCC will accept.
When you say 80% of max. Are you talking Vmp or Voc?

Here is what I'm struggling with.

Specs for (4) 200w panels.

Maximum Power(Pmax): 200W
Maximum Power Voltage(Vmp): 37.6V
Maximum Power Current(Imp): 5.36A
Open Circuit Voltage(Voc): 45.4V
Short Circuit Current(Isc): 5.83A

I could do 2s2p and have like 75v Vmp and 90v Voc. But 90v Voc is pushing it for Victron 100/30 controller. So I could go to 150/35 controller, but I'm not that near 80%.

Specs for different (4) 200w panels.

Maximum Power(Pmax): 200W
Maximum Power Voltage(Vmp): 20.4V
Maximum Power Current(Imp): 9.80A
Open Circuit Voltage(Voc): 24.3V

I could put all in series and be at 80v Vmp, but 99v Voc and again have to move up to 150/35 controller and not be 80%.

What would you do?
 
When you say 80% of max. Are you talking Vmp or Voc?

Here is what I'm struggling with.

Specs for (4) 200w panels.

Maximum Power(Pmax): 200W
Maximum Power Voltage(Vmp): 37.6V
Maximum Power Current(Imp): 5.36A
Open Circuit Voltage(Voc): 45.4V
Short Circuit Current(Isc): 5.83A

I could do 2s2p and have like 75v Vmp and 90v Voc. But 90v Voc is pushing it for Victron 100/30 controller. So I could go to 150/35 controller, but I'm not that near 80%.
My 300w panels have a Vmp of 32.41v and are in 2s2p = 64.82v. BUT.... the MPPT will typically lower this a bit - and I'm getting 58.5v'ish actual.
So in 2s2p your numbers would be 37.6v * 2 = 75.2v and so MPPT will lower that to around 68.5v.

Specs for different (4) 200w panels.

Maximum Power(Pmax): 200W
Maximum Power Voltage(Vmp): 20.4V
Maximum Power Current(Imp): 9.80A
Open Circuit Voltage(Voc): 24.3V

I could put all in series and be at 80v Vmp, but 99v Voc and again have to move up to 150/35 controller and not be 80%.
Yes - you don't (typically) want Voc to go above the specs. And in very cold weather, it can even go higher that 99v.... (that's what the coefficient is all about).

What would you do?
I went 2s2p and risk loss of power by being 'just a bit' below specs but I might risk the 99v Voc OR search for a bit higher Charge Controller. Its your situation and $ and I wouldn't fault you either way. :) At least you have a good handle on the issues!
 
And other MPPT only require about 2V over battery.

Vmp and Voc will vary with temperature and amount of sun. Your design should have Vmp enough over minimum MPPT range to handle those variations.

That 80% is probably Voc at room temperature is 80% of SCC max input voltage. That would give 25% of margin for cold days. You can sharpen your pencil by using record cold location for your temperature and PV panel data sheet's temperature coefficient of Voc. You may find that Voc would rise about 16% worst case, or perhaps less than that. Then you can push voltage closer.

But you're right 90V is probably too close to 100V. Unless temperature coefficient is fairly low.

It is perfectly fine not to be up to 80% of maximum voltage. Pick the best balance of parts. Midnight has a 250V SCC but it costs more per watt, than their 200V or their 150V, so best fit saves money. For long wire runs, higher voltage saves power loss and cost of copper.

Look up temperature coefficient of Voc. Efficiency of SCC vs. voltage. Put it in a spreadsheet and make sure you have safe margins first, cost and power efficiency second.
 
The Victron 150/35 SCC manual says +5v over battery bank and MPPT range 30V to 130V

If I go with the 150/35 SCC, Voc shouldn't ever be an issue.

The efficiency of SCC vs. voltage, I have no clue.
 
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The higher the voltage conversion for MPPT will be less efficient. Controller should have a graph somewhere if you keep digging. I like the simplicity of all in series so I think I would be in the 150/35. Actually I have the 60 amp Tristar that is good to 150v.
 
What would you do?
All the comments from OfGridInTheCity and Hedges make sense and have merrit.

I was torn for and answer until stepping back and wondering if you have a 24v battery.
Doing the math on that,
800w / 24v = 33.3a charging.

So i would lean towards 150/35 instead of maxing out (over paneling) a 100/30.

Running a little cooler should help the SCCs longevity. But Victron has a 5 yr warranty.

Nearly a coin toss, i give slight edge to 150/35. If money is super tight, 100/30 is a great second place option.
 
The higher the voltage conversion for MPPT will be less efficient.
I would be interested in seeing the Victron efficiency curves for input voltage and MPPT range.
My impression was that SCCs were optimized for upper end of MPPT range.
By that, coming full circle with RogerD’s astute question whether Voc or Vmp is used for this: Voc for input limit compliance, Vmp for working efficiencies.
 
All the comments from OfGridInTheCity and Hedges make sense and have merrit.

I was torn for and answer until stepping back and wondering if you have a 24v battery.
Doing the math on that,
800w / 24v = 33.3a charging.

So i would lean towards 150/35 instead of maxing out (over paneling) a 100/30.

Running a little cooler should help the SCCs longevity. But Victron has a 5 yr warranty.

Nearly a coin toss, i give slight edge to 150/35. If money is super tight, 100/30 is a great second place option.
My bad, I will have a 24v battery bank.

I just don't see having a choice but to go with the 150/35 due to Voc and the difference in price isn't that big of a deal.

The second set of panels that I would put in all series are a good deal. But they are on sale only till tomorrow. The first panels (24v) I listed are 50% more. Since you said in a post that you didn't think there was much difference between all 200w panels that exist. I want to give the lower-priced ones a shot. As I will be buying a second set of 4 at a later time.

I just wanted to make sure my choice in putting all in series and the SCC chosen was sound.
 
FM80 (no reason to believe Victron MPPT is any different):


1624413145009.png


This chart is why I indicate that 1.5X the battery voltage is the "sweet spot" for an MPPT controller. Worst case is about 2% difference vs. PV 34V and 100V.

There are more losses when taking larger steps in voltage.

It's also the reason that I say the efficiency loss in moving to higher PV is likely offset by reduced wiring losses with higher PV voltage.
 
PV panels differ in size. Some deliver 140W/mm^2, others 200W/mm^2
They also have various voltages, some a better fit to your particular SCC's specs.

Series vs. parallel - any constraints on mounting location and orientation?
All in one series string need to be same or very similar orientation. Shading of a panel or diode-bypassed portion changes the output voltage.
 
I would be interested in seeing the Victron efficiency curves for input voltage and MPPT range.
My impression was that SCCs were optimized for upper end of MPPT range.
By that, coming full circle with RogerD’s astute question whether Voc or Vmp is used for this: Voc for input limit compliance, Vmp for working efficiencies.
Here are some examples. Victron apparently does not publish this. I doubt Victron has some magic.


TriStar%252520MPPT%252520efficiency%252520graph.JPG
 
FM80 (no reason to believe Victron MPPT is any different):


View attachment 53734


This chart is why I indicate that 1.5X the battery voltage is the "sweet spot" for an MPPT controller. Worst case is about 2% difference vs. PV 34V and 100V.

There are more losses when taking larger steps in voltage.

It's also the reason that I say the efficiency loss in moving to higher PV is likely offset by reduced wiring losses with higher PV voltage.
Based on that. I could do 2s2p (40.8 Vmp / 48.6 Voc) and go with the 100/35 controller.

However, if I need 24v + 5v for the SCC to start. Wouldn't the higher Vmp (all series) be better?
 
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So, the charts imply mppt controllers operate more efficiently at voltage close to the battery voltage…
However, the chart does not show how well across the board different voltages will operate in low light situations, cloudy, dawn or dusk… if only the sun would cooperate and stay in one place relative to the panels!
I vote, overpanel the controller, setup the panels for best average solar aiming, and so the load won’t drop the voltage below 150% of battery voltage…
 
My VoC is 142v with a nominal 48v battery. I'm getting over 7 solar hours in Connecticut, USA right now ... flat mounted panels. Makes me wonder whether my high VoC is contributing to my ability to charge even in bad conditions (clouds, early/late in the day, etc). I'm only supposed to get 5.5 here.
 
So, the charts imply mppt controllers operate more efficiently at voltage close to the battery voltage…
However, the chart does not show how well across the board different voltages will operate in low light situations, cloudy, dawn or dusk… if only the sun would cooperate and stay in one place relative to the panels!
I vote, overpanel the controller, setup the panels for best average solar aiming, and so the load won’t drop the voltage below 150% of battery voltage…
That is why I asked if the higher Vmp of putting panels all in series VS efficiency of SCC be slightly better with the lower Vmp of 2s2p ... which would be better in the long run.
 
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