PVM Solar Controller Voltage vs PV Max Input Voltage and Battery Rated Voltage

[itsmaxdd]

Hello gentlemen,
I hope to be in the right place.
I'm about to build a PV system and i have some questions.
As the title says i've a doubt regarding the design of a Solar panel system.
Right now my equipment has the following specs:

Solar Panel:
Max Power 6KW (250W*24)
Voc = 43.2 V
Vmax = 34.4 V
Isc = 7.85 A
Imax = 7.26 A

Battery Pack:
Rated Cap = 300Ah
Rated Voltage = 96V (12V*8)

Now the inverter doubt, assuming those two possibile scenarios

Inverter A:
Rated Output Cap = 5KW
Peak Power = 10KW
Battery Voltage (DC) = 48V
PWM Solar Controller Voltage = 48V
PWM Solar Controller Current = 60A
PV Max Input Voltage = 100V

Inverter B:
Rated Output Cap = 7KW
Peak Power = 14KW
Battery Voltage (DC) = 96V
PWM Solar Controller Voltage = 96V
PWM Solar Controller Current = 50A
PV Max Input Voltage = 200V


So my questions are:

1. Is it true that Inverter A can't be use if the Battery Voltage (assuming 96V) is greater than the allowed by the inverter? Will it be convinient to perform a parallel series so that the Battery Rated Voltage should be 12V*4 = 48V with a Rated Cap = 600Ah ?
2. Is it true that Inverer B can be used in both the configurations mentioned above?

Now i'm assuming that those specs are for each single panel (i've some doubts though)

3. Inverter A has a PV Max Input Voltage = 100V so 100V/43.2V leads to a maximum of 2 panels in series (with some margin). Is it true? To me it's not because the Solar Vmax (34.4V) * 2 panels = 68.8V which is greater than the PWM Solar Controller Voltage (48V). If what i'm saying is correct i can only use 1 panel.
4. Inverter B has a PV Max Input Voltage = 200V so 200V/43.2V leads to a maximum of 4 panels in series (with some margin). Is it true? To me it's not because the Solar Vmax (34.4V) * 4 panels = 137.6V which is greater than the PWM Solar Controller Voltage (96V). If what i'm saying is correct i can only use 2 panels in series.

Is it true i need a charge controller?

What could it be the best configuration then?

Regards,
Max

[sensij]

If the inverter has a solar charge controller built in, you do not need to buy an external one. You might choose to do so if you want the better yield an mppt controller can offer over pwm.

The inverter input voltage and battery voltage generally have to match. 96 V inverter requires 96 V battery, 48 V inverter requires 48 V battery. No mixing.

You use the Voc of the panels to test against the max input voltage of the controller. So, 2 and 4 panels in series, respectively. Your observation on the difference between the V at the maximum power point and the battery voltage is good... It means that the pwm controller may not be extracting the maximum possible power from the panels. This is why you might consider buying an external mppt controller.

[itsmaxdd]

If the inverter has a solar charge controller built in, you do not need to buy an external one. You might course to do so if you want the better yield an appt controller can offer over pwm.

The inverter input voltage and battery voltage generally have to match. 96 V inverter requires 96 V battery, 48 V inverter requires 48 V battery. No mixing.

You use the Voc of the panels to test against the max input voltage of the controller. So, 2 and 4 panels in series, respectively. Your observation on the difference between the V at the maximum power point and the battery voltage is good... It means that the pwm controller may not be extracting the maximum possible power from the panels. This is why you might consider buying an external mppt controller.

Thank you for your answer.
By reading more in details (chinese manual) it is clearly stated that the controller is an MPPT type.
The Specs above were probably wrong.
Can you take a look at my excel sheet to see if what i'm calculating is correct?

https://docs.google.com/spreadsheets...=0&single=true

From your answer i can assume i can't double my total battery pack capacity.
At the same time my observation doesn't hold anymore in this new case since a 5x4 and 4x5 lead to a maximum voltage of 181V and 145V respectively which are below the Voltage input range of 120-250V.

Regards,
Max

[itsmaxdd]

Thank you for your answer,
I've found new spec since the manual wasn't the one of my inverter.
It is stated that mine is MPPT.
Can you give your opinion regards this excel sheet i've done?

https://docs.google.com/spreadsheets...=0&single=true

If what you are saying is true then it seems that i can't go for a doubled capacity since i need to maintain 120V. Moreover if those are the new specs i have no problem regarding my last observation as well.

[sensij]

I'm glad to see you are considering the temperature effect on Voc. I can not see the content of the formulas in the spreadsheet, and some of the calculations look off on superficial inspection.

What is the model of your inverter? What panels are you planning on using (first post said 250 W, 2nd post said 300 W)?

[itsmaxdd]

I'm glad to see you are considering the temperature effect on Voc. I can not see the content of the formulas in the spreadsheet, and some of the calculations look off on superficial inspection.

What is the model of your inverter? What panels are you planning on using (first post said 250 W, 2nd post said 300 W)?

Sorry for the double post.
Try this link

https://docs.google.com/spreadsheets...it?usp=sharing

The panels are indeed 300W * 20
The model is an ALLRUN Qingdao model (pretty unknown).
Can you be more precise regarding the mistakes?

Thank You,
Max

[sensij]

I don't understand your Voc temperature adjustment calculation. Typically, the coefficient would be published as something like -0.32% / deg C, which translates in this case to -0.32% * 43.2 = -0.14 V / deg C. In the extreme case of -10 deg C, relative to the published STC value @ 25 deg C, that is -35 deg C difference. -35 * -0.14 = 4.9 V per panel increase in Voc. 44.8 + 4.9 = 49.7. With 4 panel strings, that is 4 * 49.7 = 199 V, while 5 panel strings would be 248 V. That is the number to compare to the max PV voltage input allowed for the charge controller portion of the system.

In your battery design section, you have "8 x 1" next to 120 V, and "4 X 2" next to 60 V. Are those voltages intended to indicate the min PV voltage that is required to charge that battery bank?

[itsmaxdd]

Hello sensij,
It is true that normally the Temp Coeff is -0,32 %/°C but in my datasheet it is reported as - 80 mV / °C hence i'm assuming that for each °C i have -80mV and since the delta temperature is 35°C / 30°C i've just multipied the two.
I've reasoned like that because it can't be 80%/°C (i doubt it is 0.80% as well) and because the Power Coeff and the Short Current Coeff are reported as you say in %/°C.
Of course the design that uses OC/SC has already safe margins because it means that all the solar panels should break at the same time (correct?), so even if i would be reaching the Max PV Voltage the effective voltage (under normal condition load) would be lower.

The second question is related to the answer you gave me, you said that "96 V inverter requires 96 V battery, 48 V inverter requires 48 V battery. No mixing" , were you referring to the fact that i can't mix 96V with 48V batteries or i can't mix (in this case) 120V inverter with 60V batteries (due to the parallel/series design of the battery)? In short, if i have those batteries with 12V * 10 = 120V can i put only 6 of them in series with other 6 batteries in parallel in order to achieve 60V (in a 120V input inverter) and so double the capacity? Is it worth in first place, powerlosses aside?

[itsmaxdd]

https://www.victronenergy.com/upload...WM-or-MPPT.pdf

From this useful paper

"An MPPT controller attempts to harvest power from the array near its Maximum Power Point, whilst supplying the varying voltage requirements of the battery plus load. Thus, it essentially decouples the array and battery voltages, so that there can be a 12 volt battery on one side of the MPPT charge controller and two 12 Vpanels wired in series to produce 36 volts on the other. "

This means that i can indeed work at different voltages.... Than i still can't understand why the Battery Input voltage is rated as 120V instead of "< 120V". Is there a minimum voltage required?

[sensij]

Please post the data sheet for your inverter.

[itsmaxdd]

This is the only thing i've found

https://drive.google.com/open?id=0B9...pjQVVVendjc1lN

https://drive.google.com/open?id=0B9...YyTGZuY3d2VEJz

The rated voltage is not reported correctly (it is indeed 120V)

[sensij]

I don't see what is reported incorrectly. The battery bank you would use with this inverter is 96V. The PV strings would be designed to provide a Vmp of at least 150 V, and keep the temp corrected Voc below 300 V.

Does that help?

[mrduongonline1]

Your observation on the difference between the V at the maximum power point and the battery voltage is good. Than i still can't understand why the Battery Input voltage is rated as 120V instead of "< 120V". Is there a minimum voltage required?



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