Moving forward with solar

[Hedges]

cells connected in parallel doesn't balance them.
Cells connected in parallel and charged part way past knee of the curve is what balances them. Typically to 3.65V.

That's parallel top balancing.

Got a BMS (or two)?
You'll need that when you series charge them.


10 panels in series x 42.04Voc = 420.4V. That is too close to 450V, just 7% rise due to cold temperature and it will exceed 450V.
You need about 16% headroom. To compute that exactly, we need:
1) Temperature coefficient of Voc from PV panel data sheet.
2) Record coldest temperature for your location.

 

[SunCatcher]

cells connected in parallel doesn't balance them.
Cells connected in parallel and charged part way past knee of the curve is what balances them. Typically to 3.65V.

That's parallel top balancing.

Got a BMS (or two)?
You'll need that when you series charge them.

Okay... so turns out I have two BMS and a bunch of wires to hook up to the cells. What should I do now? I did look into a charger but as I recall it was $130.00 and it would never be used again. I am willing to spend that if it is important enough.

10 panels in series x 42.04Voc = 420.4V. That is too close to 450V, just 7% rise due to cold temperature and it will exceed 450V.
You need about 16% headroom. To compute that exactly, we need:
1) Temperature coefficient of Voc from PV panel data sheet.
2) Record coldest temperature for your location.

So as far as temperatures go for here. We grow a garden all year long and while it may freeze overnight in Dec. Jan. but most of the time it is back up above freezing by the time the sun gets up enough to make any electricity. In an normal year it would hit 25 as a low but always back up past freezing and usually 40 degrees by lunch. Its rare that it doesn't hit 40 during the day. However... I did check and found out that in 1990 it went way down and hit -3 so that would be that once in 30 - 50 years. So that keeps things interesting.


I have 28 Aqua-Blu325 panels

Nominal Power (Pmpp): 325 Watts

• Nominal Power Voltage (Vmpp): 34.36 Volts
• Nominal Power Current (Impp): 9.32 Amps
• Open Circuit Voltage (Voc): 42.04 Volts
• Short Circuit Current (Isc): 9.77 Amps
• Panel Efficiency: 18.87%

I am trying to learn and understand, So the question that I have is this. Open circuit voltage... when determining how many panels that can be safely connected lets say that I have 10 panels at 42. 04 volts totaling 420.4 volts. That number is determined with "No load" However the Nominal Power voltage is 34.36 and perhaps I do not understand what that really means but it obviously means that it is under load. So if the temperature drops down to say 20 degrees that means the furnace will be running a lot, the batteries will be charging as soon as there is PV voltage. After all we just had 15 or so hours of darkness, and adding to that we likely will only have 3 hours a day for solar gain meaning our batteries a running very low. even if they get fully charged, which wont happen here because of winter clouds, we then will be selling power back to the grid. So in the real world how would 10 panels, for example, be even close enough to harm the Inverter in winter months? There is a lot of demand and so in reality it would seem that the array would be operating somewhere between Nominal Power Voltage of 34.36 per panel and well below the Open circuit voltage of 42.04. I would love to hear the answer to that question as it would help me make a lot more sense out of this.

In the end I really don't care how many panels per string and am hoping with help from here to come up with a practical solution. We are VERY close to energizing the system.

Les

 

[Hedges]

Here are the instructions for top balancing:

Explanation for Beginners of Top and Bottom Balance

This is a beginners explanation of top and bottom balancing cells that should help you understand what it is all about. To get the document, click on the orange button at the top of the page. NOTE: This is not intended to be a tutorial of how...

diysolarforum.com

Cells as received are a partial state of charge. To charge them in parallel with a 3.65V supply takes forever.
By assembling the series pack with BMS and charging the battery until BMS disconnects, they get much closer to full. PV + SCC can do that, or a bench supply.
You then need a way to charge low cells up to match the high cells (a CV/CC supply set to 3.65V), or a way to bleed off high cells toward the lower ones (any resistor, even a light bulb. But you lack an automatic control mechanism). Following a bleed-off, PV + SCC can again charge towards full.
It is very important to never over-charge any cell, which would ruin it.

If well balanced, battery can be charged by PV to some voltage where all cells are relatively close to 100% SoC and BMS passively balances. If not close enough, BMS disconnects and your system shuts down. Depending on how close you get them, may be able to set a reduced charge voltage and over a long time BMS will passively balance them. If far apart (e.g. 30% SoC for some, 50% SoC for others), that may be less practical, would limit usable capacity.)

Some people report just building and using a pack without balancing. So try that (after reading balancing instructions) because it is the first step of balancing.

 

[Hedges]

So as far as temperatures go for here. We grow a garden all year long and while it may freeze overnight in Dec. Jan. but most of the time it is back up above freezing by the time the sun gets up enough to make any electricity. In an normal year it would hit 25 as a low but always back up past freezing and usually 40 degrees by lunch. Its rare that it doesn't hit 40 during the day. However... I did check and found out that in 1990 it went way down and hit -3 so that would be that once in 30 - 50 years. So that keeps things interesting.


So in the real world how would 10 panels, for example, be even close enough to harm the Inverter in winter months? There is a lot of demand and so in reality it would seem that the array would be operating somewhere between Nominal Power Voltage of 34.36 per panel and well below the Open circuit voltage of 42.04. I would love to hear the answer to that question as it would help me make a lot more sense out of this.

In the end I really don't care how many panels per string and am hoping with help from here to come up with a practical solution. We are VERY close to energizing the system.

Assume 50-year or 100-year record cold when designing for VoC. The consequences if you guess wrong is wrecking the electronics. Many people have done that (if not due to cold, just due to wrong voltage.)

Vmp, temperature, and MPPT voltage range are less critical. If not correct it just produces less power or none until conditions change.

Out of 28 panels you can just set one aside as a spare (or use it with another charge controller to maintain a car battery for jump starts.)
With 27 panels you can do 9s x 3 into three inputs.
With 28 panels you could do 7s x 4 into four inputs, if inverter/charger has that many.

I like strings of multiple orientations, one to catch morning sun and one for afternoon. More flat production throughout the day.

 

[SunCatcher]

I like strings of multiple orientations, one to catch morning sun and one for afternoon. More flat production throughout the day

Yes... I see that is VERY commonly used around here. Because this is a ground mount the panels are oriented the same.

I decided to call Solark to see what they had to say. This time I got a guy that him and I communicated well. Last time that didn't happen...probably my fault. So my questions were answered and that the solution was the same one that I had come up with but I didn't put the pieces together in the correct way. So Solark says that I am fine with 10 panels totaling 420 volts even in cold weather since the Inverter is designed to handle 500 Volts plus. Problems arise after 550 Volts. SO then the other two legs can each have 9 panels and they hook to the other MPPT using the dual inputs. The Voltage will be 9x 42 but the amperage is still only 9.32 since it doesn't multiply like Voltage. So I double checked to make sure I had understood correctly and so I think I have this right.

So here is what I am looking at

MPPT #1 - 10 panels in series for 10x 42.02 = 420,2 volts having 9.32 Amps
MPPT #2 - Leg 1 is 9 panels in series for 378 Volts having 9.32 Amps
- Leg 2 is 9 panels in series for 378 Volts having 9.32 Amps

From Solark Manual

A. Sol-Ark has DUAL MPPTs for two separate PV input pairs
B. MAX PV input = 13kW (± 5%) / system | 6.5kW / MPPT | MAX 500VOC PV | MAX ISC /MPPT 25A (limiting to 20A) Damage will occur if PV VOC > 550V
C. Parallel strings per MPPT must be the same Voltage i. PV1 A/B must be the same voltage if using both strings ii. Panels on the same MPPT CAN face different directions
D. Ground the panel MOUNTS/FRAMES to any ground in the Home via 12AWG wire E. IF using Y-Connectors: Running two strings in parallel, totaling 20A (self-limiting)

The reason that I struggled so much with trying to understand this is that since Voltage multiplies when hooked in series... I was thinking that Amperage multiplied as well. But after realizing that is not the case... as long as the Voltage on two legs are the same then you can hook them together and the Inverter is happy.

Well... I came here to learn... LOL

Les

 

[SunCatcher]

AND more good news. This time Solark really came through for me. I asked them about the 120% rule and here is their solution. Thought I would share it here hoping it will help others. They said this unit can only supply 9000 watts maximum so they are recommending putting in a 40 Amp circuit breaker. What a simple solution. Once we get down the road a bit I may come back and update how that went. I love how clean and simple that is and right from the manufacturer. The Inverter cannot supply more than that breaker can produce. So when I first called Solark last fall I was not able to communicate with the guy and was left kind of discouraged. That is why I joined here. But sometimes it pays to call back and this time things went VERY well. But I do want to thank you guys for the help and encouragement along the way. For that I am grateful.


Les

 

[Hedges]

If a 200A panel with 200A main breaker, 120% rule allows 40A PV breaker installed at far end.
It is only supposed to carry 80%, 32A, continuous, which is about 7700W at 240V.
If sol-ark exported 9000W continuous, could trip on a hot day. Marginally sized wire could contribute to that; larger is better.

I forget if we went over panel/busbar rating. If your panel has 225A busbar and 200A main breaker (like Square-D QO), then you can have 70A PV breaker.

Because yours is ground mount you could (have?) design(ed) with two orientations. That would help stay under max current.