Solar panel math assistance

DickyDck · August 2019

Need some second person double checking to make sure my math is right. I'm trying to understand the equations that were used to make up my current solar array, here is my info as far as it pertains to what I'm working with...

2 x Schneider MPPT 60-150

24 Hanwha QCell Panels - Specific info below
Nominal Power = 305 Watts
Short Circuit Current = 9.84 Amps
Open Circuit voltage = 40.05 Volts
Current at maximum Power = 9.35 Amps
Voltage at maximum power = 32.62 Volts
Maximum System voltage = 1000 Volts
Weight 41.45 lbs (not that this matters but it's on the sticker)

so the array is set up in 8 sets of 3 panels in series. I'm not sure how to figure out amps/watts/volts for this, so this is my guess based on what the youtube has tried to explain to me...

3 panels in series
Nominal Power value x 3 panels = 915 watts? (this is hopeful max of course)
Voltage at maximum power x 3 panels = 97.86 Volts?
Total Watts / total Max Volts = 9.35 amps?

If this is correct, and I have 8 strings total, 4 strings going to each charge controller, meaning each charge controller is doing it's things with the following numbers...

1 x MPPT 60-150 handling
Max 3660 Watts
Voltage at maximum power = ??? not sure if it stacks to 391.44V, or stays the same across all at 97.86V
Amps = unknown if it stays the same or adds up? Stays at 9.35a or adds to 37.4a

since all the breakers in my combiner are 30amp, the math has me all kinds of confused.

Also, what gauge wire should I have based on these numbers going from the combiner to my charge controllers? There are 2 Positive and 2 negative (Red and Black) one set going to each charge controller, I haven't looked close enough to see what gauge it is but it seems thin to me...

all in all, I'm looking into adding panels, to over provision since you can only really expect 70% of rated power output, I have some room to add to get a little bit more out of my rig.

Photowhit · August 2019

DickyDck said:

Need some second person double checking to make sure my math is right. I'm trying to understand the equations that were used to make up my current solar array, here is my info as far as it pertains to what I'm working with...

Cool!

DickyDck said:
so the array is set up in 8 sets of 3 panels in series. I'm not sure how to figure out amps/watts/volts for this, so this is my guess based on what the youtube has tried to explain to me...

youtube =

Easiest is to use a string sizing tool, but since you appear to be okay on the Max volts (3x VOC=120 volts, max is 150, so unless you are in a very cold climate you should be safe)

You did not tell us the system voltage (battery bank) I'll assume 48 volts for a large system like this.

Charge controllers are rated at their output current so these are 60 amp output, lets see how you match up.
Since MPPT charge controller reduce the voltage presented to the optimum charging voltage, again assuming lead acid batteries, 48 volt system charging at 58 volts.

You have

DickyDck said:
3 panels in series
Nominal Power value x 3 panels = 915 watts? (this is hopeful max of course)
Voltage at maximum power x 3 panels = 97.86 Volts?

Correct!

DickyDck said:
1 x MPPT 60-150 handling
Max 3660 Watts
Voltage at maximum power = ??? not sure if it stacks to 391.44V, or stays the same across all at 97.86V
Amps = unknown if it stays the same or adds up? Stays at 9.35a or adds to 37.4a

In series the voltage adds and the amperage stays the same.
In parallel the amperage adds and the voltage remains the same.
So 97.86volts (potential) at 37.4 amps.
Amps x volts = watts so
97.86 x 37.4= 3660 watts.

DickyDck said:

since all the breakers in my combiner are 30amp, the math has me all kinds of confused.

That's not good, The panels have a series fuse/breaker rating of 20 amps. Each string should have a breaker of 20 amps. It's in the specs;

https://www.tentensolar.nl/images/producten/Zonnepanelen/Datasheet_zonnepanelen_Hanwha_Q-CELLS_300_305Wp_Mono_White_sheet_black_frame.pdf

DickyDck said:

Also, what gauge wire should I have based on these numbers going from the combiner to my charge controllers? There are 2 Positive and 2 negative (Red and Black) one set going to each charge controller, I haven't looked close enough to see what gauge it is but it seems thin to me...

Higher voltage can run on thinner wires, usually we look for 2-3% voltage drop across the distance. You can plug in the voltage, current(amps) and distance into a "DC voltage drop calculator"

Here's one;

https://www.calculator.net/voltage-drop-calculator.html

DickyDck said:

all in all, I'm looking into adding panels, to over provision since you can only really expect 70% of rated power output, I have some room to add to get a little bit more out of my rig.

The Normal Operating Cell Temperature numbers are about 75% of the Standard Test Condition rating (STC). And during the morning and cool temps you could plan on getting 80-85% and can get STC in very cold clear sunny days.

Looked to me like you were over the max cost effective size array on each charge controller now. Let's run the numbers.

You have 3660 watts going to each charge controller, charging voltage for a 48 volt flooded lead acid battery bank is around 58 volts. 3660watts ÷58 volts = 63amps. 63 x .8 = 50amps. So squeezing on another string of panels likely would not be cost effective, but close. Would help on overcast days...

DickyDck · August 2019

Thanks!!!! So yes my battery bank is 48v, but running AGMs so I'm not charging at 58v but 55v. Even on the brightest days I get right around 6kw -/+ 500w, and I was planning to add 6 more panels (2 strings of 3 panels, 1 string added to each CC) which would as you said help on overcast days which average me around 4.5kw - 5.0kw for the sun hour peaks between 10:00 to 16:00. I can get the panels, the cabling, the mounts, for about 1600. The only thing I'm worried about is the gauge cable, I think it is 2 awg, but I need to verify physically with my own eyes.

mcgivor · August 2019

What capacity are the batteries? There are limitations to the amount of current they can accept, furthermore running charge controllers at maximum capacity shortens life expectancy by running hot, they also reduce their output over 40°C heat sink temperature, so pushing the limits may result in less than expected results.

littleharbor2 · August 2019

Find the specs for charging your batteries. Hopefully they are deep cycle. Best to charge them at the mfg. specs. 55 volts sounds a bit low. 58 seems about right but that will be proven in the mfg. specs.

DickyDck · August 2019

All my solar gear is in my garage which is climate controlled, keeping it around 75 degrees or less so keeping the heatsinks cool has been easy enough, and keeping the batteries cool has been pretty good, I took them off the wooden sheet and the concrete is helping them stay between 23-26 celsius. I'll double check the battery specs, I read on a generic site and came up with 55.x but you are correct the manufacturer will probably have specific charge specs.

littleharbor2 · August 2019

Curious as to what the numbers are when you find out. Here is the numbers for CSB batteries. X4 would be as high as 60 volts.

Image: https://us.v-cdn.net/6024911/uploads/editor/fk/7bbf0dqn4gir.jpg

DickyDck · August 2019

you are correct! 57.2 is what I should be at...

https://www.wholesalesolar.com/cms/four-star-solar-crown-48-vdc-10-560-wh-with-mnbe-d-enclosure-8-battery-bank-agm-battery-maintenance-2936570746.pdf

crown 6v agm x 16 of them making 440ah, I can set the Inverter charger to 57 but the charge controllers doesn't seem to allow it to go that high, I'll have to investigate further

Photowhit · August 2019

DickyDck said:
crown 6v agm x 16 of them making 440ah,...

I opened your link and it directly posts info on charging;

If I understand what you are saying, you have purchased, 16 of the 'golf cart size batteries that would be 220 amps each, and you will have 2 strings for a 440ah 48 volt battery bank?

I would think this is undersized. I don't know your loads but just one of your 2 arrays could provide up to 50 amps or about a 11% charge rate. While it appears they can handle up to 25%, that might be over kill for you. I certainly wouldn't add to the arrays for this battery bank.

DickyDck · August 2019

yep, this is definitely undersized for the battery bank no doubt. I should have double at around the 800+ mark, which initially the idiots who set up my system had me only on 220ah. I got the second string to make 440 and a lot of the weird abnormalities in my system went away. I do manage to sell back a decent amount since I'm grid tied, which does go back into the main panel and save me some money overall on my power bill. A lot of guys here said I should go FLA batteries which I might do when these 16 start showing degradation, but for now it seems to be working pretty good.

Photowhit · August 2019

Grid tied changes things a bit...

I didn't get around to suggesting @mcgivor was wrong, and that off grid systems don't work at maximum capacity enough to worry about, but now I fully agree and wouldn't add more array to each charge controller, but rather add another charge controller if you wanted to increase the array.

Where are you located?

DickyDck · August 2019

I’m In Virginia, I was thinking about possibly doing another CC as well, though it would be interesting where I would place it on the wall, could figure it out though. I wouldn’t be able to add an entire 12 panel string though, would having a third CC with only 6 panels on it mess with the overall system, or would it be intelligent enough to work it all out on its own?

Photowhit · August 2019

I don't deal with hybrid systems personally, but since they would basically be feeding the battery bank/system. I don't think it would be a problem. I take it it's a Schneider hybrid inverter? @Dave Angelini is likely to know also if you post the model, there may be minimal specs for the battery bank. Dave does do this for a living...

You could add 1 string and spread them out 3 to a charge controller.

I asked because I have an old 60-150 that is suppose to work. If you were local, I'd let you set it up and agree on a price, if it worked for a year...lol.

DickyDck · August 2019

How old is old? Depending on price I might be interested, and yes all my stuff is Schneider, minus the blatant under sizing that the company did with my system it’s been good for me so far. Dave has helped me out with other things I’ve been wondering about with my system, it’s been really helpful. Ping me direct about that CC you have, if it was working when you stopped using it in theory it should still work?

Photowhit · August 2019

Sorry, I wouldn't sell it that way. I don't know the history, never used it, just came with some stuff I purchased and so I could only do something where I got paid after it worked for a while. If I get a chance I'll set it up in my system in a week or 2. I probably shouldn't have brought it up. If it makes you feel any better it's a pretty early version and isn't very pretty. I think I had asked Dave about something about it, that made me feel a bit better about the version or date made...

Estragon · August 2019

IMHO, controller capacity shouldn-t be calculated using absorb voltage (when current will be tapering anyway). Max current would likely be in bulk, so a lower voltage (eg 48v, roughly 50% SOC seems more appropriate, but worst case could be ~LBCO of 42ish volts).

3660/48v=~76a. Depending on location, the array isn't likely to put out full STC rating, but could still be running the CCs near full capacity. Especially with the grid tie, having some more CC capacity to let them run cooler makes sense to me

DickyDck · August 2019

sounds like the general consensus is a third CC, which means I'll have to get another Combiner since it looked like it is only configured for 2 strings. I'll take a pic later and someone can correct me if I'm totally out in left field.

Estragon · August 2019

With 8 strings and 3 controllers, you'd likely do 3 strings into each of 2 controllers, and 2 strings into the third. You probably don't really have to use a combiner for the third controller, as you can likely parallel the strings with mc4 'Y' connectors. Breakers/fuses are needed with 3 or more in parallel. All subject to local codes though.

Of course, having the breakers and box can be handy as disconnects for troubleshooting etc., and would be needed if adding a third string to that CC.

littleharbor2 · August 2019

Estragon said:

Of course, having the breakers and box can be handy as disconnects for troubleshooting etc., and would be needed if adding a third string to that CC.

If nothing else it will be a transition box to change from PV wire to less expensive wire, albeit in conduit.

Photowhit · August 2019

Estragon said:

Max current would likely be in bulk, so a lower voltage (eg 48v, roughly 50% SOC seems more appropriate, but worst case could be ~LBCO of 42ish volts).

But that's NOT how charging lead acid works.

The transfer of current is pretty much 1:1, but there must be a higher voltage, usually 10-15% to get current to flow to the battery bank. So if your battery is at rest at 50% SOC around 48 volts, you charge controller would limit voltage to around 48+4.8(10%) for a voltage of around 52.8 volts or even higher!

That's the reason batteries can be held in 'float' with a voltage of 13.6 volts for a 12.7 volt (at rest) battery, without over charging them. A 14.5 volt charge rate at absorb is 12.7/14.5= about 14% above the resting voltage.

AGM should be somewhat more efficient, but the charge controller will want the current to flow into the battery so will present a higher voltage.

DickyDck · August 2019

I went out to the box on my lunch break and took some pictures, I could in theory add a 3rd metal plate contraption while getting 2 smaller ones, or 3 equal size ones.... I'm not sure..... to avoid having to get a new combiner box. but the space would be tight getting 3 plates, but it "could" work. And I would just have to get another 40 yards of red and black copper to run through the underground pipe to my garage for the third CC. I'm a horrible estimator of gauge though, and it looks like the # rubbed off on the cables here, so I will have to get something to gauge the cable diameter before I go out and purchase some.

Image: https://us.v-cdn.net/6024911/uploads/editor/j7/k95r61924zqc.jpg

DickyDck · August 2019

anyone know what those things are called, that I have circled in the picture? or do I have to custom make these things somehow?

Photowhit · August 2019

I think they are commonly known as "Comb bus bars". I did a quick search, maybe finger bus bar is better...

This might work, they give no dimensions only for a link to a din rail pass through block and a 125 amp rating. I'd like them to sayit's tinned copper...

Soladeck Negative Bus Bar - 4 String

https://www.altestore.com/store/enclosures-electrical-safety/electrical-enclosures/combiner-pass-through-boxes/soladeck-negative-bus-bar-4-string-p9533/

DickyDck · August 2019

Photowhit said:

I think they are commonly known as "Comb bus bars". I did a quick search, maybe finger bus bar is better...

This might work, they give no dimensions only for a link to a din rail pass through block and a 125 amp rating. I'd like them to sayit's tinned copper...

https://www.altestore.com/store/enclosures-electrical-safety/electrical-enclosures/combiner-pass-through-boxes/soladeck-negative-bus-bar-4-string-p9533/

SInce I'm only looking to add 6 panels, which would be 2 additional breakers, this would probably work! now to figure out the gauge cable to get...

Estragon · August 2019

Photowhit said:

Estragon said:

Max current would likely be in bulk, so a lower voltage (eg 48v, roughly 50% SOC seems more appropriate, but worst case could be ~LBCO of 42ish volts).

But that's NOT how charging lead acid works.

The transfer of current is pretty much 1:1, but there must be a higher voltage, usually 10-15% to get current to flow to the battery bank. So if your battery is at rest at 50% SOC around 48 volts, you charge controller would limit voltage to around 48+4.8(10%) for a voltage of around 52.8 volts or even higher!

That's the reason batteries can be held in 'float' with a voltage of 13.6 volts for a 12.7 volt (at rest) battery, without over charging them. A 14.5 volt charge rate at absorb is 12.7/14.5= about 14% above the resting voltage.

AGM should be somewhat more efficient, but the charge controller will want the current to flow into the battery so will present a higher voltage.

My 48v nominal bank is at 87% SOC this morning, which would have a resting voltage of ~50v. Classics are charging in bulk at 51.5v. Yes, a bit higher than resting for that SOC so current flows to the bank, but not by 10%+.

If the bank was at a lower SOC, the controller output would be at a lower voltage and higher current, which is why I'm suggesting Vabs may be too high for a conservative estimate of required controller capacity.

When they float, the batteries will be held at ~52.1v. The bank and controller will both be at or near that potential, so little current will flow.

DickyDck · August 2019

Estragon said:

Photowhit said:

Estragon said:

Max current would likely be in bulk, so a lower voltage (eg 48v, roughly 50% SOC seems more appropriate, but worst case could be ~LBCO of 42ish volts).

But that's NOT how charging lead acid works.

The transfer of current is pretty much 1:1, but there must be a higher voltage, usually 10-15% to get current to flow to the battery bank. So if your battery is at rest at 50% SOC around 48 volts, you charge controller would limit voltage to around 48+4.8(10%) for a voltage of around 52.8 volts or even higher!

That's the reason batteries can be held in 'float' with a voltage of 13.6 volts for a 12.7 volt (at rest) battery, without over charging them. A 14.5 volt charge rate at absorb is 12.7/14.5= about 14% above the resting voltage.

AGM should be somewhat more efficient, but the charge controller will want the current to flow into the battery so will present a higher voltage.

My 48v nominal bank is at 87% SOC this morning, which would have a resting voltage of ~50v. Classics are charging in bulk at 51.5v. Yes, a bit higher than resting for that SOC so current flows to the bank, but not by 10%+.

If the bank was at a lower SOC, the controller output would be at a lower voltage and higher current, which is why I'm suggesting Vabs may be too high for a conservative estimate of required controller capacity.

When they float, the batteries will be held at ~52.1v. The bank and controller will both be at or near that potential, so little current will flow.

do you have a battery monitor doing your SOC readings or old fashioned hydrometer thingy?

Estragon · August 2019

That SOC reading was from the (Midnite Classic) charge controller via smartphone app. I also have a trimetric battery monitor, which generally more or less agrees with the Classics.

Both meters read overall bank voltage, which is useful for day-to-day monitoring, but doesn't tell me much about individual battery/cell charge. For that I use the old fashioned hydrometer thingy to check cell SGs every month or two.

DickyDck · August 2019

if the damn Schneider batt monitor was so damn expensive I would get one, but for the amount of batteries I have I don't see it as necessary. Just more for good informational data which I like

Photowhit · August 2019

Estragon said:

My 48v nominal bank is at 87% SOC this morning, which would have a resting voltage of ~50v. Classics are charging in bulk at 51.5v

Lithium batteries?

If flooded lead acid check your setting against SG reading, at 87% you should be charging at much higher voltage and would not be in bulk.

I did specify battery chemistry. Lithium are much more efficient and some AGM will be as well, but wouldn't be in Bulk.

Estragon · August 2019

Batteries are flooded L16s.

The reading was early morning. Charging (ignoring Classic skip days) always starts in bulk regardless of SOC. It stays in bulk until voltage rises to Vabsorb (currently just under 60v).

At a low SOC, it can stay in bulk for hours. On a day when the sun doesn't emerge from gloom until late morning, voltage won't rise much until the sun comes out, at which time max current at the lower voltage will start slowly increasing the voltage. At 50% SOC it would take ~3hrs with decent sun to get there.

From todays SOC, it can usually hit Vabs fairly quickly, even on a mainly cloudy morning like today.

Solar panel math assistance

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