SCC Sizing, What is VOC Temp Coefficient and Pmax Temp Coefficient?

[PeteW]

So, I'm just minding my own business trying to learn all I can about installing a decent solar system on our rig when along comes MPPT sizing and kicks my A**. Okay kidding aside, I can't seem to find understandable answers (for me) to this problem. Nate Yarbrough puts in a college try with his video about Sizing Solar Chargers. He even provides a calculator, but I'm still struggling. Primarily I do not understand how to arrive at or find the VOC temp coefficient and the Pmax temp coefficient (his calculator requires ) to arrive at the right size SCC for my (8) Rich Solar 100W noncrystalline solar panels. I know I need to factor in the coldest temps I expect to be using my panels in. Since I'm putting them on my travel trailer, I want to plan for the possibility of being in extremely cold weather even though I hope that never happens. If anyone can help me with these questions I'd be grateful. Not that it matters much to answer my above questions (I don't think), but I'm planning on building a 24V battery bank.

 

[snoobler]

Voc temp coefficient is the percent rise in voltage for a corresponding temperature change from 25°C:

-0.33%/°C means that if the temperature increases to 35°C (+10°C), the voltage will DECREASE by 3.3%; additionally, if the temperature drops to 15°C (-10°C), the voltage will INCREASE by 3.3%.

The Pmax temp coefficient is similar to the above, but it's a % loss in power based on an increase in temperature:

-0.4 %/°C means there is a 0.4% loss in power for every °C above 25°C

They are part of the panel specification - not necessarily on the sticker, but they should be on the datasheet from the manufacturer.

 

[PeteW]

snoobler,
Thanks for coming to my rescue. This is really interesting and helpful. If you can tolerate some additional questions, Please explain to me how you arrived at the -0.33%/°C for the VOC and the -0.4%/°C for the Pmax. And are these the coefficient? Is there a place I can find the formulas for this to study them. I tried to write down what Nate was saying, but honestly, he talks so fast that I really had difficulty following him. Hahaha

 

[rmaddy]

Please explain to me how you arrived at the -0.33%/°C for the VOC and the -0.4%/°C for the Pmax. And are these the coefficient? Is there a place I can find the formulas for this to study them.

Most solar panel manufacturers provide data sheets for their solar panels. And listed among all of the other specs for the panels you will normally find those coefficient values. I couldn't find any such values on the Rich Solar website. But the following link does have them:

ENF Ltd.

Rich Solar Inc Solar Panel Series Mega 100 Watt. Detailed profile including pictures, certification details and manufacturer PDF

www.enfsolar.com

There you will find:

Temperature Coefficient of Pmax	-0.39 %/°C
Temperature Coefficient of Voc	-0.29 %/°C

There's no formula to obtain those values. They are values provided by the manufacturer just like all of the other numbers they provide.

But there are calculations based on panel arrangement, system voltage, expected temperature range, etc. where you can determine min and max voltage and current when used with a given MPPT charge controller. Victron provides a spreadsheet to help with all of that. You can download it from:

Matching solar modules to MPPT charge controllers - Victron Energy

Life used to be so simple; in a 12V battery system you took a ‘12V’ solar module, watched carefully that the maximum PV current would not exceed the charge controller maximum current and the system would work. Unfortunately due to the fact, that with PWM controllers the PV module is not feeding...

www.victronenergy.com

You can add your own solar panels and their specs to the spreadsheet to see the results. You can also add your own charge controller specs so you don't have to use Victron components to make use of the spreadsheet. The calculations tab shows all of the nitty gritty of how everything is calculated.

 

[MisterSandals]

along comes MPPT sizing and kicks my A**

So what is your issue? What are your panels (watts and Voc)? Do you have a plan on how your are going to arrange them in parallel/series?
Do you have an MPPT SCC in mind or a brand or a dollar range?

The temp coefficients are usually considered when getting around to situations when the Voc of the array is getting within 10-15% of the SCC's max input voltage. And then, you need to look at record low temps for the area(s) the panels will operate.

 

[MisterSandals]

(8) Rich Solar 100W noncrystalline solar panels.

Is there a sale on little solar panels? Did you buy these already? Are you constrained by space on top of an RV where big cheap panels will not fit?

Just checked out the link: 100w and 22.8Voc.

Do you have a preferred SCC or brand or dollar range? With 8 of these, you'd probably need some combination of series and parallel, like 4S2P.
This would put your Voc at 91 which is too close for a 100v max input SCC, especially since you mention extremely cold weather. But good for something like Victron 150/35.

Amazon.com

You could put a 8 in series for 182v. A Victron 250/50 would leave plenty of expansion rpm.

800w / 25.6v = 31a charging.

Need something cheaper? Different color? Different something else?

 

[snoobler]

snoobler,
Thanks for coming to my rescue. This is really interesting and helpful. If you can tolerate some additional questions, Please explain to me how you arrived at the -0.33%/°C for the VOC and the -0.4%/°C for the Pmax. And are these the coefficient? Is there a place I can find the formulas for this to study them. I tried to write down what Nate was saying, but honestly, he talks so fast that I really had difficulty following him. Hahaha

Those are the values for my existing panels. If you can determine the make and model of your panels, you should be able to find a datasheet online that lists those values.

 

[PeteW]

Sorry for my delay in replying. I had something come up. It's definitely not do to indifference. I very much appreciate all of the help offered and all of your time and knowledge shared. Snobber, thanks again for getting the ball rolling.

Most solar panel manufacturers provide data sheets for their solar panels. And listed among all of the other specs for the panels you will normally find those coefficient values. I couldn't find any such values on the Rich Solar website. But the following link does have them:

You can add your own solar panels and their specs to the spreadsheet to see the results. You can also add your own charge controller specs so you don't have to use Victron components to make use of the spreadsheet. The calculations tab shows all of the nitty gritty of how everything is calculated.

Thanks a lot, much appreciated!

But there are calculations based on panel arrangement, system voltage, expected temperature range, etc. where you can determine min and max voltage and current when used with a given MPPT charge controller. Victron provides a spreadsheet to help with all of that. You can download it from:

Is there a sale on little solar panels? Did you buy these already? Are you constrained by space on top of an RV where big cheap panels will not fit?

Just checked out the link: 100w and 22.8Voc.

Do you have a preferred SCC or brand or dollar range? With 8 of these, you'd probably need some combination of series and parallel, like 4S2P.
This would put your Voc at 91 which is too close for a 100v max input SCC, especially since you mention extremely cold weather. But good for something like Victron 150/35.

Yes, I know how I want to wire them, I just get confused as to how to put it correctly. As stated, It's 8, 100W panels. I plan to connect them as pairs in parallel then connect the 4 pairs in series (2P4S? or 4S2P?). Does that seem reasonable? I have not selected a solar charge controller yet. That is what I'm trying to determine. I do think I want a 3KW Hybrid Inverter. I've only looked at the Victron 3k 24V and it's a monster. Can some an inverter be mounted on it's side? I'm outfitting a 29' Airstream travel trailer so space on the roof is limited to the 8 panels I'm installing. Space on the inside is also tight. I'm locating the battery bank and therefore the SCC and the Inverter in the very front section under a horseshoe dinette setup I intend as part of the build out. There are no basements on Airstreams.

 

[MisterSandals]

I plan to connect them as pairs in parallel then connect the 4 pairs in series (2P4S? or 4S2P?).

If you connect 2P4S you will need 4 sets of Y connectors. If you connect 4S2P (4 in each series first, then 2 in parallel) you only need one set of Y connectors.
What the array produces will be the same in the end either way.

 

[mitiempo]

The Victron 100/30 or 100/50 would be good options for controllers.

 

[PeteW]

If you connect 2P4S you will need 4 sets of Y connectors. If you connect 4S2P (4 in each series first, then 2 in parallel) you only need one set of Y connectors.
What the array produces will be the same in the end either way.

I totally see this. Thank You! Is there a some good material on making Series and Parallel connections for PV modules and also batteries / battery cells. I obviously need some better learn'n in this area. If I didn't mention it, my intention is also to build a 24V 460AH LFP battery.

 

[PeteW]

The Victron 100/30 or 100/50 would be good options for controllers.

Thank you for your advice. At this point I'm very interested in learning how to do the sizing. That way I can make sure I leave myself room for expansion, or alternatives. I'm very hands on.

 

[MisterSandals]

Is there a some good material on making Series and Parallel connections for PV modules

Are you looking at 2P4S or 4S2P?

I can provide links to the bits if i know what you're doing...
And do your panels have MC4 connectors or ??

 

[PeteW]

Are you looking at 2P4S or 4S2P?

I can provide links to the bits if i know what you're doing...
And do your panels have MC4 connectors or ??

4S2P Is what I'm going to do. All of my modules have MC4 connectors on 31" leads. I'll need to buy some branch connectors for the pairing and I plan on making the span MC4 connector wires. I've watched a number of videos on doing this including a couple of good ones. ?

 

[MisterSandals]

4S2P Is what I'm going to do.

Thats the easy way! You take 4 panel and connect + of panel 1 to - of panel 2. Connect + of panel 2 to - of panel 3. Connect + of panel 3 to - of panel 4 and you are done with a 4S string.

Do this with the other 4 panels.

You will need this Y connector (pair):

Amazon.com

You will need to buy or make a cable to connect to the Y connector to go to your SCC.

Amazon.com

or I set of these connectors to put on your wires:

Amazon.com

 

[PeteW]

Thats the easy way! You take 4 panel and connect + of panel 1 to - of panel 2. Connect + of panel 2 to - of panel 3. Connect + of panel 3 to - of panel 4 and you are done with a 4S string.

Thanks for your help and the links. Any suggestions on where I can get good S & P connection info for battery cells, preferably in writing and with diagrams. I really do understand it, but it helps me a lot to see it in multiple ways and details to gain a firm grasp.

 

[MisterSandals]

Use the download button here:

Cell Configurations for 12V 24V and 48V LiFePo4 Batteries

This short deck shows 1P and 2P cell configurations for 12V, 24V & 48V LiFePO4 batteries. To get the deck, click on the orange button at the top of this page. Note: Reviews, Comments and corrections are welcome Document Revision History...

diysolarforum.com

Come back if you want a sanity check. Good luck!

 

[PeteW]

Come back if you want a sanity check. Good luck!

At a glance it looks like just what I had in mind. Thanks. All of this mental force feeding is causing things to get a bit gooey between the ears, so at this point I'm using ear plugs to hold it all in. If or probably when it all gets to much, I'll be back for that sanity check. ??
Thanks again.
Pete

 

[PeteW]

Use the download button here:

Cell Configurations for 12V 24V and 48V LiFePo4 Batteries

This short deck shows 1P and 2P cell configurations for 12V, 24V & 48V LiFePO4 batteries. To get the deck, click on the orange button at the top of this page. Note: Reviews, Comments and corrections are welcome Document Revision History...

diysolarforum.com

I just finished a deep dive. Super helpful! Thanks again.