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Old 03-24-2020, 09:30 PM   #1
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Sizing Solar System

Hello All, I am trying to figure out the solar system for my 2003 Chevy Express bus. I purchased 3 Valence U27-12XP 138Ah batteries that I am planning on using. My original thought was to buy 6, 100 watt solar panels. The open circuit voltage of each panel is 21.3v and the short circuit current is 5.83A

I was planning on using a victron 100V/ 50A solar charge controller. After research I am hearing conflicting thing on whether this solar charge controller would work or not. I assumed you simply multiplied 6 (panels) X 100W to get 600W total. 600W/ 12V (battery bank voltage) = 50A. This lead me to believe that this controller would work for 600W of solar. However this would mean that I would need to wire the panels in parallel correct? In series the panel max voltages (21.3V) would add to a total of 127.8V and this controller is only rated for 100V.

Am I thinking about this correctly? What am I missing?
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Old 03-25-2020, 09:06 AM   #2
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There are always pros, cons, and compromises. In some cases, a combination of series/parallel is a good design plan, this might be one of them (partial shading is another). You are correct about the limits of the CC (charge controller).

Remember that a a "12 volt" battery is not 12 volts. It is basically discharged at 12 volts. Resting, it should be in the 12.6 range. Charging is a minimum of 13.5 volts (more or less, depending on chemistry). Also, it will be nearly impossible for 600 watts of panels to produce 600 watts of output - in real world conditions. If you flat mount them and are in an area of very good insolation, they will produce something closer to 80% of rated output.

With all of that in mind, a 600 watt array is likely to produce a maximum of about 500 watts. With the panels connected in some combination of series/parallel, you may be feeding a maximum of something like 16 amps at 63 volts into the CC (or 24 amps at 42 volts if connected another way).

One advantage of higher voltage is a smaller gauge wire from the roof to CC. A negative is that the current in that wire is more dangerous.
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Old 03-25-2020, 09:30 AM   #3
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Originally Posted by mitchk View Post
Am I thinking about this correctly? What am I missing?
I have the Victron 150V/100A, more or less the same deal.

The thing to be careful of is that 50A is both the input and output limit. If you wire your panels all in parallel, you're going to end up with a 21.3v 34.98A (5.83A*6) array, which is fine. I would wire parallel wherever possible as to reduce the impact of shade on individual panels.
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Old 03-25-2020, 10:31 AM   #4
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Originally Posted by JDOnTheGo View Post
There are always pros, cons, and compromises. In some cases, a combination of series/parallel is a good design plan, this might be one of them (partial shading is another). You are correct about the limits of the CC (charge controller).

Remember that a a "12 volt" battery is not 12 volts. It is basically discharged at 12 volts. Resting, it should be in the 12.6 range. Charging is a minimum of 13.5 volts (more or less, depending on chemistry). Also, it will be nearly impossible for 600 watts of panels to produce 600 watts of output - in real world conditions. If you flat mount them and are in an area of very good insolation, they will produce something closer to 80% of rated output.

With all of that in mind, a 600 watt array is likely to produce a maximum of about 500 watts. With the panels connected in some combination of series/parallel, you may be feeding a maximum of something like 16 amps at 63 volts into the CC (or 24 amps at 42 volts if connected another way).

One advantage of higher voltage is a smaller gauge wire from the roof to CC. A negative is that the current in that wire is more dangerous.
Thanks for the help, I appreciate it! It seems like most people say that wiring panels in series is more efficient if partial shading is not an issue. So maybe I should wire 2x 3 panels in series and then wire the 2 groups together in parallel. So I would still have some of the benefits of being wired in series and if one group went down due to partial shading the other 3 panels should work fine correct? So that would be the 16amp/ 63V scenario you were talking about. In that case I shouldn't have any issues using the victron 100V/30A correct?

Actually disregard that last sentence since the panels are 600W nominally the charge controller would be sending the power to the battery at 600W/Charging voltage (not really sure what this is but lets say 12V nominally) = 50A. So I would still need the 100V/50A charge controller.
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Old 03-25-2020, 10:34 AM   #5
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Originally Posted by kazetsukai View Post
I have the Victron 150V/100A, more or less the same deal.

The thing to be careful of is that 50A is both the input and output limit. If you wire your panels all in parallel, you're going to end up with a 21.3v 34.98A (5.83A*6) array, which is fine. I would wire parallel wherever possible as to reduce the impact of shade on individual panels.
So I would need to stick with the 100V/50A even if I wire the panels as explained in my previous post because the output to the batteries would be 50A and 12V = 600W nominally. Does that make sense to you?
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Old 03-25-2020, 11:57 AM   #6
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Originally Posted by mitchk View Post
So I would need to stick with the 100V/50A even if I wire the panels as explained in my previous post because the output to the batteries would be 50A and 12V = 600W nominally. Does that make sense to you?
Correct.
Charge controllers are rated by output amps (regardless of output voltage - within reason). You will never see 50 amp output from a 600 watt array but you may see 40-ish (13.5 volts) if you get REALLY lucky.
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Old 03-25-2020, 12:34 PM   #7
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Originally Posted by mitchk View Post
So I would need to stick with the 100V/50A even if I wire the panels as explained in my previous post because the output to the batteries would be 50A and 12V = 600W nominally. Does that make sense to you?
Right. How you wire the panels is largely up to you, the one configuration you can't do is 6 in series. 21.3*6 = 127V.

Since I'm debating the same kinds of restrictions right now with my own controller, I'll give you my examples as well.

I have a 24V battery bank. 3050W of panels (305W*10) are arriving tomorrow. They're 36.6V panels pumping out 8.33A each.

Scenario 1: with a 24V battery bank I'm current limited to 2400W input from the charge controller (24V batteries, 100A current limit). I'll probably never see all 3050W in full sun mounted flat, but I might see 2600W. Just not going to get all there is.

Scenario 2: If I switch up to a 48V battery bank, now I can't run all my panels in parallel. 8.33A times 10 is 83.3A, but the panel voltage of 34.98 is not enough to charge the 48V battery bank.

Scenario 3: If I wire all my panels in series I'm way overvoltage at 366V.

I think what I'll end up doing is wiring 2 panels in series, times 5 in parallel (2S5P). That gives me 73.2V at 41.65A, and I can choose to switch my battery bank to 48V later. I probably would have been better off with 12 panels, or with just 9 because I could have done 3S4P, 4S3P, or 2S6P (or 3S3P if I got 9).
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Old 03-25-2020, 01:56 PM   #8
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Thanks for the help everybody. I think I am going to go with the following setup:

6x HQST 100W solar panels (2 parallel sets of 3 panels in series)
3x Valence 12V U27-12xp 138ah batteries
1x Victron 100V/50A solar charge controller

I think that the battery bank is oversized compared to the rest of the system. I estimate that I will only use around 60Ah a day so I'm assuming the system should be able to keep up with that. The extra Ah in the battery bank would just ensure power for a couple of days with little or no sun.

Any comments or concerns? Has anyone had any experience with the Valence batteries or setting up the victron charge controller for Lithium iron phosphate batteries?
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Old 03-25-2020, 02:06 PM   #9
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Originally Posted by mitchk View Post
Has anyone had any experience with the Valence batteries or setting up the victron charge controller for Lithium iron phosphate batteries?
Just know the specifications of your batteries and set the charge controller accordingly. Do not rely on profiles, specify exact voltages. And if you can, monitor the state of the batteries internally to see if there's cell drift.

For instance, the maximum voltage of my packs is 25.2V, which is compromised of 6 cells internally. Each cell maxes out at something like 4.2V. Well, if one cell lags behind at 3.8V the charge controller (not knowing the internal state of the batteries) will continue charging the other cells beyond 4.2V trying to get to 25.2V total. That will ruin the pack, ruin your day, and/or cause a fire.

I'm using Tesla packs, which have absolutely zero margin of error. I limit charge to 0.25V under maximum, and limit bulk charging only to 0.5V under maximum to allow for unbalanced cells to catch up.
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Old 03-25-2020, 10:26 PM   #10
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Originally Posted by kazetsukai View Post
Just know the specifications of your batteries and set the charge controller accordingly. Do not rely on profiles, specify exact voltages. And if you can, monitor the state of the batteries internally to see if there's cell drift.

For instance, the maximum voltage of my packs is 25.2V, which is compromised of 6 cells internally. Each cell maxes out at something like 4.2V. Well, if one cell lags behind at 3.8V the charge controller (not knowing the internal state of the batteries) will continue charging the other cells beyond 4.2V trying to get to 25.2V total. That will ruin the pack, ruin your day, and/or cause a fire.

I'm using Tesla packs, which have absolutely zero margin of error. I limit charge to 0.25V under maximum, and limit bulk charging only to 0.5V under maximum to allow for unbalanced cells to catch up.
What is the difference between charge and bulk charge? Do mean charging one battery pack vs multiple?

From my research I believe that the Valence batteries have an internal BMS in each battery that makes sure the cells are leveled. The BMS's in each batter do not communicate with each other though. You need to get an external BMS but the problem is Valence doesn't sell them to the general consumer so they are hard to find. I think I'm just going to monitor the voltage of each battery so that I can make sure that each battery is level with one another. If they become uneven I guess I could unplug them all and charge each one to a specific value individually in order to re-level them.
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Old 03-26-2020, 12:24 AM   #11
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Originally Posted by kazetsukai View Post
Since I'm debating the same kinds of restrictions right now with my own controller, I'll give you my examples as well.

I have a 24V battery bank. 3050W of panels (305W*10) are arriving tomorrow. They're 36.6V panels pumping out 8.33A each.
I think what I'll end up doing is wiring 2 panels in series, times 5 in parallel (2S5P). That gives me 73.2V at 41.65A, and I can choose to switch my battery bank to 48V later. I probably would have been better off with 12 panels, or with just 9 because I could have done 3S4P, 4S3P, or 2S6P (or 3S3P if I got 9).

Perhaps scenario 4: Only use 9 of the panels in 3S3P and have a spare.


Heck, the spare could be used to keep your chassis batteries topped off.
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Old 03-26-2020, 09:16 AM   #12
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What is the difference between charge and bulk charge? Do mean charging one battery pack vs multiple?
Bulk vs Absorption vs Float:
https://www.leadingedgepower.com/bat...k-1235505.html

I have Absorption and Float set to the same cutoff value, basically eliminating float altogether. I do NOT want a float charge, when my batteries are charged they are done.

That being said, I always have 20W-50W of draw on my bank, so the controller will almost always be in either absorption or bulk.

Quote:
Originally Posted by mitchk View Post
From my research I believe that the Valence batteries have an internal BMS in each battery that makes sure the cells are leveled. The BMS's in each batter do not communicate with each other though. You need to get an external BMS but the problem is Valence doesn't sell them to the general consumer so they are hard to find.
There are users on this forum that I believe monitor their Valence banks with an Arduino. Search around a bit for the details on that if you are interested. If they have an internal balancing you're probably fine... but I would look into at least gaining visibility of whats going on inside your batteries if you can.

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Heck, the spare could be used to keep your chassis batteries topped off.
I think 2S5P is fine for now. When I expand, I'll definitely do multiples of 3 or 4.
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Old 03-26-2020, 04:48 PM   #13
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Ok, I will need to make sure all my settings are incorrect when I install the charge controller. I think I need to buy some cable extension cords to go from my solar panels to the charge controller. Since on the solar panel side I will have about 63V and 16Amps I just need to the charge controller for those values correct? Then from my charge controller to batteries I will need a bigger cable since the amperage jumps up to a little less then 50A? I don't need the thicker cable on both sides of the charge controller correct?
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Old 03-26-2020, 07:07 PM   #14
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Originally Posted by mitchk View Post
Ok, I will need to make sure all my settings are incorrect when I install the charge controller. I think I need to buy some cable extension cords to go from my solar panels to the charge controller. Since on the solar panel side I will have about 63V and 16Amps I just need to the charge controller for those values correct? Then from my charge controller to batteries I will need a bigger cable since the amperage jumps up to a little less then 50A? I don't need the thicker cable on both sides of the charge controller correct?
The charge controller will figure out the PV side of things, that's automatic. You just need to tell it how to treat your battery bank.


Yes, you will likely need bigger cables going from the charge controller to the bank than what you have going from the charge controller to the panels. The bigger the better, really. The largest cables typically go to the inverter- I have 4x (two cables for positive and two cables for negative) 4/0 cables going there.



Use a wire gauge calculator first, then test your cables under stress. I highly recommend inline breakers on both the PV side going into your charge controller and on the side going to your battery bank, sized appropriately.
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Old 03-26-2020, 08:23 PM   #15
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The charge controller will figure out the PV side of things, that's automatic. You just need to tell it how to treat your battery bank.


Yes, you will likely need bigger cables going from the charge controller to the bank than what you have going from the charge controller to the panels. The bigger the better, really. The largest cables typically go to the inverter- I have 4x (two cables for positive and two cables for negative) 4/0 cables going there.



Use a wire gauge calculator first, then test your cables under stress. I highly recommend inline breakers on both the PV side going into your charge controller and on the side going to your battery bank, sized appropriately.
Awesome sounds good. It looks like 10AWG cable will work on the solar to controller side according to online tables. I think I'm finally starting to gain an understanding of this stuff. Thanks again for your help!
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Old 03-27-2020, 01:57 AM   #16
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Originally Posted by kazetsukai View Post
Yes, you will likely need bigger cables going from the charge controller to the bank than what you have going from the charge controller to the panels. The bigger the better, really. The largest cables typically go to the inverter- I have 4x (two cables for positive and two cables for negative) 4/0 cables going there.

Cable size, current capacity, and cable length are all interrelated. How long are the dual 4/0 cables between your battery bank and the inverter?
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Old 03-27-2020, 06:30 AM   #17
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Cable size, current capacity, and cable length are all interrelated.
^^^ This
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How long are the dual 4/0 cables between your battery bank and the inverter?
Well, there's more to the picture than just the cables.
  • Each Battery -> 175A breaker -> Bus Bars using 2 AWG, about 4' - 5' length, two cables (one positive, one negative).
  • Bus Bars -> Inverter (4x 4/0, two per pole) about 4' - 5' as well.

The rationale for sizing is that the inverter is rated for 8000w with surge load capacities about double that, but 4000w is probably its maximum for a 75%+ duty cycle. 2000w-3000w is 100% duty cycle. My numbers:
  • 8000W draw / 24v battery bank = 333A. Divide this by the three batteries, 111A for surge loads.
  • 4000W draw / 24v battery bank, per battery that's around 55A draw.
  • 2000W draws would be 27A.
Given the above, I thought 2AWG to be plenty suited for the use case. I know internal resistances, etc come into play so loads don't divide up exactly like that, but if you put a heavy draw on an array like this where all of the batteries are at the same voltage eventually pulling more from the battery doing the most amount of work is going to have more resistance along the way than the ones doing less work.

This setup is not ideal for a number of reasons I go over in my build thread. I'm changing over to 48V either this weekend or the next.
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