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Charge Controller Sizing Issue

Vicad88

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Joined
Jul 14, 2023
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Hello,
I am in the process of building my first box truck conversion. Unfortunately, necessity dictated that I set up a system and see how it went after some real world testing. Ideally, I would have all the components I needed to do a proper power audit, but that wasn't possible in my case. So, I started with 4x455W panels, and positioned them as tightly as I could on the roof as to leave room for expansion. It turns out I will need extra power, and I'm in the process of sizing a charge controller for the extra pieces. I have purchased an additional two 455W panels, and after measuring I have room for 2x175W panels next to the roof fans. I left room in my cabinet for a single extra charge controller, and I'm having difficulty with the math using mismatched panels. Before I make a costly mistake, I want to ask folks here for their opinion.

Specs:
  • Power: 455W
  • Maximum Voltage (VMP): 41.51 V
  • Maximum Power Current (IMP): 10.96 A
  • Open Circuit Voltage (VOC): 49.35 V
  • Short Circuit Current (ISC): 11.44 A
  • Maximum Fuse Rating: 20 A

  • Power: 175W
  • Optimum Operating Voltage: 18.1 V
  • Open Circuit Voltage: 21.6V
  • Short Circuit Current: 10.31 A
  • Optimum Operating Current: 9.67 A
  • Maximum Fuse Rating: 20 A
The first 4 455W panels are on a 150/70, 2 and 2 series parallel. The calculator I used for that gave a value of:
  • Solar Array Wattage: 1820W
  • Temperature Compensated Array Voltage: 123V
  • Controller Output Amperage: 63A
  • Array Short Circuit Amperage: 21.92A
  • Max Series Fuse Rating: 20A
Now the issue, that original calculator does not allow for input of multiple sized panels, so I went looking for another. I found two that gave identical values (2x455w + 2x 175W in series parallel):
  • Max power output: 1220.55W
  • Max power voltage (Vmp): 63.11V
  • Max power current (Imp): 19.34A
  • Loss: 8.07%
That's a jump down from the 150/70, and it made me curious. I plugged in the info from my original 4x 455W(2 and 2 series parallel), and it gave me this:
  • Max power output: 1819.8W
  • Max power voltage (Vmp): 83.02V
  • Max power current (Imp): 21.92A
  • Loss: 0%
That seems to suggest that I could have done a 100/30, and the 150/70 is too big. If that's true, then it would make sense to add some to it, and get a smaller controller for the remaining portion. That said, the info from my charge controller shows the highest Voltage recorded was 96.30V, which is higher than the 83.02V suggested above. One of these is wrong, and I'm not sure which. The original calculator asked for VOC and ISC values, while the other two asked for VMP and IMP, which I think accounts for the mix up. I am looking to get as small of a controller as I can. After this, I will not have any more roof space, so there will be no option for expansion. I'm not interested in ground deploy, and I think 3,040W should be plenty. In any case, this was my first build, and I'm still learning the ropes. Can someone walk me through this? Please let me know if I left out a value you need. This is for a 24V system. Thanks!
 
24V system.

Note that MPPT current is OUPUT. They have different PV INPUT current limits. Thus the array POWER is determined by MPPT output current and battery voltage.

28V * 70A = 1960W - about the peak power array you can install without overpaneling. You are right-sized for an 1820W array.

A 100/30 would only output 30A * 28V = 840W, and your 2S Voc is too close to 100v.

You would need another controller to add 2X 175W panels. They can't be combined with your 2S2P array without penalizing its performance. you would only need a 350W/28V = 12.5A controller.
 
Everything sunshine said is correct, he just missed you are also adding 2 more 455W panels.

I agree with him that you leave the existing system as it looks good. Now you just need to work out how to get the most out of the new 2x 455W + 2x175W.

Mixing panels can be done but you need to be careful so generally isn’t recommended, but it’s also probably a bad idea to add two additional charge controllers.

I would put each 455W in series with a 175W and then put this in parallel with the other string, this will reduce the Imp of the 455W to match the Imp of the 175W which means you lose 1.3A x 41.5V = ~54W from each of your 455W panels, which is where the 8% loss you were talking about comes from. But you are still ~250W better off than not having the 2x 175W.

So for a charger you would need:
V 49 + 21 = more than 70V + temp compensation*
A 10 x 2 = about 20A (input)
W 400x2 + 175x2 = ~1150W

*Don’t forget to check the temperature coefficient of your panels as the voltage will go up considerably when it gets cold, There are calculators to work out what this is.

For a 24V system (1150W/28V = ~40A)
Assuming you are looking at Victron I’d have a look at the 100/50, remembering what sunshine_eggo said about the charger amps being output not input.
 
Thank you for the detailed replies!

For now, I'm just interested in getting the two 455's on the roof. If I really, really feel like I need the extra 175w panels, I'll buy them at a later time.

I'm hesitant to buy them though, the 'price per watt' is pretty bad. I paid $255 for the 455's, two 175's are $355. $100 more for 105w less, plus a 108w efficiency loss. Hopefully it won't come to that, but it's nice piece of mind knowing the controller could handle it, if it comes to that.

I checked the coefficient numbers, and the calculator was giving me some pretty hefty recommendations of a 250/60. I plan to travel full time, I suppose I'll have to stay out of the cold!

Thank you again, being my first post I didn't know what kind of reception to expect. I really appreciate you guys taking the time to lay it out for me.

Cheers!
 
Happy to help.

Is the 250/60 for all 6 panels? If what you have works I wouldn’t replace it unless you don’t have room for two chargers. It is ok to have two chargers on one battery and redundancy can be handy if one fails.

Temperature coefficient shouldn’t be more than 1.25x voltage unless you are really really cold!!
Try this calculator
 
2S3P would work on the existing controller. Would simply be over-paneled and you'd lose your peak output while benefiting from the larger array the rest of the day, but flat panels on a roof rarely output max power when you consider the totality of the year of use. Summer is the only time you're going to lose anything significant.
 
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Good point. it would be interesting to see what the output of the current system is on a good day, if it rarely goes over 1500W then you really don’t lose much to the over-panelling at all.
 
I took a look at the calculator you sent over.... I'm not quite sure how to input the values with the values being different for two sizes?
The values for the 455W are:
Pmax -0.35%/C
Voc -0.29%/C

And the 175W:
Pmax -0.37%/C
Voc -0.28%/C

I was interested in the value for just under freezing, say 30F(-1.11C)

Would I do each panel individually and then add the values together after?

The panels have been working quite well so far, on good days it records a pull of 12.5Kwh and I've seen a 1823W high. I'm having a separate issue of the SOC resetting daily that I haven't figured out yet. It seems that the Smart Shunt sets the SOC to 100% each night. It seems to be around the time the controller has no more input for the day. I kind of suspect that the full SOC value may be set too low? Perhaps it gets to a certain voltage, and based one what I set it at it thinks full SOC is achieved?
Specs here:
My charger configuration:
Absorption voltage: 28.10V
Float Voltage: 27.00
That was from the manual page 5:

1. Bulk/Absorption: 28.1V (+/- .2V)
2. Float: 27V (+/- .2V)
3. Low DC cutoff: 23.5-22V*
4. Battery Charge Temperature Range: 32°F - 122°F
5. Battery Discharge Temperature Range: -4°F - 131°F

Shunt Config:
Battery Capacity: 600Ah
Charged Voltage: 26.4 (I don't see this value anywhere, maybe the issue? It could have been the default, and I didn't change it since I didn't see a matching value.)
Battery SOC on Reset: Keep SOC

Thanks!
 
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Thank you again, being my first post I didn't know what kind of reception to expect. I really appreciate you guys taking the time to lay it out for me.

This forum is really awesome when it comes to new people who have no idea but are humble and wanting to learn. Like yourself :)

It’s less tolerant of people who think they know more than they actually do and act like arrogant twats. Or just the arrogant part alone doesn’t make friends lol.
 
I took a look at the calculator you sent over.... I'm not quite sure how to input the values with the values being different for two sizes?
The values for the 455W are:
Pmax -0.35%/C
Voc -0.29%/C

And the 175W:
Pmax -0.37%/C
Voc -0.28%/C

I was interested in the value for just under freezing, say 30F(-1.11C)

Would I do each panel individually and then add the values together after?

The panels have been working quite well so far, on good days it records a pull of 12.5Kwh and I've seen a 1823W high. I'm having a separate issue of the SOC resetting daily that I haven't figured out yet. It seems that the Smart Shunt sets the SOC to 100% each night. It seems to be around the time the controller has no more input for the day. I kind of suspect that the full SOC value may be set too low? Perhaps it gets to a certain voltage, and based one what I set it at it thinks full SOC is achieved?
Specs here:
My charger configuration:
Absorption voltage: 28.10V
Float Voltage: 27.00
That was from the manual page 5:

1. Bulk/Absorption: 28.1V (+/- .2V)
2. Float: 27V (+/- .2V)
3. Low DC cutoff: 23.5-22V*
4. Battery Charge Temperature Range: 32°F - 122°F
5. Battery Discharge Temperature Range: -4°F - 131°F

Shunt Config:
Battery Capacity: 600Ah
Charged Voltage: 26.4 (I don't see this value anywhere, maybe the issue? It could have been the default, and I didn't change it since I didn't see a matching value.)
Battery SOC on Reset: Keep SOC

Thanks!

When charging with solar, it's important to set your shunt charged voltage to 0.2V below ABSORPTION voltage, not float. The Shunt is seeing a current at less than whatever % you've specified while remaining above 26.4V for 3 minutes.

Set charged to 27.9V and tail current to 6%. This should prevent false sync.
 
Got that updated, tail was at 2%. After a minor headache, I got the BMS software to connect without the program freezing and manually reset the SOC in the shunt. I'll report back tomorrow, hopefully that fixed that issue. I found you can't always count on the batteries' lights being accurate. I'm fairly sure that they're not intervals of 25%, but I can't find that info anywhere. It's currently at about 42.6% SOC with 2/4 lights on, so I guess it rounded up.
 
Last edited:
I took a look at the calculator you sent over.... I'm not quite sure how to input the values with the values being different for two sizes?
The values for the 455W are:
Pmax -0.35%/C
Voc -0.29%/C

And the 175W:
Pmax -0.37%/C
Voc -0.28%/C

I was interested in the value for just under freezing, say 30F(-1.11C)

Would I do each panel individually and then add the values together after?

Yes, do them separately then use those values in your other calculator to get the combined output at -1C. But -1C isn’t too bad, you are looking at about 10% increase in voltage so as long as you leave yourself a 20% buffer you should be fine.

So you shouldn’t put more than 2 panels in series on your current controller, but 3 parallel strings of 2 panels (2S3P) would be fine. You will lose a bit of output when the sun is directly overhead but this may not happen often.
 
Temperature coefficient shouldn’t be more than 1.25x voltage unless you are really really cold!!
Try this calculator
Calculation Form
I took a look at the calculator you sent over....
If you want an old-school way to do it, this resource shows how to calculate it. It also has charts that let you look up a multiplier based on the temp and temp coefficient.

 
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