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MPPT calculation

JustDanD

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May 28, 2021
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I had purchased a Victron MPPT 100/50 already for my build. Then stuff happened with my solar panels and I ended up getting LARGER panels than I had planned, for the same price. Now, when I used Will Prowse's formula in his book, it seems my MPPT is not big enough. I am OK with adding a second MPPT if need be and splitting the load, but would like some input on this.

4 solar panels
320W max power at STC
40.1 Open circuit voltage
10.8A short-circuit current
33.7v Optimum Operating voltage
9.5A Optimum operating current
15a Max series fuse size rating

Solar Panel Array Wattage / Battery Bank Voltage = Min Solar Controller Amp Rating
(320x4)= 1280 / 24 = 53.33amps ** over the 50 of the MPPT

So if I series/parallel to make 67.4 volts (but plan for 80.2 short-circuit?). would it be?
(320x2) = 640 / 24 = 36.67amps ** still under the 100/50 MPPT rating (80.2 v / 36.67 amp)

Thanks
Dan (JustDanD)
 
Solar Panel Array Wattage / Battery Bank Voltage = Min Solar Controller Amp Rating
(320x4)= 1280 / 24 = 53.33amps ** over the 50 of the MPPT

So if I series/parallel to make 67.4 volts (but plan for 80.2 short-circuit?). would it be?
(320x2) = 640 / 24 = 36.67amps ** still under the 100/50 MPPT rating (80.2 v / 36.67 amp)

Thanks
Dan (JustDanD)
1280w ÷ 24 = 53.3A > 50A
Why do you think that is a problem?
You can overpanel that Victron 100/ 50 by a lot more than that.
Read the manual
Also your battery voltage is more than 24V
That 100/ 50A SCC will work just fine with your 1280 watts of panels.
Connect your 4 320w panels 2 Series 2 Parallel. 2S2P
80Voc is safely under 100V max input.
 
Last edited:
Thanks Forbisher, Sometimes is just takes a re-wording of things in order to understand. In doing the formula for the SCC, I was thinking the amps from the panels applied to the capacity of the MPPT. I was not applying it to MATCHING an MPPT to the amps coming IN to charge the battery.

I also understand about the battery being more than 24V and how that would change the numbers. All the book examples are always on the general voltage (12, 24, 48) and don't speak to their actual higher value.

A great Light-Bulb moment for me.
 
1280w ÷ 24 = 53.3A > 50A
Why do you think that is a problem?
You can overpanel that Victron 100/ 50 by a lot more than that.
Read the manual
Also your battery voltage is more than 24V
That 100/ 50A SCC will work just fine with your 1280 watts of panels.
Connect your 4 320w panels 2 Series 2 Parallel. 2S2P
80Voc is safely under 100V max input.
I went to question your post and then quoted it. Look at the difference between my quote (above) and your reply: "Connect your 4 320w panels 4 Series 2 Parallel. 2S2P". LOL.
 
I went to question your post and then quoted it. Look at the difference between my quote (above) and your reply: "Connect your 4 320w panels 4 Series 2 Parallel. 2S2P". LOL.
Yeah i made an obvious typo then corrected it.
4S2P would be 8 panels.
2S2P is correct.
 
OP post #1:
"(320x2) = 640 / 24 = 36.67amps ** still under the 100/50 MPPT rating (80.2 v / 36.67 amp)

That is error, it should be 26.67A, so two strings in parallel then the total Amp is 26.67A x 2 = 53.34A, same result as 1280W/24V.
 
Here are some real-world details that soften this issue a bit. Most likely, unless totally, totally depleted, your batteries will not ever be charging at 24V. Most likely they will be charging at ~26V. The second issue is that panels almost never put out their rated wattage under real-world conditions. 90% is doing really, really well. Plug those numbers into your formula and you see it's not critical

{ (320W X 4 Panels)/ 25V charging } X .90 fudgefactor = 46A

So, you will be fine.
 
Bud Martin. Yes..I did the math wrong. Thanks for doing it the right. It does make more sense.

MichaelK. Those factors are all true and helped me understand this better. I tend to plan for extreme examples so that I sleep better at night. I am a database programmer so I have to plan on every possibility, no matter how unlikely. LOL

thanks all. I feel good that my original choice of MPPT was valid... even though I was not 100% sure what I was talking about.
 
I am a database programmer so I have to plan on every possibility, no matter how unlikely. LOL

thanks all. I feel good that my original choice of MPPT was valid... even though I was not 100% sure what I was talking about.
Did you read the Victron manual?
Max PV array size and specs.
 
I did and the array size is under the manual max. Reading manual specs make more sense as I get more familiar with battery/solar information. You guys help out a lot by stating things in different ways. I have background in electronics and electricity but NO background in this area or in design of systems.

This forum is awesome!
 
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