kevin-ferguson avatar image
kevin-ferguson asked

IMP & ISC for MPPT 450/200

I am looking to purchase the victron mppt 450/200 solar charge controller. The Max PV operating current is 18A/tracker and Max PV short circuit current is 20A/tracker.

I have 36 qcell qplus-G4.1/SC solar panels and their specs are as follows:

Pmpp 280
Voc 38.97
Vmp 31.67
Imp 8.84
Isc 9.41

For each tracker, I was planning to set up 18 panels. 2 arrays in parallel. Each array having 9 panels in series.

I live in NJ and our record low is -26C and record high is 42C.
Our mean low is -13.9C and mean max high is 37.8C.

I've made the following calculations (per tracker) and for cold I used -30C and for hot I used 45C.

Voc (STC) - 350.73
Imp (STC) - 17.68
Isc (STC) - 18.82

Voc (coldest) - 447.18
Imp (coldest) - 13.86
Isc (coldest) - 14.76

Voc (hotest) - 315.66
Imp (hotest) - 19.64
Isc (hotest) - 20.92

Are my calculations correct? If so, I think I am fine during STC and when it's coldest. My concern is when it gets really hot. I think I run the risk of the amps being too high and thus damaging the controller. Can I get help confirming this?

Thank you in advance for your help.

mppt rs
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Alexandra avatar image Alexandra ♦ commented ·

@Kevin Ferguson

What are the temperature co efficients for the panel?

And you are supposed to aim for 8x float voltage if you batteries. What batteries are you using?

You can go over on the input short circuit amps. You can void the warranty however.

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kevin-ferguson avatar image kevin-ferguson Alexandra ♦ commented ·
Hi Alexandra, Thank you for your response.

I forgot about the temp coef. I was using -.5%. I come to find it's actually -.29%. This changes my Voc (Hot) to 36.71 and Voc(cold) to 45.19V.

I also found a temp coef for Pmax -.4% and a temp coef for Isc .04% which leads me to believe I was calculating those numbers incorrectly. I was applying the same equation for Voc to Vmp and Vsc and then dividing watts (280) by each to get Imp and Isc. Can I apply the same equation to Vmp and Vsc? Should I apply that same equation for Pmax before dividing?

I was looking at the EG4 lifepower 12v 400ah batteries. Float Voltage is 13.5. I was going to connect 4 in series and 2 in parallel. 8x would put this at 432V. Should I aim to get Voc as close as possible to that? What's the impact if it isn't?

I remember reading that in the manual and thinking to myself, make sure the polarity is correct and I am should be ok regarding Isc. Does this also apply to Imp?

Thank you for all your help and guidance.

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Alexandra avatar image Alexandra ♦ kevin-ferguson commented ·

Just the short circuit amps of the panels is important. Sometimes to prevent charging the mppt can short the input. So it needs to be within the rating. I think the manual says 30A on the max. But warranty is voided above 18\20a.

Tracking amps is different. So don't stress about that.

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2 Answers
nickdb avatar image
nickdb answered ·

Have you used the mppt calculator on the victron site? Best place to start.

You have 4 trackers available to you, so no need to risk any overloading.

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kevin-ferguson avatar image kevin-ferguson commented ·
Great idea - I forgot about that - checking it out now.

Thanks for pointing out the 4 trackers - for some reason I had it in my head that it only has 2 trackers.

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kevin-ferguson avatar image
kevin-ferguson answered ·

Alexandra helped to point out a flaw in my calculations - I had forgotten about the temp coeff and was using a generic average value rather than the actual one. I adjusted my calculations to use the actual value and based on my math, the numbers appeared to work out.

I then used the mppt calculator on victron's site to validate my calculations and it recommended the 450/200 - even with only using 2 of the 4 trackers. That being said - I am going to use all 4 for now, but it's good to know i have room for expansion in the future.

Thank you both Nick and Alexandra for your help.

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