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rattus avatar image
rattus asked

Smart Solar MPPT ratings and calculations: am I wrong?

Good day all.

So I'm trying to make sense of numbers going up and down. As I understand it the common wisdom is to divide your PV power input (W) by the Voltage of your battery to determine a suitable Amperage rating for your MPPT Solar Charge Converter (SCC), is this right?


I've been studying the datasheets and here are my findings...

With the 100|20:

Nominal PV power @ 12V =290W @ 24V = 580W @ 48V = 1160W.

But if I divide those numbers 1160/48...580/24...290/12 I get 24,16666.

Which is higher than the 20A rating for the SCC.


(this seems to suggest to me that the actual PV power rating of the device allows more than the Amp rating)

Also, what exactly does Nominal mean?

(to me it means a small amount)



What's going on here?

Please set me straight...

Regards.

MPPT Controllers
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4 Answers
klim8skeptic avatar image
klim8skeptic answered ·

Look at it thee other way round.

290w / 20a = 14.5 battery volts.

580 / 20 = 29v.

1160 / 20 = 58v.

Those are the battery voltages you are going to charge up to.

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pwfarnell avatar image
pwfarnell answered ·

In this case the nominal means a generic value that the actual value will vary from by a small amount, i.e. you have nominal 12V batteries, but the actual voltage will not be exactly 12V but will vary depending on the state of the battery and how much current is flowing in or out. The same with the power, for a nominal 12V system, the nominal power is 290W, but depending on circumstances it may be more or less.

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rattus avatar image
rattus answered ·

@klim8skeptic

Thanks, though to be honest I'm not always sure when to "look at it the other way round". Learning, one bite a time.


So is the right way to understand it, (continuing with the 100|20 example at 24V, 580W)...

"The Solar Charge Converter will limit the amount of Amps going out to the battery to 20A, thus the battery charging voltages will jump up. Hence one must make sure this voltage must not be so high as to damage one's battery? And all this is to happen at a PV input VOC lower than 100 (in the case of the 100|20)?"


@pwfarnell

Cheers. That helps.

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klim8skeptic avatar image klim8skeptic ♦ commented ·
Yes, the mppt will limit its output to 20a.

It is normal to over dimension the pv array, 20-30% would be normal.

And yes, keep the array Voc below 100v.

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kevgermany avatar image kevgermany ♦♦ commented ·
Also make sure you set the battery up correctly in the MPPT
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rattus avatar image
rattus answered ·

Thank you for the feedback.

@klim8skeptic

I find the statement " It is normal to over dimension the pv array, 20-30% would be normal. " interesting.


Is this because we cannot expect optimal performance from the PV array, eg around 70%? I have seen elsewhere on this community individuals (I think it was @WKirby ) say "you can never really over pv array" (or something similar). It would seem the datasheets and manuals stick to precision but in reality things are a bit more wavy. That, at least, is the picture developing in my head.


I suppose what I'm driving at is this:

Where in one's system must one absolutely stick to the limits of the items stipulated and where can one allow room?

Or am i now in the "how long is the piece of string" or indeed "where are you measuring the piece from" territory?


It looks like science. But feels like love.

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klim8skeptic avatar image klim8skeptic ♦ commented ·

@Rattus

Where in one's system must one absolutely stick to the limits of the items stipulated and where can one allow room?

You absolutely have to stay below the mppt's pv voltage limit @ array low temperature. And not go above the mppt's pv Isc limit.

Or am i now in the "how long is the piece of string" or indeed "where are you measuring the piece from" territory?

Solar PV is inherently lossy. High temps, clouds, any shade, and less than directly tracking the sun angle, results in losses. Additional pv capacity above the mppt "nominal rating" will give a higher energy harvest, but at the cost of theoretical peak production figures.

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