is the calculator wrong, or something i dont know?

Question

question

valvetdk asked Jan 22, '24

is the calculator wrong, or something i dont know?

Am I mistaken, or is the calculator wrong?

I used the calculator to determine the size of my MPPT. My solar panels have the following specifications, and I used two units of this model in the calculation, resulting in an indicated MPPT of 100/30 for my 24V system:

Pmax = 405
Open circuit voltage (Voc/V) = 37.15
Short Circuit current (Isc/A) = 13.78
Vmp = 31.18
Imp/A = 12.99

I connected both panels in series, so the formula for my voltage is 37.15 + 37.15 = 74.3, and the current remains the same at 13.78 A. However, when I use the calculator again and input that I want to connect 2 strings in parallel, the calculator suggests upgrading to an MPPT 100/50 model.

But why? Doesn't the voltage remain the same, 74.3 Volt , and the current becomes 13.78 + 13.78 = 27.56 A? Shouldn't I still be able to use my MPPT 100/30 with 2 series + 2 strings, or why does the calculator recommend upgrading to 100/50?"

Solar Panel mppt calculator

2 |3000

Attachments: Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Modifications Space

Node Red Space

Answer 1 · 2024-01-22T07:39:35Z

klim8skeptic answered · Jan 22 at 07:39 AM

@ValvetDK Shouldn't I still be able to use my MPPT 100/30 with 2 series + 2 strings, or why does the calculator recommend upgrading to 100/50?"

It comes down to the array power and mppt output current at battery voltage.

The 100-30 can handle an array of about 880w, with its output current of 30a into a 24v battery. 2 of your panels is 810w, a good fit for the 100-30.

The 100-50 can handle an array of about 1400w, with its output current of 50a into a 24v battery. 4 of your panels is 1620w, a good fit for the 100-50.

2

2 |3000

Attachments: Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

valvetdk commented · Jan 22 at 07:43 AM

Oh, so it's not enough to just look at the maximum voltage and current an MPPT can handle; one also needs to take wattage into account, right?

0 ·

klim8skeptic ♦ valvetdk commented · Jan 22 at 08:04 AM

@ValvetDK Yes. You dont want to hook up an 1620w array, and only get 880w out of your 100-30 mppt, into your battery.

0 ·

Answer 2 · 2024-01-22T08:14:38Z

prein answered · Jan 22 at 08:14 AM

Yes, the 30 amps is not only on the mppt side, but also on the battery side.

That means that with 24V battery you can only charge with 30A= (*24=720 watts)

Nothing breaks when you do connect them, because you stay within spec. But there is much solar energy unharvested

2 |3000

Attachments: Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

question