question

francesco-marzulli avatar image
francesco-marzulli asked

Panels number for MPPT 250-70Tr

PV panels: 390W; Vmpp=40,8V; Impp=9,56A; Voc=49,3V; Isc=10,03A; -0,27%/°C; 0,04%/°C.

System battery:48V

With this panels, already with Tmin = 0 ° C, it would be Voc = 52.6V (with Tmin= -10°C it would be Voc=54V, only with Tmin=20°C it would be Voc=50V) and therefore a maximum of 4 panels can be connected in series for MPPT 250-70Tr: con you confirm this?

For MPPT 250-70Tr, the (Isc)max = 35A, so a maximum of 3 strings (of 4 panels) can be connected to it: it's right?

I know my question is a weird one but I ask (to understand some things better): what would happen by connecting 5 panels in series instead (for Vmpp: 40,8Vx5=204V < 250V)?

What problems would arise? Can there be any damage to MPPT? The system will not work properly?

MPPT Controllers
2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

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

1 Answer
klim8skeptic avatar image
klim8skeptic answered ·

What problems would arise? Can there be any damage to MPPT? The system will not work properly?

You can never, ever expose your mppt to higher array voltages than it's design is rated for (at expected local low temperatures).

Once the blue smoke escapes the blue box, it is an expensive paper weight.

>Victron online MPPT calculator. <

4 comments
2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

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

francesco-marzulli avatar image francesco-marzulli commented ·

Thanks @klim8skeptic, I fully agree with your considerations, of course... I only ask a confirmation for the calculations I have made...

0 Likes 0 ·
klim8skeptic avatar image klim8skeptic ♦ francesco-marzulli commented ·
My calculations would be..


Voc @ 0c..

Temp below STC x temp coeff. 25 x 0.27% = 6.75%.

Voc @ 0c = 1.0675 x 49.3v = 52.63v per panel.

0 Likes 0 ·
francesco-marzulli avatar image francesco-marzulli klim8skeptic ♦ commented ·

Thanks @klim8skeptic , so the limits I identified earlier for MPPT 250-70Tr are correct... max 4 panels in series; max 3 strings (total 3x4=12 panels).

Moreover: 390Wx12=4680W -> 4680W/48V= 97,5 A, but current limit for battery charge is 70A...

0 Likes 0 ·
klandos avatar image klandos francesco-marzulli commented ·
Depending on orientation the 70A Limit is only sometimes a Problem.

You Could use the 250/100 to gain everything. The 250/100 could also handle 70A input Id you would like to overprovision.

0 Likes 0 ·