Determining MPPT to battery bank wire size (48v system)

[AgroVenturesPeru]

Hello again forum,

I'm about to move off-grid, and trying to figure out how to put all my components together. I bought a custom kit from Novum Solar here in Peru based on my wants/needs.

I think they sent about 90% of everything I need to put the system together (they told me that I had better buy certain things on my own, once I know the measurements and configuration of where I will be doing the installation... One question I have is about the cables.

On the roof there will be 12, 400w panels arranged in 4s3p

Each string producing ~196V and ~10.5A will go to a combiner box for paralleling the strings, and, from there, output roughly 196v and 32 amps.

I've got 4mm squared wire for the roof to the combiner box (max distance on one string is 5m from the combiner box). I'm also thinking I can get away with using the same 4mm-squared size wire from the combiner box to the charge controller (about one meter of length).

The charge controller is a Victron MPPT 250/100 with screw terminals.

Does anyone know how to calculate what the MPPT charge controller does to the 196v and 32amps that enter it? I know the MPPT lowers the volts and increases the amps, but I don't know exactly to what degree nor how many amps will be coming out of the MPPT. Is that programmable?

Battery bank:
4, LiFePo4 batteries wired in parallel. Each one is 48v and 74ah.
The manufacturer of the batteries says not to exceed a charge/discharge current of 37A on each battery for warranty purposes.

Basically I just want to know what is the correct wire thickness from MPPT to battery bank (assuming the distance between the two is less than 1 meter or 3.2ft) in this situation. Please advise. I did a number of internet searches before asking here.

Edited to add:

I should have mentioned that Vmp = 40.16V
and Imp = 9.96A

to be more precise.

 

[JoeHam]

Let’s assume your MPPT is 100% efficient to provide some margin of safety.

196v times 32 amps is about 6300w.

Divide that by 48v and you’re around 131 amps. I would round that up to 150 for a greater margin of safety.

Of course the 250/100 may never put out more than 100 amps but I’m a fan of larger conductors when in doubt ?

 

[Diysolar123]

well, lets try this....

input power is 196v * 32amps so a max of say, 6400watts
your 48v battery bank will have a
max voltage of 16cells * 3.65v = 58.4 volts
and a min safe voltage of 16 cells at 3.0v = would be 48volts

6400 watts / 58 volts = max of 110 amps available to the batteries.
6400 watts / 48volts = max of 134 amps

so at the low end the cable would carry 134amps.

as this is for power transmission I would go with at least 1guage copper wire.

 

[AgroVenturesPeru]

Here are the actual stats of the 400W panels:

I should have mentioned that Vmp = 40.16V
and Imp = 9.96A

40.16v x 4panels = 160.64v
9.96A x 3 parallels = 29.88A

160.64V x 29.88A = 4,800W

4,800W divided by 48V battery bank = 100 Amps

Okay....but what does this mean?

Apparently 4 AWG or 2 AWG according to this table? https://www.explorist.life/wp-content/uploads/SOLAR-WIRE-SIZE-CHART-1024x932.jpg

OKay, that makes sense with what's included in the kit. The batteries in my kit came with these extra battery cables: https://voltaconsolar.com/pylontech-battery-cables.html

Those, however, look like they're meant for an inverter connection, and wouldn't even be thick enough in my case.

Also I received six, 1.25m sections of 2 AWG, 4 of them have circle terminals on both ends, but two of them have circle terminals on only one end, the other end has another kind of terminal that seems kind of weird:

What do you call that kind of terminal?

 

[JoeHam]

I’ve always heard them called pin terminals.

 

[Bud Martin]

Ring terminals, and wires with ferrules (Balck and Red wires to the left of the pictures) installed or pin terminals?.

 

[JoeHam]

Ring terminals, and wires with ferrules (Balck and Red wires to the left of the pictures) installed.

Actually ferrules are crimped sleeves over wire while pin terminals are solid at the pin part and crimped onto the wire at the “non-pin” part.

With a ferrule you will see the copper wire at the end.

With a pin terminal imagine the ring of a ring terminal is replaced by a solid piece. The rest is the same.

 

[Hedges]

On the roof there will be 12, 400w panels arranged in 4s3p

Now that you know this array might be able to deliver 110 or 135A to battery, but SCC will clip that to 100A,
Consider slanting 1 string of 4 panels toward the west, and one string toward the east. That will reduce the peak maybe enough to eliminate clipping, but deliver good power for more hours in the morning and afternoon.

 

[Bud Martin]

Actually ferrules are crimped sleeves over wire while pin terminals are solid at the pin part and crimped onto the wire at the “non-pin” part.

With a ferrule you will see the copper wire at the end.

With a pin terminal imagine the ring of a ring terminal is replaced by a solid piece. The rest is the same.

I cannot tell 100% from the picture if that is ferrule or solid pin in the beginning, it does look like solid pin when I zoomed in the picture.

 

[AgroVenturesPeru]

With a pin terminal imagine the ring of a ring terminal is replaced by a solid piece. The rest is the same.

Yes that's what those two terminals are. Any idea where they go? It doesn't look feasible to attach them to the charge controller's screw terminals.

 

[JoeHam]

Yes that's what those two terminals are. Any idea where they go? It doesn't look feasible to attach them to the charge controller's screw terminals.

I believe Samlex uses those for their inverters.

Where did you get the cables? Did they come with something you bought??

 

[AgroVenturesPeru]

I believe Samlex uses those for their inverters.

Where did you get the cables? Did they come with something you bought??

Samlex is another inverter company. My inverter is a Victron.

I got the cables from this company:

Soluciones energéticas - Distribución y Proyectos - Novum Solar

Energía renovable descarbonizando la matriz energética peruana, Distribución de Equipamiento Solar, Ejecución Integral de Proyectos EPC.

novumsolar.com

Part of this Kit: