Basic question on max MPPT capacity to batteries

[phemens]

I have a total of 630 ah of batteries (3x 12v). My plan was to wire up 3 solar panels 1 existing 250w plus 2 new 300w panels) at 24v to a Victron Blue Solar 50A MPPT controller. I spoke to the guy selling me the panels, he tells me that my first limitation in the max wattage the batteries can accept from the controller at 12v (manual says 700w at 12v or 1400w at 24). After that point he says that the MPPT will cap the output anyways.
He says I’m better off putting 2 x 72 cell 335w panels on the Victron 50A MMPT (670w total) and adding a separate controller for the 250w panel wired separately to the batteries.à
Is this correct?

[time2roll]

MC4 is slam dunk reliable used on millions of homes at far higher power levels than our little RV systems. The wire is very UV resistant.

[BFL13]

Yes, I much prefer the wiring box that can be opened and you attach your own wires. There can be "issues" with too small terminals for the desired wire gauge (to be "worked around"), but way better than a pair of 3 foot al #12 pigtails IMO.

[brulaz]

BFL13 wrote:

...
Good price for the #8. IMO use that! Some panels don't have wiring boxes on the back, just the wires with MC4s. (and the wire is al not cu on mine) I wonder about the store-bought with MC4 wire if it is al too 😞 I use the cu #8 connected to the cut-off al pigtails with wirenuts for my portable set-ups

Yes, my portables came with MC4 connectors. And the boxes (hopefully with diodes inside) look like they're sealed/glued pretty tight. So I'm just using MC4s to connect/disconnect them.

I'd just be sure that the cable you get is designed for outside runs with UV and weather protection.

[BFL13]

phemens wrote:
I'd be wiring in series for 74v, not 24v. I have a local place where I can get 8 gauge for 65 cents/foot, thanks for the link though. I love Princess!

In Bulk the voltage on the panel to controller wiring will be set by the MPPT near the Vmp of the panels (added in series) so that would be more like 60 volts with the two panels. (You still use the added Vocs for controller voltage limit though)

In Float you can also get a higher current to run any loads but no more than one panel's Isc if they are in series.

Good price for the #8. IMO use that! Some panels don't have wiring boxes on the back, just the wires with MC4s. (and the wire is al not cu on mine) I wonder about the store-bought with MC4 wire if it is al too 😞 I use the cu #8 connected to the cut-off al pigtails with wirenuts for my portable set-ups

[phemens]

brulaz wrote:

phemens wrote:
I'll check with the supplier I'm buying the panels from. I'm sure he has them, but whether he has them at a better price than me just buying some regular 16awg, I don't mind wiring them up myself.

I can pick up 50' of 10ga with MC4 connectors for C$48+tax around here.

I usually just wire up to the connectors on the panel directly without the MC4 adapters. Bulk 10 gauge I can get for 40 cents/foot, so it's cheaper that way.

[phemens]

I'd be wiring in series for 74v, not 24v. I have a local place where I can get 8 gauge for 65 cents/foot, thanks for the link though. I love Princess!

[brulaz]

phemens wrote:
I'll check with the supplier I'm buying the panels from. I'm sure he has them, but whether he has them at a better price than me just buying some regular 16awg, I don't mind wiring them up myself.

I can pick up 50' of 10ga with MC4 connectors for C$48+tax around here.

[red31]

2% of 74v is 1.5v

http://www.genuinedealz.com/voltage-drop-calculator

16g x 30' x 9A ~2.25v

If they come with 10g pigtails, use 10g.

[BFL13]

phemens wrote:
yep, I just checked, they're Canadian Solar panels, about 37 volts. The MPPT can handle up to 100v. Website calculator says 2% loss at 30 feet at 75v with 16 gauge.

Note that you do the ampacity at Vmp for max amps when using MPPT

Also confirm the calculation is both ways (round trip)

This calculator (scroll down) says you get 17% drop with #16 at 24v and 50 ft round trip with 10 amps. #8 is 2.x %

https://www.powerstream.com/Wire_Size.htm

I use this but wait till it us on sale for $16. It comes on sale every so often, was on sale recently, so have to wait a while now I guess. Comes in red or black.

https://www.princessauto.com/en/detail/8-gauge-25-ft-primary-wire/A-p8216871e

[phemens]

I'll check with the supplier I'm buying the panels from. I'm sure he has them, but whether he has them at a better price than me just buying some regular 16awg, I don't mind wiring them up myself.

[brulaz]

phemens wrote:
Maybe on amazon.com, but not amazon.ca. Unfortunately we don’t have all the same low prices here,cheaper to run my own.

Surprised there's not some local solar shops around Montreal that can sell you 8 or 10 ga solar panel wire with connectors. There are quite a few around Toronto.

[brulaz]

BFL13 wrote:
Not sure going 24v has any advantage with inverters after that last thread about how the batteries at 24v have such a high R. You get little relative R advantage just from using thinner wire-- in fact, you might lose on the deal. Plus you then still need 12v to run the rig.

That is inverter-battery, not solar panel-controller, where I think we have that thinner wire advantage established as fact.

Ohm's law is not selective. There's a "thinner wire advantage" at higher Vs everywhere. From the solar panels to the charger for sure, but also from a higher V battery bank to the inverter, from the charger to the battery bank, ... everywhere there is less lost wattage over wire at higher voltages.

Where there does not seem to be any advantage at higher voltages is in the internal V drop under load and resistance of the lead acid battery bank itself. Reconfiguring a battery bank from 12 to 24 or 48V does increase the bank's internal resistance and internal V drop under load but not the *percentage* V drop (or wattage loss), as the bank's V is higher and current is lower at the same load. So, no disadvantage either.

AFAIK there is no disadvantage to higher Vs electrically, but cost, availability and complexity are certainly factors to consider.

[phemens]

yep, I just checked, they're Canadian Solar panels, about 37 volts. The MPPT can handle up to 100v. Website calculator says 2% loss at 30 feet at 75v with 16 gauge.

[lawrosa]

phemens wrote:
The original plan was 8 gauge wire run from the roof to the MPPT controller, roughly 25 feet, 4% loss at 24v, for the 3 panels in parallel. Am I correct in thinking that for the 2 new 72 cell 335w panels, would be better to wire in series at 48v (@9A)using say 16 gauge wire to the Victron 50A MPPT, and keep the 8 gauge for the single 24v 60 cell 250w panel to the existing 30A Rogue MPPT? If ever I added another panel to pair with the 250w, would run in series as well.

Are not those panels like 38 volts? They sound like residential panels like mine. Two in series I push 70 volts to the controller. I use 8 gauge some 4o plus feet..