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.àIs this correct?

You can easily get 50 amps into those batteries when low on charge. If you expect to actually get 50 amps your panels would have to actually produce 700 watts. Reality is with sun angles, clouds and temperature etc your 850 watt rated panels may only produce 50% to 90% depending on conditions.Need more info on the 250 & 300 panels to decide if it is better to have separate controllers. Basically if the cell counts (60 or 72) are the same they should work fine in parallel feeding a single controller. Yes it is best to have same panels that match so if you have a good controller for the 250 consider leaving that in place.

phemens wrote: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.àIs this correct?The batteries will accept 30% charging rate no problem, but not worth going higher than that ( except with certain batteries) So your 630AH bank can accept 190 amps no sweat.However, the MPPT controller will cap its amps to its rated "size" in amps. Eg my Tracer 20 amper will do that and no more even if it is on a bigger panel (in watts) than it is rated for (260w)So you need a 20 amper for that 250w you have now. Add a pair of 300s you need about 25 each? So that 50 amper looks about right. So the guy is right that you need both controllers in parallel on the one big battery bank, but you will not go over what the bank will accept in total amps.If the 335s are a good price and the 50 amper will do it, sounds good. You can go a bit over on panel watts (as often recommended with MPPT) anyway, because most of the day you will be under the max watts and be getting more AH haul, but 250w over is way too much IMO.

phemens wrote: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.àIs this correct?The controller can accept up to 100 volts. This will dictate if you wire series, you need to make sure you dont go over that.Whatever watt panels you get if you go over the 700 watts the controller will just cap the amps. 700/12v = 58 amps.72 cell panels are big.If as you suggest your 850 watts of panels, you will not actually get that out of them during full sun. maybe 700??Id rather have more solar and let the controller limit the power, then not enough. Again up to the 100 volt the controller is rated at.Dont know where you are installing these. But to save on splitters and such, I would do your 3 panels in series for a 60 plus volt total. I would get matching panels with what you have. 3-250 watt panels.But the two 72 cell panels will have the higher volts. ( probably 36 v short circuit) So in series 70 plus volts to controller.So 3- 250watt or two 335watt on a voltage perspective are really the same. Its only an 80 watt difference.So it comes down to how much real estate you have to mount panels.

Can’t get the same 250 panels, they are discontinued. Have plenty of space for the panels, so 60 cell vs 72 isn’t a space equation.I have the old Rogue 3024 MPPT from my previous trailer that I can hook the 250w panel up to that, was trying to avoid 2 wire runs from roof to front if possible.

Just set both controllers to the same Vabs and time at Vabs and same Float voltage.With two separate solar sets you have the option of splitting the bank to put your inverter on one and run the rest of the rig on the other like I have posted about elsewhere--if it is any use to you of course. I do have to choose which bank the Trimetric is reading though. ( I have a shunt on each and swap the display as needed, so I don't need two displays the way I do things--YMMV)Easiest to just put both controllers on the one bank--both controller negs go on the one shunt same as all the other negs.With different controller to battery wires, they will have different voltage drops and their controller displays will show slightly different voltages, so they won't be exactly in synch for when they think they reach Vabs, but will be "close enough" if you use fat wire for each.

Basic question on max MPPT capacity to batteries

time2roll
Nomad
Mar 08, 2018
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
Explorer II
Mar 07, 2018
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
Explorer
Mar 07, 2018
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
Explorer II
Mar 07, 2018
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
Explorer
Mar 07, 2018
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
Explorer
Mar 07, 2018
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
Explorer
Mar 07, 2018
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
Explorer
Mar 07, 2018
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
Explorer II
Mar 07, 2018
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
Explorer
Mar 07, 2018
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.

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