Which solar controller for off-grid system?

[Awsmits]

Hi, I’m designing an off-grid system, planning on three 450W panels (but may expand to 6 eventually), 12V LFP batteries (three in series, but could buy 1 more and go to 24V). The batteries will have BMS protection from under/overcharging. What solar controller do you you recommend? Is series or parallel panel connection better?

 

[Rednecktek]

You mean 12v batteries in Parallel, right? Series would be 36v and that's a real oddball. With that much panel it sounds like you're going for a pretty hefty system so you should probably shoot for 24v right out the gate.

Lets do some rough math for the differences in charge controllers and panels:
3x 450w panels = 1350w
6x 450w panels = 2700w

1350w / 12v nominal = 112.5 amps
2700w / 12v nominal = 215 amps
1350w / 24v nominal = 56 amps
2700w / 24v nominal = 112 amps

So, right out the gate bare minimum you're going to need 2 SCC's in the 60a range to handle the 3 panels, and 4 SCC's to handle all 6. Even stepping everything up to a 24v system will require 1 or 2 60a SCC's for 3 or 6 panels respectively.

 

[Bud Martin]

Did you do power audit first to find out what your power consumption each day will be, and what kind of loads you will be using?

 

[Awsmits]

Yes, parallel, of course, my bad. Yeah, 36V would be odd. I agree, going to four batteries (24V) would give me more flexibility with solar components, so I think that’s the new plan. And yes, I did the power audit, using this for a bare minimum residential backup system, I think I need about 3,600W per day, but there’s always that desire to expand the system down the road, so I’d like to buy components that will handle 6 panels eventually.

 

[Rednecktek]

Sounds like a combination of going to a 24v system and a pair of 60a MPPT's in parallel, 1 for each string, would be the optimal setup. That's assuming the VoC of the panels and fudge factor allow you to do a 3s string. Worst case you're going to have to do 2s strings to their own MPPT but that's going to depend on the VoC of the panels.

 

[Awsmits]

The VoC and ISC of the panels are about 49.3V and 11.4A. Could I run 3 panels in parallel to keep the voltage low, and the amps would be low enough for the 60A MPPT controller? Second string of three panels in parallel to a second 60A controller?

 

[Rednecktek]

The VoC and ISC of the panels are about 49.3V and 11.4A. Could I run 3 panels in parallel to keep the voltage low, and the amps would be low enough for the 60A MPPT controller? Second string of three panels in parallel to a second 60A controller?

Yup, that would be the way to go. Make sure your SCC can take 150v of PV input and you should be fine for a 24v system.