New 2 Panel System: Series or Parallel?

[messydeer]

Hi!

I have a Ford Transit EL cargo van I'm converting. I just bought two Solaria PowerXT-400R-PM solar panels and am designing my electrical system. Voc at 25C for the panels is 51. I have a Victron 150V/85A controller Victron Energy SmartSolar MPPT Tr VE. Can 150V 85 amp 12/24/36/48-Volt Solar Charge Controller (Bluetooth) and a 12V 300Ah Lithium LiFePO4 battery which has a 200A BMS Ampere Time 12V 300Ah Lithium LiFePO4 Battery, Built-in 200A BMS, Max 2560W Power Output, Easy Installation, 4000+ Deep Cycles, FCC&UL Certificates, 10-Year Lifetime, Perfect for Off-Grid, RV, Solar…

What are the pros and cons of mounting parallel vs series? I understand the voltages would add in series while the amps would add in parallel. I also am aware a series connection is more efficient since the current would be cut in half compared to 2 panels in parallel.

I have read that if I have partial shading, a series wired system could give me an open circuit. Solaria says these panels have "...Sub-strings...interconnected in parallel, within each of the four panel quadrants..." which would perform better in partial shade.

I have also read that having when the sun is low series wired panels are more efficient. The following is from Camper Solar Panels - Series vs Parallel .. "...To charge a 12v battery bank, you’ll need closer to 14 volts (depending on battery chemistry. 14.4 for Lithium) to actually start charging. Most single solar panels in the 100 – 200 watt range have voltages in the 18-20 volt range. If you wire in parallel, this means you’ll only have a range of 6 volts to work with where your solar panel can make power. Basically, this means that your solar panel needs to be operating at 75% capacity before it will even charge your batteries at all. If you wire the same solar panels in series, using the same panels as above, you’ll have about 57 volts to work with. This means that, through the power of the MPPT controller, each panel only needs to be able to muster up 25% of its voltage capacity before it can start making amps in a 3 panel series..."

I plugged in the numbers for my two panels wired in series into the calculator on explorist.life https://www.explorist.life/solar-charge-controller-calculator/ and got 115V max with 9.4A going into the controller and 75A coming out.

It sounds like wiring in series would be the way to go for me. Thoughts? I'd appreciate any feedback related to other issues in my proposed system as well.

Thanks!

 

[chrisski]

You really mentioned all the pros and cons. Do you have roof obstructions, or do you plan on parking in the shade that will make paneling better, than parallel will be better. In fact each panel has voltage so high, I doubt any loss to a 12 volt system with two panels in parallel would be noticeable.

I'd wire in parallel with a 2 to 1 MC4 branch connector on the roof, but I think you know the right answer for you better than anyone.

 

[MisterSandals]

I'd wire in parallel with a 2 to 1 MC4 branch connector on the roof, but I think you know the right answer for you better than anyone.

If you do this, you can easily switch to series (and back if you want) to see if one works better for your system. It’s an interesting experiment.

Your results may vary on cloudy vs sunny days, winter vs summer…

 

[rmaddy]

I plugged in the numbers for my two panels wired in series into the calculator on explorist.life https://www.explorist.life/solar-charge-controller-calculator/ and got 115V max with 9.4A going into the controller and 75A coming out.

That charge current doesn’t seem right. You are talking about 800W of panels on a 12V system. 800W / 12V is about 67A. Since your batteries will be more like 13.5V then it’s 59A. And since your panels will be flat mounted on the roof of a van, you will rarely get 800W so your charge current will be less than that. You could probably use a 50A controller. But Victron doesn’t have a 150/50 so I think there is a 150/60 that would be more than enough.

 

[messydeer]

Thanks guys.

Where I'll park at home, there is a lot of shade. And it's north of Seattle, so there is only half the year that I can even see shade, hehe! On the road...good question. Mostly full sun I suppose. And where it'd be shady, I'd think I could find full sun if it were important.


I like the idea of trying both methods. Parallel might want thicker cables, but for this small amount of copper it shouldn't break the bank.

One other thing. I'm now living on my building lot in a big RV. It has a couple nearly dead 6V batteries in series. I've been using shore power but the 12V demand (aka refrigerator) overdraws their capacity in only a day or two. Instead of troubleshooting, I just hooked up a battery charger to run full time. I know. Shame on me....

....anyhow, I think I'd be able to charge the RV batteries with my van panels, right? Assuming the problem is the batteries, and not the inverter.


And rmaddy, I ran those numbers on 0 deg F, which would almost never occur. And it wouldn't be at noon at the equator, although I will be picking up my Equadorian GF in Quito ;-) To get your amperage, I plugged in 68F for the lowest daytime temp I'd encounter. Is that your assumption? But if I use 30F instead, I get 72A. And 15F would be 73A. These are more realistic temps and but I'd still need the 150/85. How does this look to you? If I did go with the 150/70 from Victron, it would save me $60 and 5 lbs.

 

[rmaddy]

You should get the 150/60 from Victron. Save even more money and weight. Voc goes up when the temperature goes down. Panel wattage is Vmp x Imp. And again, with flat mounted panels you will never see 800W. Also keep in mind that when it is colder it is usually during the time of year when the sun is lower meaning you are getting even less power from the panels. Going with a 60A SCC versus an 85A SCC means that maybe two days a year your panels had the potential to provide more charge current that you can't take advantage of.

60A x 12V = 720W
60A x 13.33V = 800W
60A x 13.5V = 810W
60A x 14.4V = 864W

If your battery is nearly depleted and down to 12.0V then you can only take advantage to 720W of your 800W of panels. How often is your battery going to be that low and your flat mounted panels are capable of actually producing more than 720W? At almost all times you battery voltage will be high enough that 800W isn't enough to max out a 60A SCC.

Plug your panel numbers into Victron's MPPT calculator and it recommends the 150/60 as well.

MPPT Calculator - Victron Energy

MPPT Calculator - Victron Energy

www.victronenergy.com