4S or 2S2P for most solar production.

[thebeavesknees]

Hi, all!

I have 4 REC Alpha 405W panels that are going to be installed on my travel trailer soon. I have no direct solar experience, so I was wondering if anyone could help shed some light (I love puns, so, beware!) on what might work best in my case.

The VOC and Isc are 48.9V and 10.3A, respectively. This puts me in a bit of an awkward spot. A 2S2P configuration would let me use something like an MPP 3048LV-MK, but if a 4S configuration would be better, I would need to go with a newer/different AIO that will support over 200V for the SCC input.

The trailer is currently located on a slightly NW/SE azimuth (and darnit, I can actually shoot an azimuth but I forgot before writing this), with heavy shade from a tree in the morning, mostly clear sky until deep winter from later morning until late afternoon. Right now, I'm not going to tilt the panels, but, I may at a later date.

I think I should settle this first before I decide what AIO to get, since that seems a bit more sensible than forcing myself into one situation because I prefer an exact AIO (based on a lot of reading and videos and generally good things, I do like the MPP 3048LV-MK, and it doesn't hurt that it is on sale now), when it isn't ideal for the panels I have now. There is a possibility to add more panels, but, that would be in a year or three, as I'd need to remove a bunch of stuff from my roof which I'm not willing to do just yet.

Thoughts and suggestions are welcome!

Thanks for your time!

 

[pollenface]

Sounds like you're on the right track, just keep in mind you will need more pv voltage headroom for cold mornings. VOC rises as temperatures fall. 250-300v on the pv input would be great if going 4s, otherwise 150v if going 2s2p

 

[thebeavesknees]

Sounds like you're on the right track, just keep in mind you will need more pv voltage headroom for cold mornings. VOC rises as temperatures fall. 250-300v on the pv input would be great if going 4s, otherwise 150v if going 2s2p

Cool!

Is there a calculator for PV voltage gains vs temp? Or would the manufacturer data sheets have anything other than the sticker voltage at STC?

 

[pollenface]

I use this random table I found on the net

The lowest temp I see is about 2C so I multiply my array VOC by 1.1x

 

[Pierre]

I would go 4s as you exchange higher voltage at the input to the SCC for a lower voltage but higher current into the batteries. Power in = Power Out ( minus losses ) Also the higher voltage gives you a bit more headroom when the light intensity onto the panels vary.

 

[SamDeleted]

2s2p is better if you suffer shading

 

[thebeavesknees]

I use this random table I found on the net

The lowest temp I see is about 2C so I multiply my array VOC by 1.1x

I found this, too. I am going to try to find the full spec sheet for my panels and try to figure this out more exactly. Thanks!

 

[thebeavesknees]

I would go 4s as you exchange higher voltage at the input to the SCC for a lower voltage but higher current into the batteries. Power in = Power Out ( minus losses ) Also the higher voltage gives you a bit more headroom when the light intensity onto the panels vary.

How would this setup in a 4S configuration perform with my less than ideal alignment and with the shading I get in the morning and afternoon? That's what I'm trying to figure out. I understand what you're seeing, but it seems to me that the 2S2P configuration would be better, but, I can really find a way to test that on one unit since the voltage input is in two very different ranges. I haven't even found a charge controller that can work down at ~80V or above ~170V. So, I'm stuck picking one out the other.

 

[thebeavesknees]

2s2p is better if you suffer shading

How much better? Do you have personal experience with trying both?

 

[zanydroid]

How much better? Do you have personal experience with trying both?

There’s a whole thread on this.

EDIT; try searching for parallel and trailer in the forum and you’ll see a lot of parallel enjoyers

You can run both sides of the parallel at full voltage without activating bypass diodes and without optimizers. The probability math says that there are fewer ways to choke 2S2P than to choke 4S

It’s hard to say how much better IRL because someone probably needs to do simulation modeling.

 

[zanydroid]

Note if you attempt to use optimizers they’re probably a better bet for the higher voltage string because there’s a higher chance of helping, and also optimizers drop string voltage in an attempt to boost current. If you are having trouble finding a SCC that can activate in 2s voltage then it just gets worse if optimizers start doing current matching.

 

[zanydroid]

Another option is going 4P, I know some people that love parallel.

But this may get into exotic/more expensive single MPPT. And required 24V battery (because standard MPPT are buck (voltage reducing)) only). So at 48V battery you need enough series voltage to satisfy the hunger of the buck.

If you must have 48V battery there are now buck boost MPPT floating around AliExpress and Amazon that can charge from 1s voltages but you may end up being a Guinea pig, as I’m not sure how many people out there are using them for mission critical stuff.

And to make it super clear, this buck issue is the reason you can’t find MPPT for 48V below 80V or so. And that 80V cutoff definitely makes buck optimizers a bad value prop

 

[thebeavesknees]

There’s a whole thread on this.

EDIT; try searching for parallel and trailer in the forum and you’ll see a lot of parallel enjoyers

You can run both sides of the parallel at full voltage without activating bypass diodes and without optimizers. The probability math says that there are fewer ways to choke 2S2P than to choke 4S

It’s hard to say how much better IRL because someone probably needs to do simulation modeling.

I've read through a lot of that thread. There's a lot of speculation, and not a whole lot of testing in a consistent manner. I guess it's just too variable and I'll have to make a decision and then stick with it.

 

[thebeavesknees]

Note if you attempt to use optimizers they’re probably a better bet for the higher voltage string because there’s a higher chance of helping, and also optimizers drop string voltage in an attempt to boost current. If you are having trouble finding a SCC that can activate in 2s voltage then it just gets worse if optimizers start doing current matching.

I'm not interested in optimizers for this install, but I can see how they would be valuable in many cases.

 

[thebeavesknees]

Another option is going 4P, I know some people that love parallel.

But this may get into exotic/more expensive single MPPT. And required 24V battery (because standard MPPT are buck (voltage reducing)) only). So at 48V battery you need enough series voltage to satisfy the hunger of the buck.

If you must have 48V battery there are now buck boost MPPT floating around AliExpress and Amazon that can charge from 1s voltages but you may end up being a Guinea pig, as I’m not sure how many people out there are using them for mission critical stuff.

And to make it super clear, this buck issue is the reason you can’t find MPPT for 48V below 80V or so. And that 80V cutoff definitely makes buck optimizers a bad value prop

That's something I had never considered, but thinking about it, I don't really want to be the test subject. I will be going 48V, and it just doesn't make much sense for me to try a 4P setup with these panels, in this orientation. If I had the roof space and the cash, I'd loved to have been able to do a 3S2P config, but, it's no use wasting my time thinking about what can't be at this time.

At this point, I've pretty much decided that a 2S2P setup is what I'll start with. If I want to try 4S,I'll need to get a separate SCC, and that's really not the end of the world.

Thanks!

 

[zanydroid]

OK. Your gain from 4s vs 2s2p in full sun is pretty predictable, it's just voltage drop from losses, which may be compensatable with going to #8.

4p will always win vs 2s2p and 4s in shading. For 2s2p vs 4s it probably depends, however thinking about this it might be possible to mathematically prove some stuff with algebra and combinatorics.

Note 2s2p may or may not be able to advocate bypass diodes, it depends on if 5/3* Vmpp is above the working voltage range (that is the voltage roughly of single diode activated). Some people see this as a good thing because bypass diodes are a bit of a wear item (thermal aging when they activate).

From your original post it sounds like you have full sun most of the day so the shading perf question is arguably academic.

 

[thebeavesknees]

I would have to completely change a system design if I want to do 4P, and since I've decided to go o 48V, that's just a hard no.

I'll set it up as 2S2P and report my results when the system is up and running!

Thanks for the ideas!

 

[SamDeleted]

How much better? Do you have personal experience with trying both?

Apparently quite a lot ,I have not tested but others have

Series string shading summed up in 4 pictures

Each row is a string. These two strings of panels are in parallel and connected to victron MPPT. You can see that the last panel is shaded: 420VDC. So plenty of voltage and the last panel in the string is shaded, but only producing 200W for both strings! Now wait 30 minutes.. The shading...

diysolarforum.com

 

[thebeavesknees]

Apparently quite a lot ,I have not tested but others have

Series string shading summed up in 4 pictures

Each row is a string. These two strings of panels are in parallel and connected to victron MPPT. You can see that the last panel is shaded: 420VDC. So plenty of voltage and the last panel in the string is shaded, but only producing 200W for both strings! Now wait 30 minutes.. The shading...

diysolarforum.com

I've read through that, and most of the examples are so far off from what I'm going to be running that I can't see them as comparable. But that's OK. I'll start with 2S2P and give it time and see what happens.

 

[zanydroid]

I would have to completely change a system design if I want to do 4P, and since I've decided to go o 48V, that's just a hard no.

Sure that makes sense. I just brought it up again because it's the easiest to establish mathematically as dominating the others. Like high school is enough provided there is understanding of how to map from the EE to the smol brain math... Probably need a few years higher math to analyze 2s2p vs 4s but not like the next level degree