RV Solar Suitcase - How Much Will MPPT Improve Output?

[Browneye]

Okay...first question...readers digest version: should I get a mppt controller?

I have a 'suitcase' system I bought a few years ago - I am now looking at putting more power on the roof - questions for another day. But I have been pondering this little array and what might make it work better, or at least as good as it can, based on what I'm learning here. It's an outstanding forum! And Will's vids are great.

The suitcase setup is two 80w poly panels hinged together in a nylon case with folding legs. It had a cheapo pwm 15a charge controller attached to one of the panels, they are wired 2p. I made up a 10awg lead 22' long so it can be placed fore or aft of the RV, connected to the house batteries - two GC2's. I don't have meters or testers, other than a volt meter, so I have no idea what it's putting out. But it does charge, and raises the volts up to charge level. Obviously it's never going to peak the 220ah bank over a couple of days of dry camping. I estimate we use about 40-60ah of power per day. The new roof array is planned to be 3-400 watts with an mppt controller, use them together when needed.

Recently I decided the suitcase controller should be a lot closer to the battery bank, mainly for current drop from the long leads. So I mounted it on the wall next to the power compartment and connected it with less than two feet of 10awg to the 12V bus bar, and then the PV lead to the exterior wall and installed a 10awg SAE port. The long lead wires now connect direct to the panels, still parallel, and can plug into the SAE port. Yes, the junction boxes were a mess - I installed ring terminals on the leads and cleaned up all the wiring. Yes I see now the SAE connector isn't all that great, but it's a pretty low power setup, and the lead wires are all 10awg. Now at least the current drop is on the raw PV power before it gets to the controller, instead of the drop in power output like it was.

What I am now wondering is what benefit I would get by connecting them in series to an mppt controller, like a victron 75/10 or similar. Would the cost involved upgrading the controller give me much of an improvement in output? Would it be worth doing?

Panels are standard 12v items - rated like 21.5v open and 4.55a.

Photo of them deployed.

 

[circus]

Wouldn't make any difference. Well, maybe a couple cents per month.

 

[12VoltInstalls]

MPPT doesn’t shine until or unless you can dump higher volts into it- then it will maximize the 12V output amps.
Like this: I finally plugged in my other two panels for 600W 3S2P the other day. Full sun hitting all panels (albeit at a “bad” angle) had me charging at 32A or so.

Unless you series the two panels mppt won’t get you anything- and even then the scale isn’t going to bring much of an obvious benefit if any.
Just get 800 or 1000W- with the real estate on that roof you will be able to get the MPPT benefits.

 

[Browneye]

Thanks guys...more reading...more reveal. For the 160 watts of panels seems I'm getting all I can get already.

The roof-top project is coming along...two 215w panels on order - HD had their Nature Power brand on black-friday sale yesterday at 65% off - black-framed mono panels were $158 each - listed online at $397 today!
Getting some strut-rack for mounting today, want to paint it.

Need to choose a SCC - the Victron or EP Tracer quandary. Suggestions welcome. Quite a cost difference.

That will give me about 20amps of charging on average, which should cover our daily usage. The big power-hog for us is the forced-air ducted furnace - about 8A when running. If it runs six hours a day in cooler temps that's about 50Ah. The rest of everything is led lighting, the fridge board, and some electronics charging. The led TV is about 50W AC on a small inverter - 5A on DC. I think we should be good for a few days off-grid. We don't mind the onan genset running some. Still need it for air conditioning when it's hot out. It powers an IOTA DLS45.

 

[12VoltInstalls]

forced-air ducted furnace - about 8A when running.

Are your batteries lithium?
These modern RV furnaces have reasonably low labeled current draw but duty cycle plus their very low temp purge shutoff and the pre-fire fan cycle and it seems they run almost constantly.

I have an old, old, old duotherm that I’m seriously considering wasting the money to fix. Same labeled 16k BTUs but the new furnace never feels “hot” and at 20 degrees runs what seems like forever at 3A. The old furnace is 8A with a standing pilot, but other than a short run of ‘cold’ once it fired you didn’t need to stick your hand by a register to see if it was actually warm air- it was HOT.

Anyway I mentioned this because if you’re not lithium the constant draw of 8A will suck more than the math out of the batteries- and faster than the math, too.
I don’t know how cold it gets where you plan to be but if you have less than 4-500Ah usable you might not like it.

 

[Cal]

MPPT doesn’t shine until or unless you can dump higher volts into it- then it will maximize the 12V output amps.
Like this: I finally plugged in my other two panels for 600W 3S2P the other day. Full sun hitting all panels (albeit at a “bad” angle) had me charging at 32A or so.

Unless you series the two panels mppt won’t get you anything- and even then the scale isn’t going to bring much of an obvious benefit if any.
Just get 800 or 1000W- with the real estate on that roof you will be able to get the MPPT benefits.

Not correct. You will see a 5% improvement over PWM in a 12V battery system when using standard 36-cell 12V panels.

 

[Browneye]

Are your batteries lithium?
These modern RV furnaces have reasonably low labeled current draw but duty cycle plus their very low temp purge shutoff and the pre-fire fan cycle and it seems they run almost constantly.

I have an old, old, old duotherm that I’m seriously considering wasting the money to fix. Same labeled 16k BTUs but the new furnace never feels “hot” and at 20 degrees runs what seems like forever at 3A. The old furnace is 8A with a standing pilot, but other than a short run of ‘cold’ once it fired you didn’t need to stick your hand by a register to see if it was actually warm air- it was HOT.

Anyway I mentioned this because if you’re not lithium the constant draw of 8A will suck more than the math out of the batteries- and faster than the math, too.
I don’t know how cold it gets where you plan to be but if you have less than 4-500Ah usable you might not like it.

Thanks. The old winnebago (itasca) we bought new in 2005, have gotten by it's whole life, mostly dry camping, with the two GC2's - about a 100Ah usable. We don't camp much in the cold, but if we move back to WA - Yakima cuz all the kids are in Seattle - we might want a gas heater. I was always gonna buy one of those Olympian things and hang it on the wall. But yeah, it does blow a lot of air, and most of the hot goes out the exhaust LOL.

Even when we've been out in 40* temps it doesn't run that much - double pane windows and 2" of foam in the walls, it's pretty comfortable. Since it's just 30' it has one 15,500btu AC and the 4kw onan, which is also pretty quiet.

Perhaps a couple of Battleborns are in our future...we'll see.

I have pretty well decided on one of the new EP Xtra SCC's - $99 on sale. Unless someone can talk me out of it. My solor-guru neighbor is all victron stuff. The panels were a steal and they're really nice - two 215w mono with black frames. The extra 20A or so we can get for battery recharging should cover us.

I painted the strut-channel today - 30' of it for racking the two panels.

 

[Browneye]

Not correct. You will see a 5% improvement over PWM in a 12V battery system when using standard 36-cell 12V panels.

Well, spending $75 for half an amp doesn't make much sense.

 

[12VoltInstalls]

Not correct. You will see a 5% improvement over PWM in a 12V battery system when using standard 36-cell 12V panels.

Ok. Sure. Maybe.

Going on my experiences the improvements feel exponential with some series panels. Math VS practical use.

In practical use I wasn't too impressed going to mppt until I started series and the early, late, and cloudy performance is way better than prior when I went to series panels. 5%, 1%, 20%… whatever. Series had me recharged way earlier in the sun day with the same watts. Math is not always an adequate prediction of practical results. Full batteries by mid AM consistently VS afternoon on sunny days is pretty stinkin practical.

 

[Cal]

There shouldn’t be any performance difference between series & parallel panels during sunny or cloudy conditions.

 

[12VoltInstalls]

There shouldn’t be any performance difference between series & parallel panels during sunny or cloudy conditions.

I wish that were true.

But there absolutely is a difference when it’s cloudy enough to cease most production and parallel panel output is only 8-12V: you get no charging whatsoever.

However, 8V x 3 (series hyperbole) is more than battery volts and you can get at least some battery charging. Same thing with early or late in the day: there’s a little bit more charge time with series connected panels. The mppt just needs to see some loaded voltage to output some amps. That is what mppt does

 

[Cal]

I wish that were true.

But there absolutely is a difference when it’s cloudy enough to cease most production and parallel panel output is only 8-12V: you get no charging whatsoever.

You're really giving out bad info. Not only does my 12V panel produce power when cloudy, it also produces power when it's raining!

The only advantage serial has over a parallel 12V system is at sunrise and sunset. Virtually no power is produced at that time and therefore no big deal.

 

[12VoltInstalls]

You're really giving out bad info.

Ok. At first I thought you just didn’t know some things. I know too little myself.

But I see what’s going on, now.
Carry on. I’ve got other stuff to do.

 

[Browneye]

My new roof-top panels will be wired in series - they're about 25v open - measured them at 23.3V right out of the box with the sun just over the horizon. Epever actually recommends at least 2 panels, and up to 4 in series with mppt, and up to 50% over the rating for over-paneling. So I'm about 40watts over the limit, but still about half the rating in volts. So I expect to get what I can out of the two 215 panels.

This chart from Will made the most sense for me, understanding the advantages. With panels flat mounted, and off-season, you get a little more out of them in less than optimum conditions with the higher pv voltage.

 

[Browneye]

Winnebagos have a fiberglass roof - over expanded foam - basically it's glued to luan-plywood over styrofoam - not much there to screw to. But ideal for the VHB tape install. Not trusting the sufficient area of bonding with the little z-brackets, I opted for the strut channel technique. With 1-5/8 wide by two 5' mounting channels, there's 1.25 square feet for adhesive for 10.5 sq-ft panels. Based on the glazing specs and the lifting wind pressure, I'll have about a ten-times redundancy in holding power, or about two-thousand pounds for these panels. Lifting pressures are estimated at up to 250lbs for 58"x26" size of panel at 90mph on a roof top. That's 60mph plus a 30mph gust with the RV. Being open all around, and 1-3/4" off the roof deck, pressure will readily equalize.

3M stats on VHB adhesive: https://multimedia.3m.com/mws/media/893360O/vhb-structural-glazing-tape-g-b23f.pdf
Wind Load calculations: http://solarabcs.org/about/publications/reports/wind-load/pdfs/Wind_Load_blanksstudyreport3.pdf


I had some old Centari acrylic enamel on hand, so painted the cut-to size struts and strut-nuts. The shorter pieces will bolt to the PV frame, then mount cross-ways to the 5-foot channels that are the length of the panels. 1/4-20 strut nuts and all SS bolts and washers. I'll blue-lock-tight them as well. Hell for stout, but not taking any chances on a RV rooftop. I have some #14 coarse-thread screws I may run in on the base channels too, at least on the leading edge. I'll wait to see how it looks once they get glued on there and sealed.

 

[Browneye]

These are the panels HD was selling for $154 last Friday. The ones I got were their 215W - they delivered next-day on Saturday. Wow!
The only thing I didn't like were the lead wires - they're just cheap automotive grade PVC 12 or 14g wires with SAE plugs, so I'll install 10awg PV leads on them with regular MC4 connectors. Lead wires go down the refrigerator chiminey along with the other leads from the 10w starter-battery maintainer that's already up there. Imagine that, the coach came with that 16 years ago, but they had it connected to the house batts which made no sense. I switched that over so it goes through a battery-tender controller and maintains the starter battery. It fried one starter battery in about a year and half with no controller. LOL.

 

[Cal]

Ok. At first I thought you just didn’t know some things. I know too little myself.

But I see what’s going on, now.
Carry on. I’ve got other stuff to do.

A little schooling might be in order...

You got this false idea that Vmp (the maximum power point voltage) drastically decreases with clouds (when the panel operates at a lower irradiance..

This graph clearly shows Vmp remains constant (@30V) regardless of power level. Vmp does not decrease when cloudy as @12VoltInstalls believes.

 

[Cal]

This chart from Will made the most sense for me, understanding the advantages. With panels flat mounted, and off-season, you get a little more out of them in less than optimum conditions with the higher pv voltage.

Why should a higher voltage panel have an advantage in your chart? I see none. This chart discusses the advantage of over-paneling. Usually a motor home has limited space and over-paneling is not possible.

 

[Browneye]

Why should a higher voltage panel have an advantage in your chart? I see none. This chart discusses the advantage of over-paneling. Usually a motor home has limited space and over-paneling is not possible.

A higher voltage array, such as these two in series, allows for an MPPT controller to better transfer charging amps verses parallel with a PWM controller, no?
And the 2S provides for lower losses over the PV transmission wires, no?
So setting up 430 watts 2S to a 30A MPPT is slightly higher than the rated 390 watt spec. I could go with a 40a and get the same thing, but I pay more for for the higher capacity controller.

Perhaps I've misunderstood the whole concept and my logic is flawed. My assumption is that flat-mounted panels are never going to generate their peak production, surely not more than 30A. Thus just slightly over-panelled, no?

 

[Browneye]

A little schooling might be in order...

You got this false idea that Vmp (the maximum power point voltage) drastically decreases with clouds (when the panel operates at a lower irradiance..

This graph clearly shows Vmp remains constant (@30V) regardless of power level. Vmp does not decrease when cloudy as @12VoltInstalls believes.

This is interesting. I too thought PV output voltage dropped in low-light conditions. This says it remains fairly constant, just increasing amps.
What causes a PV to put out more or less voltage? In testing my panels with a volt-meter in the sun, I did notice the voltage would go up or down as tilt and shading was changed. Although I also noticed voltage only dropped one volt or so when the panel was half shaded.