RV Solar Suitcase - How Much Will MPPT Improve Output?

[Cal]

There's no difference between 2 serial vs. 2 parallel panels in a mppt controller. They will output the same wattage.

I've already mentioned you will see about 5% more power from a mppt controller vs. a pwm controller. It doesn't matter if the mppt panels are in parallel or series.

Regarding cables going into controller: A mppt controller is sensitive to cable losses. A pwm controller is to-an-extent not sensitive to cable losses. If you want an efficient mppt system then you must keep cable losses (from panel to controller) at 1%. You can have 3% cable losses when using a pwm controller and it won't affect output power.

Panels usually don't output rated power because this rating is accomplished in lab conditions: 25C temperature and 1000 W/m irradiance. You won't see that in real life. 50C to 75C and 800 W/m is typical.

 

[Cal]

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.

Without getting into much detail, the equivalent circuit of a pv cell looks like this:

Each cell will output the diode drop voltage when current flows. This is about 0.5V at the maximum power point. You can have a broad range of currents, controlled by solar irradiation, with very little change in the diode voltage. It changes slightly, but not significantly.

 

[Browneye]

There's no difference between 2 serial vs. 2 parallel panels in a mppt controller. They will output the same wattage.

I've already mentioned you will see about 5% more power from a mppt controller vs. a pwm controller. It doesn't matter if the mppt panels are in parallel or series.

Regarding cables going into controller: A mppt controller is sensitive to cable losses. A pwm controller is to-an-extent not sensitive to cable losses. If you want an efficient mppt system then you must keep cable losses (from panel to controller) at 1%. You can have 3% cable losses when using a pwm controller and it won't affect output power.

Panels usually don't output rated power because this rating is accomplished in lab conditions: 25C temperature and 1000 W/m irradiance. You won't see that in real life. 50C to 75C and 800 W/m is typical.

So all the talk about improved charging production with series panels to an MPPT is just BS? I should wire them parallel and get a $40 quality PWM controller? Why did I waste my money on a MPPT controller?

10awg is about as heavy of wire I want to put out to these panels, the run is 20'. And I've already ordered PV cable and controller.
The calculator I found says I would see .91% drop to MPPT with 2S - 46V @ 10A. For parallel, 23v at 20A I would need to run 3awg wire to get 2.63% loss.

Without getting into much detail, the equivalent circuit of a pv cell looks like this:

Each cell will output the diode drop voltage when current flows. This is about 0.5V at the maximum power point. You can have a broad range of currents, controlled by solar irradiation, with very little change in the diode voltage. It changes slightly, but not significantly.

I'm a layman hobbiest - this stuff is like greek to me. LOL
It is fascinating as hell though.

 

[circus]

The calculator I found says I would see .91% drop to MPPT with 2S - 46V @ 10A. For parallel, 23v at 20A I would need to run 3awg wire to get 2.63% loss.

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

Answers become dangerous when you change the question.....

 

[Browneye]

Answers become dangerous when you change the question.....

Sorry, just not following you. Perhaps I should start a new thread - initially I asked about the solar suitcase, then moving on to more panels on the roof. So yeah, they're different specs.
Really, I'm not trying to confuse anyone on purpose.

 

[Cal]

So all the talk about improved charging production with series panels to an MPPT is just BS? I should wire them parallel and get a $40 quality PWM controller? Why did I waste my money on a MPPT controller?

10awg is about as heavy of wire I want to put out to these panels, the run is 20'. And I've already ordered PV cable and controller.
The calculator I found says I would see .91% drop to MPPT with 2S - 46V @ 10A. For parallel, 23v at 20A I would need to run 3awg wire to get 2.63% loss.

Depends who you're listening to weather you're getting BS. There's a slight serial benefit when dealing with shade. If the panels contain bypass diodes then serial will provide more power in shade.

You got a math error in the cable calculation comparison.

You have 2 panels: Vmp = 23V, Imp = 10A, P = 230W

When in series, Imp = 10A
When in parallel, Imp = 20A
Both series and parallel provide 460W of power

Let's use 10 awg cable and calculate power loss for series and parallel configuration.
10 awg cable has 1 mOhm/ft resistance
Total cable run is 40 ft.
A 20 ft cable run using 10 awg cable has 40 mOhm resistance.

The power dissipation of a cable can be calculated using: P = I^2 * R
Series power dissipation = 100 * 0.04 = 4W
Parallel power dissipation = 400 * 0.04 = 16W

Series % cable power consumption = 4W/460W = 0.9%
Parallel % cable power consumption = 16W/460W = 3.5%

The 10 awg cable is adequate for both configurations. The 0.9% loss is impacted when going serial. The 3.5% parallel loss is not seen at the output of the pwm controller. The panel has more than 8W headroom when going pwm.

 

[Browneye]

You're right Cal - I count a wire distance from source to source as for one wire length, either wire. So when doing these calculations, if your distance from point to point is 20 feet total, with DC current, that's 40 feet of wire? I had no idea. Really bad assumption on my part then. And that means all my calcs are way off for measuring wire resistance and voltage drop.

The panel specs are here again, as posted above:

If you could help me choose a SCC I would greatly appreciate it. I can return the 30A MPPT I ordered.
I did not understand "The panel has more than 8W headroom when going pwm." What is 'headroom', and what does that affect?

Last night I ordered a pair of red and black 20foot 10awg PV leads with mp4 connectors on one end. And two sets of 3' ones to replace the connect wires on the panels. They installed 12-14 gauge non-PV wire with an SAE connector. I want waterproof disconnects up on the roof in case they need to be serviced or replaced.

I also ordered a 2-breaker shut off box to install before the controller so I can disconnect the panels to the controller inside the coach. Evidently this is a new NEC rule as of last year. And I like the idea of being able to disconnect the panels without going up on the roof - both feed legs.

On the battery side I ordered a 50 switching breaker like this, both for circuit breaking as well as being able to switch off the SCC:



The other main component was a new 'Xtra' series controller from Epever, a xtra3210a 30A MPPT. It was on sale for $99.
I was going to order a Victron Smart 100/30 but it's another $126. Seems the BT output is pretty cool, but after looking at it a bit I probably never would again. I can get a cable to connect a laptop to the EPever one if needed. I think they have a wifi or bt module too.

I hope I'm not way off track. Thank you for your advice.

 

[Cal]

The 215W panel you are showing is not a 12V panel. 12V panels have 36 cells. That might be a 44 cell panel. You would be wasting a lot of power if you were to go pwm. You are stuck with mppt with those panels.

Regarding pwm headroom. Say we have a 12V panel specified as: Vmp = 17.5V, Imp = 10A, P = 175W

The battery is charging at 14.4V using pwm. Charging current is a little less than Isc (short circuit current), but a little more than Imp. Let's say charging current is 10A.

Solar power going into the battery is P = 14.4V * 10A = 144 W
Panel pwm headroom = 175W * 144W = 31W

I real life the headroom is quite a bit less than 31W because the 175W panel is rated when operating at 25C. When the panel is hot the power level drops significantly but the pwm current remains the same. In other words, pwm isn't affected by panel temperature, but mppt is affected. 12V panels are specifically designed to contain 36 cells to provide headroom so that they can operate under all temperature conditions.

Regarding the 2-breaker shut-off box: There shouldn't be any requirement for a breaker between the panels and charge controller when using one or two series strings. There might be a shut-off requirement, but no circuit breaker. A simple toggle switch would work that's rated for 15Adc.

 

[12VoltInstalls]

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..

A little manners might be in order.

I have no vmp fantasies but I did observe that I got charging voltage way earlier in the sun day once I moved things from 400W parallel to 400W 2S2P series.

If I listen to you it’s all in my imagination!!

Maybe I describe it incorrectly but as I read the chart posted I’m seeing just about exactly what I see reflected on my charge controller display.
I never mentioned vmp, just output performance.

So I don’t know what you’re arguing about! I just think you are rude.

I’ve run my stuff in a couple ways on pwm for several years and now at least three ways on mppt from 200W to now 600W. I haven’t tried 600W parallel...
After a while you get a sense of what to expect under various conditions including an extended period with no direct sun except 12:30pm to sundown due to location issues.

Going bi-directional made a big impact on my charging as did going to series panels. Early morning sunrise fog/haze we have this time of year here in Vermont: charge controller showed panel output of 12-14V, 0A. Plug in the third panels (series) and immediately show 14/15V and 2-4A. Every time I played with it. Later, it creeps 40 to 55V and 10-12A and eventually in good sun I’m getting 55-58V and 32A+ but I sure don’t get that on cloudy days. My panels are currently mounted vertically btw.

Math is not always an adequate prediction of performance. I’m no EE and you’re telling me I’m full of toast but dang, nobody told my charge controller about it.
I must have gotten an accidentally shipped experimental AI model charge controller.

I do know what my charge controller displays, and I recall the times in the past I didn’t recharge in one day. And the difference between 4P 100W panels from 2P 100W panels was way less than 2S2P 100W from 2P. 12.5V-12.6 at 10pm VS 12.9V at 10pm was the effective average difference. That’s useable power.

I may not be the most astute, knowledgeable participant in this forum but 12.9 is better than 12.5

 

[Browneye]

Yeah, I can't find anything on cell type or size, number of them, nuthin. I was a little surprised when Voc was 25v. I just assumed they were 36 cell panels like everybody elses.

Okay...MPPT it is. Can you comment on the EP Xtra or Victron SmartSolar controllers? Other suggestions?? I'm just shooting in the dark here, looked at Will's plans and such and came up with what I thought would work well.

I get it on headroom. Controller is blocking down the power to 14.4 or 14.6 volts or whatever charging mode it's in. With such a high Voc I'd be throwing a lot of watts away.

The two-breaker is an easy way to configure a PV switch. It's not so much the breaker as it is the switch. The box and breaker was $35 - I thought this guy's video below had a good idea for panel feed switching. Not knowing any better, I ordered a 2-pole box and DC breaker set. A 60A breaker is never gonna break. LOL

On my other portable panel set controller I did just install a single pole toggle switch on the positive lead. It even lights up when power is on. For the roof-top panels I wanted something a little more industrial, surface mounted, with good through-put.

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[Cal]

Sorry, I can't comment on charge controllers. I'm not up to date on them. It's been over 15 years since purchasing my last mppt controller for my Winnebago. It still works well.

The video is good. The guy says a dual breaker is required since the negative solar cable is not earth grounded in a vehicle. I'm not sure what problem NEC is trying to prevent with dual disconnects. Perhaps a lightning strike exactly when working on the vehicle and touching the ground cable? Or some panel static charge generation? I'm satisficed with my single wire solar disconnect.

 

[Browneye]

$35 is a little spendy for a switch, but it does cut both legs, and easy to wire in under the controller. So I thought it might come in handy.

I'll hook everything up and see how it works. We are actually a couple of years before retirement and can use it a lot. Trying to get everything paid off and ready to go. We got a new chevy spark toad early this year right before 'carmageddon'. It tows easier than our colorado 4x4 and hauls the electric mountain bikes.

I tried running the lithium charger for the Giant ebikes and they claim 4A input, but was only drawing 1200 watts with the inverter. So the charger tapers off when the packs get full too. Installing a Xantrex 600w inverter kind of prompted all this. I got tired of those little plug-in inverters that never seemed to be enough power. And I wanted some pure sinewave power for electronics. I put three duplex receptacles front, rear, and outside, plugged into the inverter.

I could see using up quite a bit of battery to charge two ebikes. Then again I wouldn't mind running the genie for an hour - it will kick out 45 amps to recharge. Let the panels top it off.

I'm thinking I won't need to be plugged into AC anymore - the panels will keep everything topped up while parked at the house.

I got a call today from BigBattery on an inquiry I made with them. Their 170a 'Owl' is in stock and really enticing. Too bad my GC2's are still working like a champ.
I asked them about the people that make their BMS - that company sells their own big batteries as well. They are not affliated. BigBattery is really nice - seems more retail oriented. I would put one under the sofa in the living room of the RV where it would stay cool or hot, depending. It means buying a DCtoDC charger instead of the alternator, and toss the IOTA for a Progressive dynamics lifepo converter/charger. I wonder if I could do without a converter? Like with just a dc to dc and a plug-in charger for the lifepo, and the solar charge controller. Could the coach run off the 12v system without a converter, especially with the solar option for charging? I would only plug in for AC power on the main panel for air conditioning, or run the gas gennie - it could power the plug-in lifepo charger on the 110 side.

With 12amps going through the inverter I did notice quit a lot of voltage sage - batteries would show 12.3 or 12.4, but would bounce right back up to 12.7 once the load was off. I think lithiums are better in this respect too - the sales girl thought so, so now I'm curious.

It's still a couple of grand no matter which-way you do it. SMH

 

[12VoltInstalls]

Could the coach run off the 12v system without a converter

Yes.

If you are not on shorepower for charging a decent solar input and battery bank can handle your 12V needs.

If you are going to be mostly leap-frogging shorepower to shorepower in your travels and not much boondocking a converter is convenient which is why they are used.

If you don’t want shorepower consistently or to be dependent on shorepower imho disconnecting or otherwise eliminating the converter can make sense. Plus, you don’t want to power the converter off your batteries/inverter anyway but it seems you probably know that

lithiums are better in this respect too

Very likely lithium won’t show the voltage sag like lead acid does. Mathematically it happens.

 

[Browneye]

I never really understood the converter/charger thing in the first place. I get it that it has programming for battery charge maintenance, which it does a great job with on the FLA's, but if you're drawing off the batteries while it's charging I assume that would be the 'converter' part of it. It just never made sense to me - you're drawing off the 12volt bank while it's charging it. They even give it a capacity rating - in my case it's 45Ah. So is that the converter part of it, or the charger part of it? Or both in combination?

I never bothered to add up all the potential 120v appliances that could operate while the generator is running, or plugged into shore power. I guess it's just the microwave, AC, and the converter/charger unit. Oh, and the tv and tuner/dvd unit. Well, and the 120 receptacles. With the air conditioner being the biggest one - I think it's about 22-25 amps. I have a 30A system, which you can plug into a 20A wall receptacle with an adapter, but it will overheat the extension cord or blow the breaker if you try to run the AC unit on it. It draws just enough more that it needs that bigger circuit to run. Newer bigger rigs have a 50A system. Most RV parks have both, as well as a 110V outlet on the site power box.

If I were to simply remove the converter-charger, in my case it's an IOTA, why couldn't I just install a lifepo charger in it's place? It would get powered up when you plugged into shore power, or when you ran the generator. BigBattery sells a special 120v 30A charger for $180. A new converter/charger, IOTA or Progressive Dynamics, is about the same price. Then a DCtoDC charger is another couple of hundred, I guess you connect that to your starter battery and when it senses 14volts it turns on and charges your house lifepo4's with the alternator. They are all different capacities too, from 15A to 60A or even more.

What starts the generator? I assumed it was the house batteries - so if they got too low and you wanted to start the genny, you would be out of luck? I guess I could disconnect my house batts and see if it starts off the starter battery. I know the slide-out motors are hooked to the starter battery - that was a surprise. Same for the hydraulic leveling jacks. You really need to have the engine running to operate them - talk about voltage sag. LOL

Sorry for rambling...I'm kind of just thinking 'out loud' here. Thanks for bearing with me.

 

[Browneye]

The 215W panel you are showing is not a 12V panel. 12V panels have 36 cells. That might be a 44 cell panel. You would be wasting a lot of power if you were to go pwm. You are stuck with mppt with those panels.

Got the panels - they are definitely 36 cell mono - 6.25x6.25 inch cells, you can clearly see them. Four across and 9 down.

I got my new 10awg PV leads today with IP67 MP4 connectors on them, a 3' pair for each panel. Unsoldered the SAE leads they had put on them - looks to be 14g wire. The MP4 seals were a little nicer on the new cables, so those went on the PV box, cut the correct connector ends off, stripped a quarter inch of really thick insulation, mounted them thru the glands, and soldered them onto the connectors. Awesome. And there's a 20' pair of leads come down the refer chimney/roof top vent and connect to the double switch going into the controller.

I might even be able get the channels and panels mounted this weekend - we'll have a couple of summer-weather days here. It was 80 today, looking at 92 and 88 saturday and sunday.

The Epever Xtra tracer SCC came in, nice looking unit. Getting there.

The solar cells look like these:

 

[12VoltInstalls]

really thick insulation, mounted them thru the glands, and soldered them onto the connectors.

Just as a long-term or maybe even short-term dependability exercise: you should mechanically connect the wires by a crimp before soldering.

While I’ve not had much luck opening the factory crimps on the connectors inside the diode/jct box even if the wire has pulled out, a repair method I have used is to crimp a 1/8” ring terminal onto the PV wire and use the solder tab (what the diode is soldered to; btw I’ve never seen one not include a slot or other mechanical hold which is then soldered at the factory) that the original PV wire was crimped to and hold it in place with the mounting screw. Don’t overtighten - there’s not much holding power!
Then I solder the ring terminal to the tab. That way it can’t go anywhere if the solder joint fails for some unknown reason, and the screw gets seized in place.
With the compression sleeve in the gland there should be no work failure from movement and the terminal crimp should be sufficient anyway.

Sounds picky uni but with the contact/mechanical/solder three step I’ve seen zero failure; but only a few situations actually benefit from soldering.
In fact, in most other cases solder isn’t even desirable. With applications where movement or vibration is going to happen, soldering can create failure fwiw. Waterproof connectors with dry crimps inside are almost always preferred over soldering in most applications.

 

[Cal]

Got the panels - they are definitely 36 cell mono - 6.25x6.25 inch cells, you can clearly see them. Four across and 9 down.

So those panels have 36 cells but they act as 44 cell panels. Panel voltage is higher than normal and current is lower than normal.

According to my calculations the short circuit current (Isc) of a "normal" 215W, 36 cell panel should be 13.3A. Your panel outputs only 10.2A. The short circuit panel spec is important for pwm applications as pwm charging current is slightly below Isc. Isc is an excellent indicator what pwm can produce.

BTW, you can keep using your Iota converter. I converted my 55A Iota from GC2 application to LiFePO4 application. The boost voltage needs to drop from 14.7V to around 14.4V (or a bit lower). There's a pot located on the circuit board. You can lower the voltage by adjusting the pot.

 

[Browneye]

BTW, you can keep using your Iota converter. I converted my 55A Iota from GC2 application to LiFePO4 application. The boost voltage needs to drop from 14.7V to around 14.4V (or a bit lower). There's a pot located on the circuit board. You can lower the voltage by adjusting the pot.

Really! Tell me more.
Did you have the IQ4 option? According to IOTA - I contacted their tech support - they said 'no', not with the IQ4 built in - that's what I have. It's specifically designed to maintain FLA's with fully four-stage charging. Ideally it would have been a standard DLS with a plug in IQ4 dongle you could remove and replace with their 'IQ for lifepo' plug in dongle. My intention was simply to replace with a standard DLS and get the lifepo dongle. Problem solved. Then I got to thinking, why do I even need a converter/charger? Just a plug in charger maintainer would be enough for a lifepo bank or large battery.

Other than too high boost AND float volts, I think the 'equalization mode' done every 28 days is also a no-no for LifePo4 - way too high of voltage for those.
Let me know what you think is the best way to address lithium charging with shore-power and generator. I would move the gen-start to the chassis batt, take the house off the isolator, and install a DC to DC charger. Solar would cover a lot of it.

The more I study the lithium option the more I want to make the upgrade. The group-buy threads are a wake-up call though. And buying thru aliexpress is like a crap-shoot at best. I'm seriously considering either a Ampere-Time 200A or BigBattery Owl 170A. Both are readily available. Seems SOK is coming back in stock about the end of the month here.

 

[Browneye]

Just as a long-term or maybe even short-term dependability exercise: you should mechanically connect the wires by a crimp before soldering.

While I’ve not had much luck opening the factory crimps on the connectors inside the diode/jct box even if the wire has pulled out, a repair method I have used is to crimp a 1/8” ring terminal onto the PV wire and use the solder tab (what the diode is soldered to; btw I’ve never seen one not include a slot or other mechanical hold which is then soldered at the factory) that the original PV wire was crimped to and hold it in place with the mounting screw. Don’t overtighten - there’s not much holding power!
Then I solder the ring terminal to the tab. That way it can’t go anywhere if the solder joint fails for some unknown reason, and the screw gets seized in place.
With the compression sleeve in the gland there should be no work failure from movement and the terminal crimp should be sufficient anyway.

Sounds picky uni but with the contact/mechanical/solder three step I’ve seen zero failure; but only a few situations actually benefit from soldering.
In fact, in most other cases solder isn’t even desirable. With applications where movement or vibration is going to happen, soldering can create failure fwiw. Waterproof connectors with dry crimps inside are almost always preferred over soldering in most applications.

Agreed. However they were soldered at the factory. They did a decent job, but the crimp tabs were pretty loaded with solder. I didn't want to heat them up too much for fear I would disturb the bypass diodes. And yeah, the glands will hold them steady. There's a LOT of contact area there with the 10g wire. They're not going anywhere. LOL

Seems like they really cut corners on the hook-up wires. The intention must have been for the consumer to connect them to the simple charge controller they sell. But 14g wire? Most consumers are going to have a difficult time making everything work. HD is big on that Renogy stuff too. And it ain't cheap.

 

[Browneye]

BTW, I anticipate that I may still have to replace that simple PWM controller for the suitcase portable set. Seems they don't play nice in parallel with a MPPT controller. Reportedly, the pwm pulses so much in the way it regulates charge power, that the mppt controller can't get a good read on the battery voltage, and won't engage.
Will have to see how they work together once everything is in place. I have a Victron 75/10 in my amazon cart. They won't connect with each other to communicate, but at least they'll both have a smooth output power and should work okay together. Another option would be another Tracer mppt - the 10A ones are pretty cheap, like $58 or something.