Cargo Trailer Camping Converting

[Wrybread]

With 640 watts charging a 12volt battery you will need a 60Amp MPPT type charge controller. That can get you about 50 amps of charging under that best conditions which means your battery system should rated between 400Ah and 600Ah to meet a C/8 to C/12 charge rate.

I've got the same 640 watts of panels, going into a 40 amp, 100 volt charge controller. Panels are flat mounted, wired in series. Works great. I have a script that logs all my voltages and I've never gone above 95 volts. It'd be nice to have the 150 volts for some headroom, and I'm not recommending you use a 100 volt charge controller, but it's been working fine for me. I should mention that I'm at sea level on the Northern California coast where it never goes below, say, 40 degrees F. I mention that because at higher temps and higher elevations morning voltages might spike higher.

Sometimes I run my panels in parallel (when one panel might be shaded), and in my real world tests I get *exactly* the same watt output from my panels when they're in parallel versus series. They're still high voltage when in parallel (46 volts), so I'm still getting a benefit from MPPT. I'm guessing in series I'd get more power in the early morning and evening, but that's no big deal since I don't get much power then anyway. Translation: don't worry too much if you need to go parallel, in real world tests it works fine. You can easily repeat those tests for yourself.

As far as the fridge, agreed that until there's some real world measurements posted, we can't really know. But I don't think it's any big deal, as the OP stated if the fridge doesn't work he'll use it for something else. He still wants and needs solar for his rig for lighting and other stuff.

As far as carefully measuring your loads before getting your panels, while that's of course how it's done when planning off-grid systems for a house, it's kind of unusual to apply that approach to an RV. Typically with an RV you just get the biggest solar system you can fit and afford and adjust your load to that, as opposed to adjusting your panels to your load. Realistically, the options for RV solar are 100 watts up to 650 watts. Anything above that is pretty unusual. You're on the right track with your ~600 watts of solar and you'll have the best RV solar system for a hundred miles, and I'm sure with minor tweaks to your loads it'll work great.

OK I cannot say this strongly enough, you will have to have an alternate source of power. Wyrbread gets away without having an alternate source of power because he never runs more than 4 to 7 days at a time.

While I generally use my RV on weekends, I also use it for weeks at a time a few times a year, and my panels have no problem keeping up then as well. Especially in summer, which is the OP's use, I can go indefinitely from just solar. In summer I never go below 12.4 volts even with 3+ weeks of super heavy use, and my batteries are fully charged up most days even as I'm using it. Summer is dead simple, even with cloudy days, since I have enough panel wattage that even with overcast I'm getting enough charge to keep my batteries up. And here on the Northern California coast summer haze is the norm.

or, you can go with a pair of cheap 6v 210ah golf cart batteries, run the snot out of them, take them way down, and just replace $250 worth of batteries every year as you kill them.

Just don't drain your batteries below 11.8 volts, which is SUPER easy to keep track of, and your batteries will be fine. No big deal and very easy. Get a battery monitor so you can easily keep track of the voltage and loads:

https://www.amazon.com/bayite-6-5-10.../dp/B013PKYILS

And learn how to check your battery fluid levels and be super vigilant about not letting them get low, and don't over fill them since boiling over causes a mess.

[Sunking]

I've got the same 640 watts of panels, going into a 40 amp, 100 volt charge controller. Panels are flat mounted, wired in series. Works great. I have a script that logs all my voltages and I've never gone above 95 volts. It'd be nice to have the 150 volts for some headroom, and I'm not recommending you use a 100 volt charge controller, but it's been working fine for me.

Then you do not mind being robbed and taking chances. I would bet you are a democrat and want to pay even higher taxes than you do now based on that.

[littleharbor]

[QUOTE=Wrybread;n351792]



Sometimes I run my panels in parallel (when one panel might be shaded), and in my real world tests I get *exactly* the same watt output from my panels when they're in parallel versus series. They're still high voltage when in parallel (46 volts), so I'm still getting a benefit from MPPT. I'm guessing in series I'd get more power in the early morning and evening, but that's no big deal since I don't get much power then anyway. Translation: don't worry too much if you need to go parallel, in real world tests it works fine. You can easily repeat those tests for yourself.


Is that 46 volts Voc? Voc. has nothing to do with charging. If your panels are in the 35 - 36 Vmp. range once they get hot they will run in the low 30's Vmp. that is just enough to charge your 24 volt batteries with a conventional charge controller. You wont get much, if any benefit of a MPPT controller.

[Wrybread]

Is that 46 volts Voc? Voc. has nothing to do with charging. If your panels are in the 35 - 36 Vmp. range once they get hot they will run in the low 30's Vmp. that is just enough to charge your 24 volt batteries with a conventional charge controller. You wont get much, if any benefit of a MPPT controller.

Yeah, the panels are 72-cells, 46 VOC. With higher voltage an MPPT controller pulls more power from the panels when the sun is low (morning and evening), so it does have to do with charging. (The other advantages to series wiring are the ability to use smaller wire and no need to fuse the panels). But my point is that in real world tests I'm finding the difference in power to be negligable, especially since I don't get much wattage when the sun is low anyway.

[littleharbor]

So why waste money on an MPPT controller?
If you were to use 60 cell modules which have a more versatile Voc. and Vmp then you have no need to worry about running too high of voltage and you gain the benefit of MPPT.

[Wrybread]

So why waste money on an MPPT controller?
If you were to use 60 cell modules which have a more versatile Voc. and Vmp then you have no need to worry about running too high of voltage and you gain the benefit of MPPT.

Because I still get a *massive* power improvement over a PWM controller. Remember, even in parallel wiring I'm at 46 volts, which is plenty of over voltage to my 12 volt batteries for the MPPT controller to work with. And I don't "waste" much money on my charge controller anyway, my Tracer 4210a 40 amp charge controller costs all of $160 shipped.

And I'm not throwing away any significant wattage, I'm still getting 95% (or more) of the benefit from MPPT even in parallel.

And I know that in theory the 60 cell panels are more versatile, but the 72-cell panels have been working just beautifully for me for over a year. That said, if someone doesn't want to take any risks and wants to wire 72-cell panels in series, I recommend getting 60 cell panels.

[littleharbor]

How do you figure you are at 46 volts? open circuit voltage does nothing. I just mentioned that. Those 72 cell modules when hot will only output in the low 30 volt range. It sounds like you don't understand the difference or how to apply temp. coefficient values.

[Wrybread]

How do you figure you are at 46 volts? open circuit voltage does nothing. I just mentioned that. Those 72 cell modules when hot will only output in the low 30 volt range. Is sounds like you don't understand the difference or how to apply temp. coefficient values.

From both my charge controller (it reports the voltage at all times, as well as logging it), and a multimeter. The panels put out a pretty consistent 36 to 46 volts each while the sun is reasonably high.

Note: I edited the above from "40 to 46 volts" range to "36 to 46 volts" range

[littleharbor]

Not possible. 72 cells at approx. .5 volts per cell is 36 volts period. Unless you have those panels in sub zero temps it just cant happen.

[Wrybread]

Huh? Attaching a picture of the MT50 remote monitoring box for the charge controller, showing typical usage. In case the graphic doesn't come through, it's showing 85.3 volts.That's in about 65 degrees F.
mt50.jpg

[littleharbor]

You are only taking only 1,2 amps from your array because your battery is nearly full. There is nearly no load on your array and it is nearing Voc. What numbers do you see when in bulk charging mode?

[Wrybread]

In bulk charging mode it dips down to ~36 volts. But my point is that it routinely goes higher than that, hence the issue with series wiring two 72 volt panels to a 100 volt charge controller.

Also, I edited my original post to reflect that.

[littleharbor]

And the reason I suggested 60 cell panels. If you find yourself in a cold clear mountain morning your Voc. will exceed 100 volts. there are 60 cell modules being made these days in the 360 watt range if you really need that much power.

[Wrybread]

And the reason I suggested 60 cell panels. If you find yourself in a cold clear mountain morning your Voc. will exceed 100 volts. there are 60 cell modules being made these days in the 360 watt range if you really need that much power.

But they probably don't cost $150 to $200 each like the 72-cell 300+ watt panels... Remember, like most RV systems I'm trying to keep mine cheap. My whole 600 watt system including charge controller and wires (but not batteries) cost under $500, which I'm guessing is less than the price of a single 300+ watt 60 cell panel. Or do you have a link to a cheaper one? That would definitely be useful for people building out their RV solar systems.

But anyway, agreed that for wiring 72-cell panels to a 100 volt charge controller, parallel wiring is a better way to go if cold mountain mornings are a possibility.

[littleharbor]

Panels are pretty cheap these days. I am forced to sell mine for 39 to 42 cents a watt to keep the phone ringing. Anything higher and I just watch them collect dust.