Upgrading an RV System

vcallaway · September 2010

I have a 37' class A. Lots of open roof space.

My current setup is a single 85W panel connected to a Xantrex C12 controller. I have a single 200Ah Trojan battery for the coach. During the summer this does a great job keeping the battery topped off. Winter not so much.

Our sponsors have this Kyocera KD135X panel that appears to be a pretty good deal. The initial plan would be to replace the 85 with one of those. The 85 would get moved to my shed to replace the HF panels there.

Since I'm flat mounting I know I'm not going to get full power out of the panels. My C12 can handle at least two in parallel. I think my goal should be four panels. I would like to not have to run a generator at all during hunting season.

With the flat mounting do you think I would be better off changing out the controller with an MPPT type and wiring series? Budget IS an issue here

I would probably use adjustable mounts anyways. Don't think I would bother moving them unless we are going to be parked more than a couple of days.

At almost 30lbs I think these panels are about as large as I want to lug up a ladder.

If you have followed my rambling thought process please offer suggestions.

Thanks.

bryanl · September 2010

Re: Upgrading an RV System

you think you have lots of space on the roof ...

Figure it out. See if you can get c/5 charge current to your batteries (i.e. 40 amps for 200 AH battery). If that 200 AH is for 12v, then it's 480 watts of solar plus a fudge factor for efficiencies and such.

Then consider your battery bank vs your usage. You can get about 10 to 15 usable watt hours per pound of battery. Your battery bank should be sized to handle 2+ days of nominal use. For most folks, this means a lot more battery than you have and that means even larger solar is needed for proper charging.

Then you need to look at charging and maintenance issues. A full and proper charge will take 8 hours or more with good equipment and that is needed on a regular basis (or you get into the equalization charge hassle). Maintenance requires keeping a proper charge on the battery with some technique to inhibit or reduce sulfation applied.

An RV is rather limited in what you can do with solar and batteries. Don't expect too much. You'll have to make some compromises somewhere. Chasing MPPT vs PWM and wire gauge and such is the small fish in this pond.

vcallaway · September 2010

Re: Upgrading an RV System

Actually, right now I can go 2 days during the summer and only use about 30% of the battery max. And then only if I fire up the TV.

An RV is not an off-grid only setup. When I plug into shore power my IOTA IQ4 charger takes care of the battery just fine. I'm not real concerned about going into float while I'm not plugged in. I'm only looking to extend my run time in the winter without running the genny.

The short time the battery is not fully charged has not seemed to bother it much. It is 4 yrs old and still going strong.

BB. · September 2010

Re: Upgrading an RV System

vcallaway,

I assume that this is your question that you wished for more discussion on (comment in another thread)...

Bryanl pretty much addressed the cost issues for a small system where price is important--a PWM charger is probably going to work well for your system.

MPPT does not improve the amount of sun on a panel (tilt vs flat). And if you only camp at lower latitudes in the summer--Flat mounted panels are not going to cost you much power... If you go north and do winter camping--tilting the panels will help--But even that is limited as there is just not much sunlight during the winter. I use PV Watts to play around with the tilt options. For example (using 1kW of panels--smallest supported by PVWatts):

"Station Identification"
"City:","Quillayute"
"State:","Washington"
"Lat (deg N):", 47.95
"Long (deg W):", 124.55
"Elev (m): ", 55
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.5 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 47.9"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 6.4 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 1.51, 20, 1.28
2, 2.21, 29, 1.86
3, 3.12, 45, 2.88
4, 4.04, 58, 3.71
5, 4.73, 69, 4.42
6, 4.74, 67, 4.29
7, 4.91, 71, 4.54
8, 4.57, 65, 4.16
9, 4.37, 61, 3.90
10, 3.31, 48, 3.07
11, 1.87, 25, 1.60
12, 1.85, 26, 1.66
"Year", 3.44, 584, 37.38

Vs Flat Mount:

"Station Identification"
"City:","Quillayute"
"State:","Washington"
"Lat (deg N):", 47.95
"Long (deg W):", 124.55
"Elev (m): ", 55
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.5 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 0.0"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 6.4 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 0.92, 9, 0.58
2, 1.55, 18, 1.15
3, 2.49, 35, 2.24
4, 3.78, 55, 3.52
5, 4.96, 74, 4.74
6, 5.24, 75, 4.80
7, 5.31, 78, 4.99
8, 4.41, 63, 4.03
9, 3.49, 48, 3.07
10, 2.24, 30, 1.92
11, 1.13, 12, 0.77
12, 0.92, 8, 0.51
"Year", 3.05, 505, 32.32

Vs Latitude + 15 degrees (winter setting):

"Station Identification"
"City:","Quillayute"
"State:","Washington"
"Lat (deg N):", 47.95
"Long (deg W):", 124.55
"Elev (m): ", 55
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.5 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 63.0"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 6.4 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 1.54, 21, 1.34
2, 2.20, 28, 1.79
3, 3.00, 43, 2.75
4, 3.71, 53, 3.39
5, 4.16, 60, 3.84
6, 4.11, 57, 3.65
7, 4.29, 60, 3.84
8, 4.14, 58, 3.71
9, 4.16, 58, 3.71
10, 3.29, 48, 3.07
11, 1.92, 25, 1.60
12, 1.95, 27, 1.73
"Year", 3.21, 538, 34.43

If you are in a very hot region (and flat mounted panels with poor air flow) will depress the output of your panels. With Vmp of ~17.5 volts, very hot panels can have the Vmp drop down to ~14 volts (or even a bit lower)--which would begin to limit current flow into the battery bank (plus voltage drop+controller drop)... Vmp is not a sharp curve--so it will still get some current flow when trying to charge the battery bank to ~14.4 volts. But it will be less than optimum.

You might want to monitor the panel temperature between flat and tilted and see if you have a heat build up problem with your flat mounting and poor airflow.

If you do go to hot climates and boon dock, then you may want to byte the bullet and go with a MPPT charge controller so you can place the two panels in series so that you will collect the maximum amount of power even in the heat of desert.

Take a look at the Rogue 30 amp 12/24 volt MPPT controller... Its Vpanel input is not as high as the other MPPT charge controllers--But it is high enough that you can place two "12 volt" panels in series and properly charge a 12 volt battery bank with depressed input panel input voltage.

Do you have other questions? Or did I miss the point of your questions?

-Bill

vcallaway · September 2010

Re: Upgrading an RV System

I read through the documentation on the Rogue and read the info here. Looks to be a pretty good unit. It also has me re-thinking a couple of things.

I started looking at the Sunwize SW100C panels. They seem to be pretty popular for RV installations. The specs say that Open Circuit Voltage is 28.5v. The Rogue manual says it can handle up to 60v. The price on these is certainly attractive.

Does this mean I can run 2 panels in series safely with this controller? Ideally I would eventually use 4 panels in 2 banks. If so then I should be about to run 8ga wire from a combiner box to the Rogue. It is about 25' of wire between the two.

Does this sound right to you guys?

If so then I guess it is time to start ordering stuff.

dwh · September 2010

Re: Upgrading an RV System

Yea..the Sunwize voltage is too high for "efficient" 12v charging, but too low for 24v charging so pretty much the only way to use them to charge batteries efficiently is by using an MPPT controller.

And if you're going to do that, you might as well rig them in series. You would have to rig them in series to charge a 24v battery, but if you are using MPPT anyway, you could rig them in parallel to avoid shading issues on series PV and still have enough voltage differential to charge a 12v battery and still get the benefit of MPPT.

mike95490 · September 2010

Re: Upgrading an RV System

28.5 x 2 is pretty close to the 60V fatality mark. Add a cold morning as the sun comes up, and you toast the controller.

BB. · September 2010

Re: Upgrading an RV System

Yea, a single Sunwise panel (or multiple single panels in parallel) charging a 12 volt battery bank is the only configuration that is "safe" for the Rogue. Otherwise, Voc on a cold day will over volt the controller.

-Bill

Upgrading an RV System

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