Advice requested re: solar for pop-up camper

RANash · November 2009

Hello everyone!

I’m very new to solar power and basically pretty ignorant of electricity itself, to tell the truth. I’ve now inherited a Coleman “pop-up” (folding tent trailer). I’ve taken the family camping in it, and they love it. Now, I’d like to extend our choices of camping places to include those that have no electricity for extended periods (boondocking).

The trailer has a 55 AH deep cycle battery which gets drawn down about 50% each night, mostly due to the trailer’s heater and water pump. I’d like to recharge it each day using solar if I can. I don't want to mount solar panels permanently on the roof of the pop up, and I don't want to haul around more than one panel if I can avoid it.

I’m thinking about buying a Kyocera KD-135 panel which produces Pmax of 135 Watts, Vmpp of17.7, and Impp of 7.63 Amps. I am also thinking that a Morningstar SS-10 (10 Amp, 12 Volt) would do the job for the charge controller.

So I have some questions:

First, would this work (assuming a nice typical California sunny day) ?
Second, do you have suggestions for different panel(s) and/or charge controller?

Also, there may be times where I would want to park the trailer under some shade and place the solar panel away from the trailer in a sunny spot. I can rig up some kind of holder for the panel, but what I don’t know is what kind of cable to use and how long it can be. I assume that the charge controller should be close to the battery on the trailer’s tongue and the cable would be long between the cc and the panel, right?

Any advice, suggestions, tips, etc. would be very appreciated.

Thanks,

Rich

BB. · November 2009

Re: Advice requested re: solar for pop-up camper

How much power you use:

55 AH * 12 volts * 0.50 = 330 Watt*Hours of power per night

Realistically, the panels will average 77% efficiency and a flooded cell battery 80% efficient.

330 Watt*Hours * 1/135 watt solar panel * 1/0.77 * 1/0.88 = 4 hours of sun per day

Assuming no other losses, you would need 4 hours of sun per day to recharge the battery bank.

You did not tell us where you are located... Using the PV Watts website for Sacramento CA area, you would get:

"Month", "Solar Radiation (kWh/m^2/day)"
1, 2.75
2, 4.23
3, 5.27
4, 6.33
5, 6.83
6, 6.93
7, 7.23
8, 7.30
9, 6.96
10, 5.67
11, 3.68
12, 2.71
"Year", 5.50 average over the year

So, outside of January, November and December, you would have >4 hours of sun per day (assuming panel in full sun and tilted at latitude angle).

How far to move the panel away from the trailer... Use a voltage drop calculator like this one here. If we assume that you don't want more than roughly 0.5 volt drop due to wiring; 8 amp load; 10 AWG (heavy gauge extension cord), and 25 foot cord length (remember is round trip distance--this calculator assumes 25'*2 for voltage drop).

We get 0.411 volt drop (10 awg wire, 8 amp, 25' cord).

If you want longer a longer cord set, you would A) have to use thicker gauge copper or

use higher voltage panel(s) and a more expensive MPPT type solar charge controller... Basically wire the panels to a 24 volt circuit which cuts the current by 1/2 and gives allows a lot more voltage drop and still charge the battery bank (~15 volts max at battery).

All About Charge Controllers
Read this page about power tracking controllers

-Bill

RANash · November 2009

Re: Advice requested re: solar for pop-up camper

Thanks Bill, this is excellent information. I live only about 45 minutes west of Sacramento. However, we will be camping here and there, but usually under sunny skies. I really hate running generators while camping.

So, of course, a higher watt panel would give me even shorter recharge times, however, it's very hard to find an affordable 12-volt panel higher than 135 watts.

Would it work to get a higher volt panel like the Kyocera KD185GX-LPU 185 Watt and use an MPPT charge controller? I know that the MPPT controller would allow the use of the higher watt panel, and would help with longer cords (as you explained), but would it charge the battery more quickly and/or more reliably (such as on less than full-on sunny days)? Or would I be wasting the extra money?

Thanks for your help!

RANash · November 2009

Re: Advice requested re: solar for pop-up camper

Hi Bill,

I just read the pages you linked to at the bottom of your post. I think they answered my new question affirmatively.

Thanks.

BB. · November 2009

Re: Advice requested re: solar for pop-up camper

Rich,

I think you got your answers!

By the way, higher voltage panels, MPPT charge controllers, etc. will not make anything better under a cloudy sky.

The solar panels (basically) reach their working Vmp voltage under weak sunlight--and then the output current is proportional to the Watts per Sq. Ft/Meter.

Under thin clouds, you can lose 1/2 of your power... Under heavy overcast you can lose 90-99% of your panel power output (regardless of brand of panels, etc.).

And that is why you will probably want/need a back up genset. Also, a small Honda eu1000i (or the larger eu2000i) inverter generator are quieter than your car and way more fuel efficient.

-Bill

RANash · November 2009

Re: Advice requested re: solar for pop-up camper

I appreciate your help, Bill.

Just a quick sanity check to be sure I'm understanding things correctly, if you would indulge me...

So let's say I buy a Kyocera KD205GX 205 watt panel that runs at 26.6 volts and 7.71 amps as per the specs. If I buy a good MPPT charge controller, I can push those 205 watts at my battery at something like 14-16 volts and 12 - 14 amps? (Of course, I understand that these figures are assuming perfect conditions which don't exist in the real world.)

Do I have the right idea here?

BB. · November 2009

Re: Advice requested re: solar for pop-up camper

Yes, you are correct. The MPPT solar charge controllers are actually power converters... Basically the DC equivalent of an "automatic" variac (variable AC transformer).

The controller is trying to maximize Pmp=Vmp*Imp ... Then power to the battery is:

Pbatt=Vbatt*Ibatt=Vmp*Imp*0.xx

Where "0.xx" is the efficiency of the charge controller (at higher currents, efficiency is well over 90%. When the sun is low in the sky, clouds, and/or a very small array on a large MPPT inverter, the efficiencies drop under "low solar power" as there are "tare" losses--the power required just to run the electronics inside the MPPT controller).

By the way, if you can set your panel(s) to tilt towards the sun, that can add significantly to your power generation during the winter months.

Of course, panels have to lock for protection against wind and for on-the-road traveling. Do not underestimate that wind forces on these large (thin glass) solar panels. The mounting frame work must be solid and not permit the solar panels to "twist". The solar panel frames themselves are lightweight and not intended to be structural mounts themselves.

-Bill

RANash · November 2009

Re: Advice requested re: solar for pop-up camper

Thanks Bill, and kudos to you and to Wind-Sun for this forum. I've been all over the Internet and read a lot of stuff, and this forum is the best.

Solar for the camping trailer is just the first project. In 2010, I'll begin building the system for a grid-tied array for my home.

BB. · November 2009

Re: Advice requested re: solar for pop-up camper

Thank you Rich,

But it is Windsun that setup the forum funds it operations--and all of you out there that provide the content that makes this site useful (and, I think, fun to read).

Neil and I (and Jim/Crewzer before me) that just try to keep the spam down and posts on topic with the occasional nudge--With Windsun working under the hood to keep it running nicely.

Sincerely,
-Bill

Advice requested re: solar for pop-up camper

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