Small RV trailer solar-system design ?'s

khotton

I recently purchased a used travel trailer 17ft. My plan is to park it on land I own in Mexico as a poor man's vacation retreat to live in while getting a more perminent structure built. The trailer has two deep-cycle "wet" lead-acid batteries 12VDC wired thru a rotary battery selector switch with options: battery A, battery B, both batteries A and B, neither battery. I would like to incorporate a solar PV system to maintain charge on these batteries (both batteries are about 6-months old). I would like to limit cost to under $1200 (trailer was only $800). My initial design was as follows:

two Uni-Solar US64 panels (mount panels flat to trailer roof)
two Morningstar SunGuard 4.5A charge controllers
Morningstar SureSine 300W inverter

Wire one PV panel thru one charge controller to each battery (ie two identical systems in parallel). I would use say battery "A" till LVD buzzer on inverter sounds then switch to battery "B". When LVD is reached with battery "B" switch back to battery "A". I don't know if this system is the best approach. One concern is the SunGuard PWM setpoint is 14.1V (non-adjustable) is recommended for sealed or gelled lead-acid batteries. How much of a problem is this with wet-cell batteries? I guess for about $40 more, I can up-grade to Morningstar SunSaver 6A charge controllers that have selectable "wet-cell" 14.4V setting.

Is this a good design or does it make more sense to use both batteries in parallel and charge both batteries with a single charge controller? I have also seen the Morningstar SunSaver Duo controller with remote meter for dual battery RV use, but I hate to spend that much money.

Please advise/comment.
Thanks, Kevin

System2

Re: Small RV trailer solar-system design ?'s

I would sell off the two batts and go with 2 or even 4 6V Golf cart batts.

if that's not an option just keep the switch to both and follow below.

Get one inexpensive but better/larger PWM controller (I think the ones you have in mind are a little small )and as much panel as you can get. use the inverter you have already, if the trailer doesn't have one then get what ever you can find dont worry about the Sine wave unless you really need the sine wave.

niel

Re: Small RV trailer solar-system design ?'s

i'm not the expert in rvs, but you do have options in how you could do it as you know. personally, i'd go with one charge controller, but whether you charge both batteries in parallel or one at a time is up to you. if it were me i'd use one battery while charging the other.
on the pvs, the only advantage the uni 64s have is a nearly unbreakable surface. if the unbreakable surface isn't the reason for the pvs then go with the standard pvs with glass fronts. i'll not recommend any pv to be mounted flat on an rv roof unless it's in motion. while parked the pvs should be angled towards the sun, but if too difficult they have to be at least raised up off of the rv roof allowing about an 8 inch airway to cool the pvs somewhat. there's also the option of mounting them remotely off of the rv so the pvs could be in the sun and the rv in the shade if you find some down there. that makes aiming the pvs towards the sun easier and the parking options better for your rv.

12vman

Re: Small RV trailer solar-system design ?'s

I would consider another charge controller..
Quote..

"The SunGuard has a slightly lower output voltage than the SunSaver and ProStar and may not be the best choice for flooded batteries that require a higher voltage."

I'm with niel on only using 1 controller. Check out the Xantrex C-12. Sweet little controller and simple to use. It'll give you the state of charge of your battery 24/7 via an L.E.D. on the front and it has a low voltage disconnect to protect the battery..

System2

Re: Small RV trailer solar-system design ?'s

12Vman, Your intro page looks interesting but I cant proceed to what ever is after it.

Is it a forum that needs a a user name or some other login?

Thanks,

12vman

Re: Small RV trailer solar-system design ?'s

Yea.. I've been zapped..AGAIN!

crewzer

Re: Small RV trailer solar-system design ?'s

I recently purchased a used travel trailer 17ft. My plan is to park it on land I own in Mexico The trailer has two deep-cycle "wet" lead-acid batteries 12VDC wired thru a rotary battery selector switch with options: battery A, battery B, both batteries A and B, neither battery. I would like to incorporate a solar PV system to maintain charge on these batteries (both batteries are about 6-months old). I would like to limit cost to under $1200 (trailer was only $800). My initial design was as follows:

two Uni-Solar US64 panels (mount panels flat to trailer roof)
two Morningstar SunGuard 4.5A charge controllers

The Unisolar Vmp spec is 16.5 V. This is probably too low for your hot environment, as PV voltage drops as cell temperature increases. The SunGuard’s absorption voltage spec is 14.1 V, which is too low for charging a flooded-cell deep-cycle lead-battery (14.4 V or higher at 77 F is typical). Accordingly, I recommend against this configuration.

An alternative approach to consider is a single Mitsubishi PV-MF125E4N PV module rated at 125 W STC. Its Vmp is 17.3 V, and the Isc is 7.9 A. At $597, it’s cheaper than a pair of Uni-Solar 64’s ($323 each; $646 for two).

An appropriate minimum controller is the Morningstar SunSaver 10 A two-stage model, although the Morningstar Prostar 15 or the Xantrex C-12 would be better, as these two offer thre-stage charging plus equalization. The Prostar includes a built-in sensor for temp correction. An external sensor that can attach to the batteries is option for the Prostar and the Xantrex.

http://store.solar-electric.com/mi125wa12vos.html
http://store.solar-electric.com/unus64wathfi.html
http://store.solar-electric.com/ss-10l.html
http://store.solar-electric.com/pros15solcha.html
http://store.solar-electric.com/c12.html

HTH,
Jim / crewzer

12vman

Re: Small RV trailer solar-system design ?'s

Quote..
http://www.solarcentury.co.uk/products/solar_photovoltaics/uni_solar_framed_solar_module_es_62w

"Outdoors, under normally higher operating temperatures, solar panel performance changes, depending on temperature, solar spectrum (light color) and related effects. Uni-Solar products are less affected by temperature than monocrystalline or polycrystalline solar technology products. The result is up to 20% more delivered energy."

crewzer

Re: Small RV trailer solar-system design ?'s

Interesting...

The published UniSolar Voc temp coefficient is -80 mV/K = -80 mV/21.0 V/C = -0.38%/C
The same spec for the Mitsubishi is -0.343%/C

Nor much difference, but -0.343%/C is better...

Regards,
Jim / crewzer

Solar Guppy

Re: Small RV trailer solar-system design ?'s

Its the ongoing myth that Asi pushes ... better at hot temps NOT

Stick with regular Si panels , twice the watts for the size and on an RV thats a big concern

BB.

Re: Small RV trailer solar-system design ?'s

If I normalize the V/°C to 1 volt (Open Circuit)--data from Xantrex sizing tool:

BP 4175 (175 watt mono crystalline): -0.16 V/°C / 44 volt = 0.0036 V/°C
US-64 (64 watt panel): -0.07378 V/°C / 23.8 volt = 0.0031 V/°C

0.0031/0.0036= 86% US vs mono crystalline temperature sensitivity...

From what I have read, ASI panels have been known to have problems with long life. Uni-Solar does have a 20 year warranty. From one of their data sheets (PDF):

During the first 8-10 weeks of operation, electrical output exceeds specified ratings. Power output may be higher by 15%, operating voltage may be higher by 11% and operating current may be higher by 4%. Electrical specifications are based on measurements performed at standard test conditions of 1000 W/m2 irradiance, Air Mass 1.5 and Cell Temperature of 25°C after long-term stabilization.

Maximum system open-circuit voltage not to exceed 600 VDC per UL.*** Actual performance may vary up to 10% from rated power due to low temperature operation, spectral and other related effects. Specifications subject to change without notice.

The BP unit has a 25 year warranty, with this disclaimer:

1. Module Warranty: 25-year limited warranty of 80% power output; 12-year limited warranty of 90% power output; 5-year limited warranty of materials and workmanship. See your local representative for full terms of these warranties.
2. These data represent the performance of typical BP 4175 products, and are based on measurements made in accordance with ASTM E1036 corrected to SRC (STC.)
3. During the stabilization process that occurs during the first few months of deployment, module power may decrease by up to 3% from typical Pmax.

Does Uni-Solar "Front Load" their specifications or actually have 15% power degredation in less than three months plus another 10% due to other reasons (vs BP's 3% initial vs 20% over 25 years)?

I believe that Solar Guppy has said that he does not see the 20% over 25 year degridation with panels in good physical condition.

He also posted on his site (back in 2005) that ASI panels should be avoided because of life issues... Uni-Solar (at least in the pass) did make ASI panels...

As for brand, all silicon panels are basically the same. What ever you do DON'T BUY ASI panels (Uni-solar and older BP panels sold on eBay) as they have really issues long term

...cut to another post on thread...

If a panel is working, the cells themselves will last 50-100 years(SI panels). The only failure modes I have seen on panels is the interconnections and that can be repaired so IMHO, panels even with failure can be made to last close to forever. worst case is you get out the knife and soldering iron and fix a bum connection.

...cut to another post on thread...

The only concern on the Sanyo's is they are a new hybrid process that is a sandwich of Si and Asi.

Asi has a horrible track record, the panels degrade much faster in performance than Si, Only UniSolar is left holding the bag on Asi, BP abandon the technology and with a handful of others

Also, was reading around about flexible solar panels (used on sailboats and other places), and they all seem to have real problems sealing the cables to the flexible panels... If you do not need a damage resistant panel and can use glass--I like the cable attachments and sealing much better than one normally sees on flexible (plastic) panels... What ever you do, make sure that there is no movement/vibration that can stress the panel connections on your installation.

-Bill

niel

Re: Small RV trailer solar-system design ?'s

you know i have a couple of the uni64s and i should see what the short circuit current is doing before the winter arrives. i don't know if the opportunity will present itself for access soon or i may forget to do this altogether by the time it is accessable. i do not remember the exact year i even got them from naws, but i believe it was before 2000.

khotton

Re: Small RV trailer solar-system design ?'s

Thank you for all your comments/Suggestions.

I am now favoring the least expensive option I can think of. I would use a single Kyocera 65W panel on a sun-directed stand used with a Morningstar SunKeeper 6A charge controller jumpered to the battery that had just been discharged from use. When I am not using the trailer I would directly connect a Uni-Solar 5W panel to both batteries connected in parallel to trickle-charge/maintain them while gone and also minimizing the potential theft of the larger PV panel which I can hide and lock within the trailer.

The AC loads I really only need to power is a laptop computer (27W) and my wife’s curling iron (15W). I am hoping a modified sine-wave inverter (Xantrex Mobile Plug 75) will work. Have any forum members had experience (trouble) powering a laptop computer with a modified sine-wave inverter?

This PV solar option is under $500 and I can later use, for additional electric power in the trailer, some of the PV panels / charger controller / inverter that I will be purchasing for use in the off-grid home I will be constructing in Mexico.

Kevin

BB.

Re: Small RV trailer solar-system design ?'s

You really also need to define how long the loads run... A curling iron may run for 30 minutes 15W*0.5Hour=7.5Watt*Hours--and not even at full load... But the computer could run 24x7 at 27 watts = 648 Watt*Hours...

By the time you add all of the inefficiencies--a 650 watt load may need 200-300 watt of solar panels to supply that power (depending on sun, weather, temperature, exact batteries, inverter, charge controller, etc.)...

The issue with the modified square wave inverters is that 80-90% of your loads may work just fine on it--the other 10% or so may get damaged in even a few ten's of minutes of use. (by the way, motors do not like MSW inverters).

So, probably your computer power supply will work OK--but you could also consider using a 12 VDC car adapter for your laptop and skip the whole AC Inverter question...

-Bill

12vman

Re: Small RV trailer solar-system design ?'s

Quote.. niel..
"you know i have a couple of the uni64s and i should see what the short circuit current is doing before the winter arrives."

Please keep us (me) updated on this if you would.
Don

System2

Re: Small RV trailer solar-system design ?'s

Where exactly are you in Mexico? We are in the mountains of Veracruz and the temp is always cool year-round, but with plenty of sun. We have 150w of panels with a completely 12 volt system--(only use 120v ac occasionally with a $30 AUTOZ
ONE inverter). Since RV's typically use 12 volts appliances and lights, why not keep it that way and save on the inverter? frankania

khotton

Re: Small RV trailer solar-system design ?'s

I am near San Miguel de Allende (about 5miles NE of center of town -- as the crow flies).

Kevin