Solar question

[Jbrowland]

Hello and thanks again in advance.

I have a small solar panel mounted on top of my roof AC that is wired to my battery for trickle charging. My guess is that it is a 10-20 watt panel. It seems to help a little when the RV is in storage but doesn't do much else. It was already installed on the RV when I bought it. There is one wire hooked up to the solar panel that disappears in the roof and runs down to the house battery.

My 2 part question:

1. I would like to install one or two 100w panels on my roof. Could I connect the newly installed panels to the already existing wire that runs down to the battery to make the install easier?

2. I also assume I would have to install a charge controller somewhere as close to the battery as I can get it. That's tricky as my battery compartment is one of those slide out models that is close to the ground/road and not element proof as it is open air style. I assume I would have to install the charge controller inside the RV somewhere so it wouldn't get damaged by the elements? I wonder if there is already a small charge controller hidden somewhere for the small panel already on my roof.

[Almot]

Hard to comment on suitability of your existing wire. "Thin" is not a term that folks here want to hear 🙂

Voltage drop should be under 2%. Use Voltage drop calculator , dial one-way length (i.e. 10ft run of a double-lead cable is 10ft). For 2*100W in parallel dial in 12A current, 19V. At high elevations and cold weather the current will shoot a bit higher.

Mr Wiz was right as usual. You will need #10 unless it's a very short run. This is - assuming PWM type controller.

You could live with a thin wire #16 or #18 if you wire panels in series and spend few hundred more on MPPT controller. There is a low-priced $100 MPPT by Ecoworthy that would handle 2*100W, though deficiencies of this model are too significant to ignore. But it will "charge".

Half-decent 20A MPPT will cost $250-300. For comparison, a very good 30A PWM controller will cost $120 or less but it won't handle panels in series, so you will have high amps and need a thicker cable.

I would try and squeeze 300W on the roof.
Possibly - a single 260-280W panel (this would require MPPT controller, PWM won't do in this case). Then you could eliminate that 100W portable add-on. Panels from 230 to 280W are practically same size 40" by 65".

Your energy budget makes sense. 300W will harvest more than 60 AH on a sunny day. With minimal needs similar to yours I found that 30-40 AH is enough, and the biggest draw is DC circuit of propane fridge. I run catalytic heater instead of furnace and night temps are above 40 usually.

I hope this was not too much info to digest.

[Jbrowland]

Thanks for the help. I'm learning.

Some information:

The wire coming out of my Eco-Worthy 100w solar suitcase (built in controller to alligator clips) is much thinner than the wire coming out of the trickle panel and entering the roof of my RV. I'm not sure of the exact gauge of either wire. The wire currently connected to the trickle solar panel and my Lifeline group 31 AGM battery is not only much thicker but is perhaps a dark grey or sun weathered black for reference. I will try to get a picture up I took the other day while examining my roof and getting ready to clean it. I need to get some calipers or a wire gauge measuring card. My guess is that the Eco-Worthy solar suitcase wire is #16 and the the wire on the roof is #10. The Eco-Worthy wire reminds me of regular speaker wire for a home entertainment speaker while the wire on my roof looks a little like my guitar amp cord. I also wonder if the thickness of the roof wire is due to a protective cover and not the gauge?

I couldn't find a controller but I did find a mess of wires behind the driver's seat.

The 100w suitcase does a decent job of making up for the basic daily use like the various detectors and fridge IF I have perfect conditions for about 5-7 hours. That's almost impossible in most campgrounds, even here in very sunny SoCal.

We use our RV like a glorified tent. We use flashlights and heavy sleeping bags. We frequently dry camp for a week at a time. We don't use a TV or other electeonics. We mostly cook outside on a portable stove. I converted all of my lights from florescent tubes to LEDs. The biggest daily drain we have is the furnace blower because we dry camp at high elevations frequently and the temps drop to the 30's and 40's sometimes even in "summer months." We also use the water heater and water pump a few times a day.

I hope to be able to add two more 100w panels (Don't have a ton of room on the roof) on the roof. My goal is to get about 300 watts combined with the 100w suitcase I have to re-charge my 31 AGM daily to compensate for my very basic energy audit above. The biggest battery drain will likely come from the furnace blower. So basically I need the sun to give me about 50ah a day. Am I making sense?

[Almot]

"one or two 100W panels".

Check the wire gauge. #10 should be enough for two, maybe even #12 for two, maybe #12 for one panel only. Depends on the wire length to controller.

Check the specs of your old controller to decide whether you need a new one. Controller has to be at least 7A rated for 100W, 12-15A for 200W.

In your case controller doesn't have to be right next to battery. You should be fine with controller to battery cable 7-10 ft long, considering your low amps.

[Jbrowland]

Really great answers! Thanks!

[CA_Traveler]

Controllers won't hold up with moisture or flooded battery fumes. So install inside or a storage area.

[Golden_HVAC]

You should be using #10 wire at least, while the trickle charger is only going to carry 1 amp and might be #16 wire or smaller. Also because it only produced about 1 amp under noon sun, there is no need for a controller, as it can not overcharge even a full battery.

Your RV will consume about 35 amp hours daily, (to run just the CO detector, propane detector and refrigerator) (35 AH) and that is about what one of my 120 watt solar panels will make in one day. It is rated at 7 amps per hour, X 5 peak hours.

So I would recommend a pair of 140 watt solar panels, and a PWM controller. It is no longer worth it to buy a much more expensive MPPT controller, solar panels are much less expensive now. I have a Solarboost 50 controller, that cost something like $400 back in 1999. My 120 watt panels where $429 back then. Now a 140 watt panel is about $229, while a 20 amp PWM controller is $15.

SunElec.com look for 12 volt panels with aluminum frames.

#10 wire is grey and at Home Depot. It is UV rated and rated to sit in the rain or underground direct burial.

Mounts are also at Home Depot, buy 3' of 2" angle aluminum. Cut into 6" long length, drill a 5/16" hole for a 1/4-20 bolt and nylonut (the difficult to remove ones) to the solar panel frame, and 3 each 3/16" holes for #10 screws into the RV roof.

I ran my wire down the refrigerator roof vent, to the controller, to the battery. #10 wire is good for 2 panels putting out about 15 amps total. Run a second wire if you have more panels.

You can hook up as many 20 amp controllers as you like. Put one on for each pair of 140 watt panels.

You can search my posts, and refine it with solar, and find pictures of my system, as well as detailed instructions. I placed the panel mounts under the panels, so they do not have direct rain on them, and makes removing the panel mounts more difficult without removing the panels first, for theft resistance.

While 250 watts is a great start, you might find that upgrading further will allow you to camp longer without need to run the generator. I have 400 watts, and could watch TV for hours each night, and charge the next day.

Good luck,

Fred.

[Shadow_Catcher]

You will need a controller and larger wire (voltage drop).

[MrWizard]

you will need to install a NEW charge controller
and possibly new wire

that small panel most likely only has a 'blocking diode' no controller