Trying to figure if a solar upgrade is practical

[kevinb]

Greetings everyone. I recently bought a used travel trailer. It has and old solar system: two 50W panels, with a GoPower 25A PWM charge controller. It has two 12v 125AH batteries, wired in parallel. They are at least a couple years old, and then I accidentally ran them down below 50% and surely damaged them. (I know, I know...)

I am considering an upgrade to the system and replacing the batteries.

The main thing I want to accomplish is to generate enough power to recharge after running the furnace all night. I've searched for any kind of rating plate, etc., but can't find squat. But I know that I can run the furnace for two nights on a fully charged battery without trouble. (Before I killed them anyway.) Maybe three nights, depending on how cold it is.

Very roughly then, I figure the furnace could be drawing as much as 1080W a day. (3000W of battery storage * 50% = 1500W, divided by two nights = 750W per day. Add about 330W / day recharge from the existing system, which is probably optimistic, equals 1080W).

If that's the case, it's not practical / cost effective to upgrade the solar system, and I should just run a generator instead.

So my first question is how I can more accurately figure out how much power the furnace is drawing?

Thanks everyone.

[NCmountainsOffgrid]

first, just because you drew the battery down below 50% a single time is not going to necessarily create a long term issue - if you are normally allowing it to charge during the day, and discharge during the night, while the furnace blower is running periodically, you should be just fine with two 12v batteries in parallel, as long as you monitor the draw, then use the generator as needed.
The solar is a different story, as two 50w panels may or may not be enough to recharge the battery, even with many hours of full sun, if you are also using other 12v draws at the same time, even minor ones.
If you are simply concerned with the furnace blower, then you'll probably be fine to stick with what you have, and run the generator as needed. The fuel costs for the short time for the generator would easily offset the upfront cost of additional solar, and/or different batteries, simply to give you a single more hour of off-grid time.

[ewarnerusa]

Hello Kevin,
The furnace is the biggest draw on our system as well. We run 2x6V deep cycle golf cart batteries in series (~200Ah capacity at 12V) and 280 watts of solar and never had battery power be an issue for our long weekend warrior style of camping. My opinion is that solar is absolutely worth the investment for boondock camping, but also explore demand-side savings like switching over to LED lighting and using propane for fridge and heat.

I think your 100 watts of panels on a 125 Ah setup is not enough to do much while camper is in use. They would do great for maintaining the batteries while in storage; no plugging in to shore power needed. I'd aim to double the solar wattage at a minimum to provide any charging while in use on a camp trip. I feel like the 280 watts we have is sufficient, but we do experience declining state of charge throughout a long weekend. Never enough to where I feel a need to use alternate charging to get through the long weekend though.

[JSchnee21]

Well, if it were me, I'd buy a nice quiet, reliable inverter generator from Honda or Yamaha. Very fuel efficient and works day or night, rain or shine. Having done a lot of camping in my day, even with air conditioning, I never park in direct sunlight unless I can help it. So I've never understood the appeal / interest in solar solutions for RV's. There's too little surface area, too much vibration, and too much shade.

Assuming your furnace is propane, if you really don't want to run the generator at night, scale your battery bank and/or inverter (if it's not DC), to carry you through the night with a minimal DOD. Then charge during the day with your genny or shorepower.

If you have lots of storage space in your RV/truck, and a desire to be handy, you could build a small ground mount solar rig out of 2x4's and commodity residential solar panels (perhaps 3 to 5 ~330W) panels. Wheel it out into the sun, and perhaps when the new islanding Enphase Micro's come out you can use it to charge the battery bank in your RV. Otherwise you'd have to run the DC lines to a charge controller proximal to your battery bank.

As ewarnerusa suggested, you could mount 2 to 4 panels on the roof of your RV, but the collection efficiency of horizontal mounts is very poor, the panels get dirty very quickly, and you have to park out in a field to get any appreciable sun shine. But if you are still interested, by all means use commodity residential solar panels (60,72, or 96 cell) -- not the silly 12V or 24V ones the camping stores sell.

[ewarnerusa]

kevinb,
The RV furnace fan draws around 10A.

[kevinb]

Thanks for the feedback guys. You already answered one of my questions, which was: why would anyone use the 12v / 24v 100W panels - they seem madly expensive compared to just a plain old panel you'd put on your house.

Here's the panels I'm looking at: https://www.wholesalesolar.com/19774...ly-solar-panel

My thought was to get a couple of those 335W panels and mount them to the roof, maybe on a bracket that you can tilt up while you're parked. (See the diagram attached) If the furnace blower is drawing 10A @ 12v, it sounds like I'd need at least 3 panels.

In the summer I will probably try to camp in the shade, but in the winter (when the furnace is important), I try to park in full sun. In the summer I expect the load would be much less, since the furnace would not be needed much, if at all.

planned_solar_systemtt2.jpg

[SunEagle]

Thanks for the feedback guys. You already answered one of my questions, which was: why would anyone use the 12v / 24v 100W panels - they seem madly expensive compared to just a plain old panel you'd put on your house.

Here's the panels I'm looking at: https://www.wholesalesolar.com/19774...ly-solar-panel

My thought was to get a couple of those 335W panels and mount them to the roof, maybe on a bracket that you can tilt up while you're parked. (See the diagram attached) If the furnace blower is drawing 10A @ 12v, it sounds like I'd need at least 3 panels.

In the summer I will probably try to camp in the shade, but in the winter (when the furnace is important), I try to park in full sun. In the summer I expect the load would be much less, since the furnace would not be needed much, if at all.

planned_solar_systemtt2.jpg

With over 600 watts of panels you will need a 60amp MPPT CC for a 12v battery system. (670w / 12v = 55.8amps) That 30amp CC is too small for 670watts.

[kevinb]

Thank you Sun Eagle. I guess I misunderstood the ratings on the panels and controller. I was just adding up the output amps on the panels. Thank you for saving me a stupid mistake.

OK, scratch that design.

Yowza! It seems like there is a big jump in price to a 60amp. Renogy sells a 40amp Rover MPPT controller for about $160, then it's $499 for a 60amp. Why the big price difference? Is the 40amp MPPT junk / fake?

So if I get a 40amp, that limits me to 480W, probably less to stay safe?

Here's a more likely, more affordable setup. Which may not be enough power to do what I want. I'll have to do some more searching for affordable ~200W panels.

planned_solar_systemtt.jpg

[Mike90250]

Your fuse values DO NOT match the wire gauge you have listed. 10ga wire needs a 30A fuse.. When you figure out what charge controller you are getting
( 600w PV at 13V = 46A of charge ) use the wire that is needed for those amps, and then select the right fuse.

If you figure things well, and use the right wire, you can manage with a Dual MRBF Fuse block mounted on the battery. One fuse for the Solar/Isolater and one small fuse for the 12V loads
Bolts right onto the battery terminal
https://www.bluesea.com/products/215...k_-_30_to_300A Duar MRBF fuse.jpg

[Graybyrd]

Our 9.5 ft cabover truck camper has a single deep cycle 12v battery (there is only one single-battery compartment). I installed a PWM 30a charge controller, with a 30-ft 10-ga extension cable, that connects to either/both two Renogy 100w panels that I carry to prop up beside the camper in the nearest sunny patch. We carry the two panels inside when traveling. The solar connectors work easily for single, parallel, or series connection to the panels as needed. We just installed a diesel air heater, that draws momentary 10a glow-plug ignition, and 0.6a fan draw for constant heating. All lights are LED. Refrigerator and stove are propane; minimal 12vdc for refrigerator ignition. Backup propane RV furnace draws 12v 4a if needed, but rarely used. 700w inverter mounted for incidental, infrequent uses. 90w 12vdc laptop power recharger used intermittently.

Just received a "MakeBlueSky" MPPT 40a unit to replace HQST PWM 30a, due to excessive overcast days/frequent shade conditions here in Island/Skagit County WA location. Intent is to maximize charge opportunity for that single Group 27 Interstate Deep Cycle battery. Can't think of any other improvements, short of rigging more battery space (highly impractical).

[kevinb]

Thank you Mike. I stole that drawing from one of the FAQs on this site, and I didn't update the fuse sizes. Good to know 30A is what I need.

Unfortunately, the more I look at it, upgrading the solar maybe too expensive to be worth it.