Putting Salvaged Solar System Components to Use

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Sep 4, 2018
This is my first post here, so let me start off by saying ... WOW! There is a ton of information here! Then let me apologize for the length of my first post. I have a couple very specific questions, but to answer them I think y'all will need some detailed information.

With out further ado ? let me begin.

My goals: To be able to effectively recharge the batteries during week long stays in the winter. If I can do this successfully, I shouldn't have any problems meeting my needs during the shorter stays in the summer/early fall. I am very frugal with my electricity usage.

My travel trailer is a 2002 K-Z Sportsmen 23ft. All interior lights have been upgraded to LED lights. This TT primarily stays at our hunting camp site. There is no water, sewer or electric. Typical stays are 2-3 nights during the spring/summer/early fall, with a couple weekly stays in the late fall/early winter.

Electrical usage:
- Lighting 4-5 hours of lighting during the longer stays. Typically this is 6 LEDs @ 3.5w each.
- Refrigerator: Propane powered, minimal electrical usage
- Furnace: Propane, but during the winter stays, the furnace runs for approximately 10hrs in a 24hr time period.
- Recharging various phones, and laptops
- Radio: A couple hours a day. Just the radio, no CD usage.
- I don't use the AC, microwave, water pump, water heater or any other electrical devices. Cooking and washing is done by heating creek water over the campfire, or over the propane stove. Drinking water is bottled and 5 gallon containers filled at home. The TT, at this point, is basically a dry box that we sleep in at night. The rest of the time we are outside, even during the late fall/early winter season.

Current Battery:
- Duralast Marine & RV Deep Cycle
- Model 24MD-DL
- Marine Cranking Amps 685
- Cycles: 65
- Reserve Capacity: 140

Past Performance:
- I have been keeping the battery at the house and on a trickle charge.
- At camp I've been using jumper cables to recharge the RV battery from my truck.
- During the winter stays, I charge it for 2 hours every other day.
- Once last season my low voltage alarm went off, it was an extremely cold night, and the furnace had been running nearly constantly.

The solar components:

I recently salvaged 9 solar panels and 5 MPPT controllers.

The panels are SolarWorld Sunmodule SW 140 Poly R6A/D. They all were made in Q1 of 2014, so they are just over 5 years old. Info from the MFGs tag is below:
- Rated Max Power [W] 140
- Open Circuit Voltage [V] 22.1
- Rated Voltage [V] 18.0
- Short Circuit Current [ I ] 8.35
- Rated Current [ I ] 7.85

The controllers are Victron Energy Blue Solar Charger MPPT 74|15. They are identical to this one on Amazon, and are also 5+ years old: https://www.amazon.com/Victron-BlueSolar-MPPT-Charge-Controller/dp/B00U3MK0CI/ref=asc_df_B00U3MK0CI/?tag=hyprod-20&linkCode=df0&hvadid=167141218295&hvpos=1o2&hvnetw=g&hvrand=14592674442494067544&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9009155&hvtargid=pla-305154557039&psc=1

With all that out of the way ? here are the things I don?t know and would appreciate any advice or suggestions.

Note: The panels will need to be mobile, I will not be able to affix them to the TT itself as it will be in a very shady spot. I plan to create a frame to get them into the sunlight so they are not just lying on the ground. If I go with a 2 battery system, the panels will be less than 15ft from the TT, if I go with a 4 battery system, the batteries will be about 40 feet from the panels.

Note 2: I?m pretty handy. This isn?t a project that intimidates me.

1. What kind of budget friendly batteries would be best suited for my application and usage?

2. Would I be looking at a 2 or 4 battery bank? I have a truck bed tool box mounted to the back of my TT, so I have room for a 4 (or more!) battery system, although it will be further from the current battery location. My initial thought was 2, but if 4 is a better solution, I?m open to that too.

3. How many of the solar panels would meet my goal of keeping my batteries charged? My initial off-the-top-of-my-head guess was to use 4.

4. If I use 4, or more, panels, are the controllers I have sufficient? Is it possible, or desirable, to wire 2 panels to 1 controller, then wire the controllers to the batteries? Should I wire each panel to a separate controller, then into the battery bank?

Thanks in advance for all tips and suggestions, and again, sorry for the length of the post. I wanted to include as much info as possible.
Hi Randy. Welcome to the RVForum. RV solar isn't very complicated, but it helps to size your system for your environment, your actual consumption, your battery capacity and reality. You seem to have a pretty good understanding of your consumption, so let's look at some other factors.

Switching to LED lighting was a good move, and while your chargers and other devices aren't big consumers individually, they do add up. Your furnace is the big consumer - pulling 8 to 10 amps every hour it runs, which is upwards of 100 amps. The battery you mentioned has an 85 AH capacity, so I'm surprised it's lasting as long as it is. It won't live long with that kind of use though.

Those marine "Deep Cycle" batteries... aren't. A true deep cycle battery is much heavier duty, and is designed to survive repeated deep discharges (up to about 50%) but only if they're fully recharged soon thereafter and maintained properly. Some examples of true deep cycle batteries are, 6 volt 210 to 220 AH golf cart batteries, and many AGM batteries, but they're considerably more expensive. When it comes to RV batteries, you get what you pay for. (Just take care of them.)

In your situation, the first thing I'd do is upgrade to a true deep cycle battery bank, with a minimum capacity of 300 AHs - 400 would be better. When you're running your furnace 10 hours a day, charging cell phones, running the fridge etc, you're probably consuming about 120 to 150 amps a day, or more. That's going to draw a 300 AH battery bank down to 50 or 60% SOC, which is considered high consumption for that size battery bank.

Under ideal conditions (which rarely exists) the panels you mentioned could theoretically generate 150 amps a day that time of year, but it's unlikely. They'd have to be kept clean and facing toward the sun at all times. (Someone's going to have to keep moving them.) they can't be shaded at all, the wire run and wire gauge would have to be adequate and it would have to be a cloudless day. It's surprising how much impact just a few clouds have on panel output.

In all likelihood, you'd probably see about 50 to 75 amps a day out of those panels, best case - much less when it's cloudy - and that's if they're kept facing toward the sun. That's not bad, but it's not going to be enough to fully recharge a 300 AH battery bank that's been discharged 50%. It's more than enough to recharge your current battery though.

IMO, there's no reason in your situation to buy/use more than one solar controller. Get a single MPPT controller that can handle the amp output of your panels, then wire the two pairs of panels together in series, or all four in series if you're sure none of them will be shaded.

You might consider buying two other things - a battery monitor (like a Trimetric 2030RV or a Victron) and a catalytic heater. Catalytic heaters are indoor safe (leave a vent cracked) they consume no electricity and compared to a typical RV furnace, they only sip propane. A battery monitor will accurately display your battery bank's condition (in percentage) so there's no guessing as to the SOC. Boondocking without one is akin to driving without a fuel gauge.

here is some design info to digest..


ditto on the battery, the so called marine deep cycle are anything but... you will be much better off even with el cheapo golf cart
batteries than a marine battery..

an estimate from your data suggests a daily w/hr of approx..

lights:    3.5*6*5 hrs              = 105 w/hr
Propane fridge approx 18 W * 12 hrs = 216 w/hr
Furnace approx 60 w * 12 hrs  = 720 w/hr
radio approx 24 w * 2 = 48 w/hr
phone/laptop an unknown,  i'll ballpark it as 100 w for 1 hrs so = 100 w/hr

total is approx = 1189 w/hrs
using method in my post you need approx 500 to 600 watts of panels
and a minimum of 400 a/hr of battery.

you have 9 panels and 5 controllers.. if you were doing this as a new design then you would only need one controller.
as you have existing gear, we'll try and fit your needs to them..

biggest issue here are the controllers.. with 12 volts you have a max charge current of 15 amps and therefore ( 12 V * 15 A ) = 180 Watts..
way below your needs..  with this controller and 12 Volts you can at best only use 2 panels, in full sun the controllers will clip at 15 amps so you will waste
( 140 * 2 ) - ( 12 * 15 ) = 100 watts  so to get your 500 watt need you will need 3 controllers and 6 panels..
each array will be 2 panels in series..

you will also have another 2 panel array for your "mobile" setup..

as an example, if you used a single controller with 4 panels then you would get 140*4 = 560 watts and at 12 V you would need a single 50 amp controller.

you can achieve your goal with your surplus gear but it's messy and you need 8 panels instead of 4

now, take a look at the watt hour figures again.. the furnace is the power hog here, so what about using a ceramic heater as Kevin suggests ?

now your daily w/hr is down to approx 500 w/hr

you could now use 2 panels in series with one controller on the roof and a second pair with one controller as your mobile panels..

four panels, two controllers and two golf cart batteries ( 225 a/h ) that's a much better fit..

even better would be to sell four of those controllers and purchase a victron 100/50 and have 4 panels on the roof in a 2S2P format.
then use the other controller with 2 panels in series as your "mobile" array..

Welcome to the RV Forum Randy

I won't try and add to the solar side of your questions because I'm still in the learning side of solar.

But on the batteries side ...

We went with 2 of the crown golf cart batteries and have not been disappointed.


You didn't ask but it doesn't hurt to have a backup plan for those cloudy rainy days. A inverter/Generator would come in handy. We picked up this one and it will run the entire TT as needed. I use it to run the AC, microwave and water heater. Maybe 3 - 4 hours a day. While it's running it also runs the on board charger for the batteries. Note it's pretty quiet in eco mode and not bad at all at full power. This would be a better option than charging your battery with jumper cables


A pair of 6 volt batteries will give you the most bang for the buck, especially if you go with lower cost units.

Consider your first set of batteries as your training wheels, if you take proper care of them they'll last a long time. But if you goof along the way, it's easier to replace a pair of $90-100 batteries than ones that are more expensive.

Sams Club, Costco and Batteries+Bulbs have good deals on Duracell brand golf cart batteries made by West Penn.

Thank you all for your replies and thank you for the welcome!

Lots of good stuff here! Frankly my mind is racing with ideas and I've twice had to erase my wall o'text. I'll try to condense to the main points though.

The furnace is the power hog, that's for sure. I had planned on a propane-only heater as a back up, but if I can cut the power usage by using it as primary, I'm all for that. That suggestion is pure gold, so thank you very much!

The TT is parked in a very shady spot and doesn't receive much direct sunlight, and I unfortunately don't have an option of parking it in a more open spot. Any panels I use will need to be hauled out, set up and remain there for the duration of the stay. The distance to the direct sunlight spot is about 10ft from the hitch of the TT. Essentially we have a 1 lane road (running east to west) into camp with the campers set off the road in the trees. Cutting a large quantity of trees down isn't an option as this land is leased from a timber company.

Typically we don't spend much time actually in the camp. Breakfast in the morning, then out on the ATVs most of the day, then back in late afternoon. My thought was to set 4 panels in an A configuration with 2 panels facing east, and 2 facing west. I've been looking into systems that will track the sun, but right now haven't decided on a way to make that work in my situation.

I'm absolutely into selling controllers and panels to offset purchasing better equipment. I have 40 Interstate DCM0035 batteries that should bring in about $150, if I can get $150 for 3 controllers, and $200-$250 for 5 panels, that would provide enough return for a better controller and batteries that would give me 500ah. I'm looking at the Interstate 2400S batteries right now. The Crown batteries that were suggested look very good, but with the quantity of recyclable batteries I have, I'll most likely go with the Interstate for now.

My plan, at this point, is to purchase 2 6v batteries, use 4 panels, and 2 controllers. That will get me a good starting point and allow me to upgrade to a single controller once the other equipment is sold. We have a generator (which I absolutely despise using) that stays at camp, so I can augment the solar charging with the generator.

I purchased a cheap battery monitor before I really got into this, https://www.amazon.com/gp/product/B01JOUZELG/ref=ppx_yo_dt_b_asin_title_o08_s00?ie=UTF8&psc=1. I've already installed this, and getting a $300 purchase past the Household CFO is going to be a bit tricky. For now, I think it will tell me what I need to know, but upgrading to a better one is definitely in the cards.

I really appreciate all the suggestions, without them I would have probably made some partially educated guesses, spent some money, been disappointed with the results and kicked myself in the backside for not looking for suggestions in the first place.

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