This is Part One of a solar series written by Scott Fox for The Virtual Campground. Scott goes through the design, installation and testing of his solar system for you. Scott and his wife are full time travelers. They own a motorhome and last year covered 8352 miles and nine big western states. They love National and State Parks and National Forests. In the west, they love to take advantage of all the Federal BLM land that is available for low and no cost camping. Scott can be reached at f[email protected] with questions and comments!
First, let’s clear up some common battery misconceptions
Boondocking or dry-camping refers to camping in an RV without electrical, water and sewer hookups. For this , we bring our own generator and solar panels to make electricity.
The purpose for a generator, in our minds, is to recharge our batteries. The purpose of solar is to recharge our batteries without running the generator. In our case, the generator can also run our air conditioner, but given the size restriction of our solar/battery system we have no hope of air conditioning from our battery.
That’s not to say it’s impossible; it’s just not for the design of our system. Our solar installation is nothing more than a battery charger.
Other designs have other capacities that I don’t care about. I did not rewire the RV to use more energy from the battery. If you don’t need a battery charger, you do not need solar. Also, my system is not just to keep a battery charged.
The purpose of my system is to recharge my battery.
Important language note: The word battery is commonly misused. Battery literally means a group of cells, combined together to provide a common energy source. Our RV has two boxes (most people would call them batteries), each containing lead-acid cells. The boxes are wired together to form a battery.
We have a 12 volt battery, but it should be noted that the happy range of our battery is about 13 volts. 12 volt battery is a misleading name, you see. Another need-to-know is that a lead-acid battery loves to be full and doesn’t give up its charge willingly.
In lectures by experts in the RV industry last January I heard things like:
- You cannot recharge a battery using solar
- You can’t use your generator to recharge your RV battery
Now don’t panic. I am glad to report these statements are not true. While both phrases have a nugget of truth in them, they are really only true if your system is incorrectly sized, installed, or operated.
The following will outline how I accomplished a fully functional solar RV battery charger using parts from the local hardware store and used, yet still very capable, solar panels.
It is not the cheapest system but also not the biggest and most expensive system. We aimed for and accomplished “just right.”
How to know what just right means in your solar system design
The answer is you have to measure.
There’s a lot of misinformation out there on the internet. Many experts will tell you to look at each electrical item in the RV in order to identify the amount of electricity they use. Then with a little estimating of how long each item will be used, you should do a little math to determine how many solar panels you need.
Instead of guessing or estimating my consumption, I decided to measure electrical consumption for almost a year of dry camping. I wanted to have enough real life data points to then design a solar battery charger that would work well.
So starting in the spring of 2018, we monitored battery use carefully. The purpose was twofold:
- So we wouldn’t kill our battery prematurely.
- So we’d have enough data to determine the amount of solar we’d need to live without electric hookups or a generator.
Running our generator to recharge our battery is noisy. We quickly discovered having the generator was not for the luxury of full electric power, but rather to recharge the battery fully.
When you turn on a flashlight, you create a circuit. The components are:
- the energy source, typically a battery
- the bulb
- a conductor that connects the source to the load.
If any of these three items are missing, you don’t have a flash light. You have a paper weight.
The word circuit describes the path the electricity takes from the battery, through the load, and then back to the battery. This is called a closed circuit.
If the circuit is not connected, typically by turning off the switch, then the path is not intact and the light will not work. This is called an open circuit.
Work is accomplished by passing the electrical energy through the load. This flow of electrons is called amps. Think of amps like the current in a river; the wider and deeper the river, the more water it moves. More water moving, more work can be accomplished.
Volts describe potential energy. Volts alone don’t do anything; they just sit there, like water behind a dam. When the valves are open (you turn on a switch) the flow starts and that’s when you can measure flow to determine how much energy.
If the reservoir (battery) is empty it does not matter if you turn on the switch. Everyone knows that by using the flashlight you deplete the battery, and if you leave it on too long, the battery will be “empty” and will then need recharged or replaced.
How to measure electrical use
So to measure my electrical use, I needed to measure how many amps flowed through my RV components. You do this with a device called an amp meter.
My amp meter not only had to measure the flow, but also add up the flow over time. I measured every amp I used from my battery for almost a year. And then also measured every amp I put back into the battery.
I arrived at some very interesting results.
Now, almost all RVs come with voltmeters and 99% of the volt meters are only used to give RV owners a warm fuzzy feeling. In other words, they are not a good reflection of battery charge or health. Here’s why:
Most owners just push the button and then feel good that the number is pleasantly high. Instead, you should disconnect the battery for a few hours before measuring the voltage.
If you push the button on the voltmeter when you are charging you will read the volts charging the battery, not what is stored in the battery. So what you’re getting is just a surface charge reading, not the true state of charge of the battery.
The only way to measure the true potential energy in a battery is to disconnect it and let it rest for a prolonged period prior to using the voltmeter.
This is also true when working with a flooded lead acid battery and a hydrometer.
Next up – how to quit wasting electricity.
Quit wasting electricity
When you look inside your RV, you’ll see a switch labeled “battery on/off.”
This doesn’t exactly turn off your battery. Instead, there is a relay that connects the major loads from the battery to the 12 volt circuits when energized to the “on” position. And just where does the energy come from to operate this relay?
You guessed it! Your battery.
I installed a true disconnect switch right after I purchased the RV.
Many small loads are connected to the battery even with the relay in the off position. This can slowly drain your battery and waste a lot of electricity–especially in terms of boondocking!
Even with my battery switch off, I still use .9 to 1 amp per hour. This is called the parasitic draw and is used to run things like the smoke detector.
If I were to leave the RV without a true battery disconnect, my battery would be completely dead in 300 hours. This is only 12.5 days! Many an RV owner has come back to the RV only to find the battery dead because it was never truly turned off. And the fact is that your RV can never truly be “off.”
Last year, I logged every amp our RV used while boondocking.
Our average summer overnight use is 66 amps. And in the winter that number changed to 96 amps.The difference is our winter heater and fan.
My measured loads (all amps per hour of operation):
- Refrigerator with parasitic draw is 1.5 amps. With the anti -frost function on, it becomes more than 3 amps. Unfortunately, I do not have control of the anti-frost function. (Key note, this is a propane absorption fridge. If you have an RV with an electric only “residential” refrigerator my calculations won’t work for you)
- Furnace fan consumes 4 amps when on. Heat between sunset and bedtime are about 40 amps give or take
- LED lights are .5 per “bulb”
- Inverter shows 4.8 amps even if doing nothing
- Overnight electrical consumption includes a CPAP machine (4.4 amps)
- 13 amps per hour in the summer
- 18 amps per hour in the winter
- 6-8 amps while sleeping
- 250-300 amps per day
Next up – how we store energy.
Our battery is AGM (lead-acid, absorbed gas mat) and holds 300 amps.
A safe bet is that you can use 150 amps without undue wear leading to premature battery failure. Further, the deeper you discharge a battery, the longer it takes to recharge it.
What I didn’t know is that by using the generator to charge the battery “back to full,” I was depriving the battery of a (very important life-extending) recharging stage called float charging. This is also sometimes trickle charging.
A lead-acid battery that is denied a trickle charge is not really full. I didn’t want the expense of replacing a dead battery, nor did I want to live as if electricity wasn’t available so we ran the generator and watched what we used and how to replace it.
(Much more on batteries later…)
We decided to shoot for 250 amps per day covered by solar to recharge the battery and run our RV during the day and evening hours. We calculated this at another 50 amps per day.
Unfortunately it seems that every solar expert calculates collector sizing on a different method. I have not found one sizing method that met my expectations. The other factor of human nature is that we want more. Sizing the panels was a series of guesses. We ended up getting the answer right, but one of the calculations was based on a non scientific theory — no one has yet to complain that the TV was too big or that they had too much solar.
Next up — how much solar to make 300 amps of electricity on a short winter day.
How much solar did we really need?
Given a good 6 hours of sunshine in the winter, and assuming our batteries were not overly discharged, we concluded that 600-700 watts of solar would be enough to recharge our battery and live daily.
We also calculated that you can’t always count on the sun. You know, clouds are a real thing. Another consideration we had to think about is after install, our panels are only tilted enough to make the water run-off, about 5 degrees from flat.
We don’t tilt the panels nor do we use any portable panels that could be pointed perpendicular to the sun.
For these reasons, we installed 700 watts of solar.
Now this is something we didn’t fully understand at the beginning:
Most of the potential energy that your panels produce will not be used. That statement applies to daily use and annual use. Most of the capability of the 700 watts is wasted – never used.
Initially, I was heartbroken but it is the nature of the beast.
And here’s why…
The problem is not with the panels, but in how a battery likes to be treated. If I were to draw an analogy to the human body I would describe it like this:
The battery is the stomach. A plate of food is the panels. It doesn’t matter how big the plate of food is the stomach can only take so much at a time. And when you are really hungry, you can eat food faster. But after a short while, you need to slow down and consume at a more reasonable rate.
The battery operates almost exactly like this. Feed it fast early in the morning then taper off as the day goes on.
A non-tracking solar panel system is most capable at noon. Unfortunately, the battery wants to be fed the fastest early in the day.
A solar panel that you can point at the sun, early in the day is much more effective than my flat mounted panel.
This is one of the reasons you need a larger-than-necessary solar panel system.
Next up – how to squeeze enough solar out of a right sized system without too much waste.
Getting the most out of your system without waste
The key item in the system is a good charge controller. Among other things, it feeds the battery the right amount at the right time.
Early in the charge cycle it transfers most available energy into the battery. This is called the bulk charge portion of the cycle.
Unfortunately when the panels aren’t aligned with the sun, they aren’t ready to put out their maximum. This means the bulk portion of the charge takes longer.
My panels and my charge controller could potentially kick out 50 amps in one hour. Reality not being the same thing as theory, it will never do so because
- the orientation of the panels
- the needs of the battery
For my design, I didn’t want panels that tracked or even tilt. I installed them almost flat on the roof. My design goal was to reduce my interaction with the system.
For reference, a panel that was (less than) half as big, if pointed at the sun and could track it, has the potential to put out an equal charge as my flat panel array.
Next up – how to install a just right system
Installing your RV solar system
This series is intentionally brief on the installation aspect of an RV solar system.
I think it is much easier than many people make it out to be, so don’t worry.
The three critical components are:
- The battery (this should already be installed and operable in your rig)
- The panels on the roof (simply attach how you’d like and so that they stay and won’t blow away as you drive down the road)
- A controller that will use the energy to charge the battery the way it wants to be treated and wires
Throw in a few switches, fuses and possibly a circuit breaker, tie it all together, and the collectors will charge the battery.
The difference between a solar system that will maintain a battery charge and one that can recharge a battery is size.
And a note on MPPT vs. PWM: If you have a big system that recharges batteries, MPPT is the answer. It is more expensive, but will save you lots of money on wire costs. If you desire a system to maintain a charge, PWM is fine.