Mobile Cabin Concepts and Issues
Climate Extremes Increased by Driving Around
Other Issues Unique to a Mobile Cabin
Separating the Vehicle and Cabin Electrical Systems
Battery Freezing and Low Temperature Performance
Conversion to Solar – An Example
There are 3 related online documents for Mobile Cabins:
Getting Started Basics for Your Mobile Cabin – provides an overview and covers the fundamentals www.skilledwright.com/MobileCabinBasics.htm
Mobile Cabin Concepts and Issues – provides in-depth discussion and analysis of the concepts behind designing and living in mobile cabins www.skilledwright.com/MobileCabinConcepts.htm
Resources for Creating and Using Your Mobile Cabin – provides links to suppliers and more information to support building and living in mobile cabins www.skilledwright.com/MobileCabinResources.htm
The whole point to Mobile Cabins is to free yourself to travel and stay in many different places, and sustain and grow your life and self-reliance while you do this.
Enabling you to create this freedom in your life is the purpose of the information on this page. Most of this information is applicable to all Mobile Cabins traveling almost anywhere in the world.
The home base for this project is near Seattle Washington USA and the expected typical travel range is west of the Rockies in North America during all seasons, so this is where the design requirements are focused. However since this area covers almost all climate types, most people will find this information applies to their plans.
Check out the page Sustainable Self-Reliance, Mobile Cabins, and Homesteading for ideas and resources that will support increasing your self-reliance while living in a Mobile Cabin.
If you haven’t already read Getting Started Basics for Your Mobile Cabin please do so now. The background supplied in the basics will enable you to more effectively use the information in this page.
There is a tendency for the solar energy industry to adapt packaged systems to existing life styles with most of the household change focused on more efficient appliances. Significant savings in the cost of solar systems can be achieved by fundamentally changing the lifestyle it is required to support. Due to the limitations of space, weight capacity, and budget for Mobile Cabins, there is a lot of incentive to reevaluate the lifestyle itself, especially since most Mobile Cabins aren’t expected to be lived in full time.
The focus below is on finding ways to sustain the key
aspects of a person’s life while minimizing the utility requirements.
Basically, the smaller and less expensive the Mobile Cabin is to build and
maintain, the more likely it is the owner will be able to travel for extended
periods. This in turn is balanced against being so frugal that it is so
limiting and exhausting to stay in the Mobile Cabin that the owner doesn’t use
it. Too many people go back home and park the Mobile Cabin so they can recuperate
and get on with life because their mobile cabin wasn’t adequate enough to live
in for extended periods.
The goal is to walk a fine line between freeing the owner to travel in a sustainable way and planning for so extensive a Mobile Cabin that they can’t afford to travel or maintain it.
A couple of acronyms used commonly in this document are vac (volts alternating current) and vdc (volts direct current). The standard household electricity is always alternating current (AC). Even in other countries it is just a slightly different flavor of AC because AC can be transmitted much further than direct current (DC) without a major power loss.
However batteries are always DC. Since it takes precious power to convert DC into AC, small solar systems use DC appliances to avoid having to produce AC. Small means both small use of power and small distances between system components. Small distances are 10 to 20 feet. Even in a 40 foot square house, by the time the wire runs up, over, down, and around, it is easy to get wire runs approaching 100 feet. This won’t work for 12 vdc because too much power is lost in these distances and using bigger wires to avoid power loss is too expensive. This is the reason why bigger solar or wind generating systems run at 24 vdc or 48 vdc, and then at the residence convert to the standard household power of 120 vac.
Also it is much harder to electrocute yourself on 12 vdc than on 120 vac. It is very easy to electrocute yourself on 240 vac, which is why only large consumers of power like full sized water heaters and stoves are wired for 240 vac.
So for all of these reasons 12 vdc works just fine for a small Mobile Cabin and is the most efficient choice in the sense of least power lost between the solar panels and appliances, and least number of expensive devices running continuously in the power system.
In the jargon of solar systems, the big goal is to “get off the grid” or avoid ever having to connect to the “grid” in the first place. The grid is the network of power lines and transmission equipment that cover much of industrialized areas in the world. While it is handy to be able to just plug into the grid, it is unreliable, unsustainable, expensive, and not available everywhere.
It is a huge step in self-reliance to be able to free yourself from the politics and economics of the grid and generate your own power, wherever and whenever you need it.
One overriding concern that affected my design concerns was the range of climates and seasons that the Mobile Cabin and its systems would be likely to function in since I tend to travel to many places at any time of the year. I’m just as likely to drive north for the winter and drive south for the summer as the other way around. I do poorly in the very cold and very hot. However there is nothing like the crystal clarity when the sun comes out after a major snowstorm and there is nothing like the deep purple of the summer sunset over the desert. So between Polartec and loose cotton clothing I am finding better ways to deal with extreme climates. The next step is to upgrade the Mobile Cabin.
Issues related to climates and seasons
About the only climates not covered in the regions I expect to travel in are humid summers and tropical rain forests. All electronic equipment is sensitive to humidity exceeding 95%, but solar systems can’t afford to supply the electrical power for dehumidification. This is one advantage of cool humidity – as soon as you heat the air, the humidity drops to acceptable levels.
The only 2 ways to dehumidify air is to run an electric air conditioner to cool the air, or an electric dehumidifier to dry the air. Household dehumidifiers use about 800 watts, but every watt turns into waste heat that heats the space that is already too warm. Actually they release more heat than that since as water vapor is condensed to water, quite a bit of heat is released – the same amount of heat it took to vaporize it in the first place.
So if you will be in warm humid areas, buy the most robust electrical equipment you can, and be prepared to replace it more often than the typical life cycle.
The components are going to be more subject to shock and vibration than in a stationary installation. The code for marine vehicles prohibits wire nuts; those twist connectors for wires because of the vibration issue. Only screwed wired connectors are acceptable. The same issues affect a Mobile Cabin.
Since normally the Mobile Cabin isn’t what the owner will live in for years and years, it is easier to get by with less because “it won’t be forever, you can always go back to living in a house”. This might not turn out to be true, but it works in your head.
If you find that the sun hasn’t come out for a week, or that it hasn’t been above freezing for a week, you can always drive to a better location, so you don’t have to be as prepared for the worst weather conditions other than being ready to drive out. I know of several hunters who had to go back after spring thaw to get their pickup because they waited too long to drive out in the fall. Also it took the US forest service and BLM several years to reopen all of the roads washed out by the last El Nino, so you also don’t want to be on the far side of dead end roads during record setting rain storms.
For weekend use, you can afford to use electricity faster than it charges because it can recharge during the week when you aren’t using it.
Small Mobile Cabins aren’t likely to have a built-in generator. The big class A RVs have a quiet diesel generator that automatically starts itself when the batteries get low, and turns itself off when they are fully recharged. These generators are big enough to even run air conditioners. What you do have is the vehicle engine that can provide some charge in a pinch. This isn’t very cost effective, but it can be a useful backup strategy.
If you have an occasional prolonged need for lots of electricity, you can just stop by a KOA campground for while. This is better than buying and hauling around the weight of excessive solar system capacity that you will hardly ever use. This is an advantage over non-Mobile Cabins which experience the flip side of the issue that batteries are very heavy and cost fuel to haul around. The non-Mobile Cabin is on its own power wise, but at least the batteries can sit on the ground once they are delivered.
Consider doing without plumbing to reduce cost, size, weight, and fuel usage. Between fresh water, gray water, and black water tanks, it is easy to be carrying 50 to 100 gallons of water. How big are a couple of 50 gallon barrels? This is how much space the water would be using. This much water (without tanks) would weigh 400-800 pounds. Think about how much fuel it would take to be always carrying around this much extra weight. If you are considering adding plumbing to an existing vehicle, and you haven’t done plumbing work before, go check out prices for sinks, faucets, and fittings. One fitting doesn’t cost much, but dozens do. A large part of the cost of a house is in the kitchen and bathrooms. Frequently miniature fixtures cost more because they are harder to make and less are sold.
Plumbing came with my travel trailer, but the annual cycle of drain and fill with antifreeze for winter, flush the antifreeze for spring was wearing me out. I had to do without running water for a late fall trip I made due to freezing temperatures, and it went well enough that I never put water in the tanks again. The vehicle ability to climb hills increased, engine overheating reduced, and fuel costs reduced.
Some RVs locate all the tanks and run all the water lines in heated space so they can be used in winter. Once the temperatures drop below freezing, these RVs must be heated 24 hours a day until the tanks and lines are drained and lines filled with antifreeze.
Use an airtight chamber pot for nighttime restroom needs, a plastic washtub for a sink, and a couple plastic jugs of water as a reservoir. I use a wide mouth water bottle for a chamber pot, and nobody notices anything odd when I pour it around the drip line of trees to fertilize them.
The most important first step is to separate the vehicle electrical system from the cabin electrical system. This will allow one system to fail without causing the other to fail. While it would be discouraging for the refrigerator to stop and the food to spoil, it would be much worse to be in a difficult situation, and not be able to start the vehicle because the stereo was played too long and ran the battery down.
Unless there is a way to charge the battery for the cabin system (other than running the vehicle engine) it might not be possible to separate the systems.
There are 2 good reasons to invest in a way to charge the cabin system:
Starting a vehicle requires a large surge of power for a short period, and rarely uses much of the stored charge.
On the other hand, a battery based electrical system uses a much smaller level of power for a much longer period and frequently uses a significant amount of the stored charge.
A single solar panel can be used to support quite a bit of small intermittent electrical use. A solar panel, used in conjunction with a battery charger running off 120 vac, provides flexibility. The battery charger can be used to bring the battery up to full charge whenever this is available. During prolonged bad weather, you can make a point of stopping by places where you can plug in and charge up.
To allow my larger power tools to be used, I’m going to install an inverter on my vehicle to produce 120 vac. In a pinch, I will be able to run the vehicle, turn on the inverter, run an extension cord to the battery charger for the cabin, and charge up the cabin battery while the vehicle is running.
This might seem a bit convoluted, but it separates the vehicle battery from the cabin battery. This is important because the batteries are different, use different charge controllers, and require different charge cycles. All batteries in a battery bank need to be identical batteries installed at the same time. Any differences between the batteries in a bank will cause premature battery failure.
Another battery issue is that they are more likely to freeze when not at full charge. Freezing will destroy a battery. Since vehicle batteries are always a full charge, this isn’t a problem for them in most locations. Just like liquids with a lot of salt or sugar in them freeze at lower temperatures, liquids with a lot of acid in them freeze at lower temperatures. As a battery discharges, the acid combines with the lead plates in the battery reducing the amount of acid in the water. As they charge up, the acid moves from the lead back into the water. Because of the risk of freezing it is good to protect batteries in a solar system from extremely cold temperatures.
Besides the freezing issue, batteries perform best as devices to store electricity in at room temperature. They don’t release or take on charges as well when they are very cold or very hot. So this is another reason to provide a more moderate temperature where the batteries are installed.
Batteries that have caps where you check water level, and add it as necessary, release explosive hydrogen gas during the charge cycle. If these batteries are installed in a compartment, it needs to be vented. There are small fans designed to work with explosive gasses that are used to ventilate these compartments.
There is another type of battery called Absorbed Glass Mat (AGM) batteries with spun glass separators between the lead plates. These are also called sealed or maintenance free batteries because there is no need to check or add water. They don’t spill acid or release hydrogen gas when charged properly. Improper charging can do either and will ruin the battery. They are much more sensitive to an improper charge cycle, and need a charge controller with setting options designed for this type of battery.
The glass mats help reduce the risk of freezing. This along with the lack of hydrogen gas release and lack of acid release is why I picked this type of battery. This allowed me to put them inside the Mobile Cabin where they will stay warmer in winter. Being inside will also allow them, and the devices connected to them, to avoid the winter road salt. The batteries cost as much as the solar panels, and only have a 5-8 year life cycle compared to the 25-50 year life cycle of the solar panels. So I wanted to do everything possible to protect them.
Quality charge controllers have an option for a remote battery temperature sensor so they can modify the charge cycle depending on the battery temperature. Since AGM batteries are sensitive to the charge cycle, and a Mobile Cabin is likely to be used in temperature extremes, installing these temperature sensors seemed to be essential to me.
Using propane for the stove and water heater are pretty common ideas. It isn’t as commonly known that you can run refrigerators and freezers on propane. Since the refrigerator runs 24 hours a day, it is very important to make sure it is running on propane, not electricity. You can get refrigerators that run on propane or 12 vdc or 120 vac. This means when the Mobile Cabin is plugged into a source of 120vac, you can use 120 vac instead of using propane that would need to be refilled at some point. Depending on how many sources of power a refrigerator uses, it is described as a 2 way or 3 way refrigerator.
My refrigerator uses propane and uses an electronic circuit board to control the mode, ignition, and temperature. This uses about 25 watt hours per day, which is less than the smallest light bulb, so this level of usage fits well into the concept of a small solar system. I’ve never used the mode where it runs on just electricity.
While fluorescent lights produce light more efficiently than incandescent lights, there isn’t such a thing as a really tiny fluorescent light. Diode lights are much efficient than incandescent lights, and can be used for reading lights for those long winter nights when you want a light on for hours, but don’t get a lot of sunlight to charge up the batteries with.
Many battery powered tools don’t run on 12 vdc, so their battery charger normally must run on 120 vac. I’m going to install a small inverter to produce 120 vac, but it will be on a switch. I will turn the inverter on, charge the tool batteries, and then turn it off so it won’t be wasting power in its idle state. I bought a cordless drill for the first time ever a couple years ago, and it works so well, and the batteries charge so fast and last so long, that I use it for almost everything now.
Another way to stretch the solar system is to charge tool batteries off the cigarette lighter while you drive. I have a small inverter that plugs into the cigarette lighter and use it to run 120 vac batter chargers while I drive.
Since much of my work and service is handling computerized information, it is important to me to be able to supply power to my laptop in the Mobile Cabin. Besides this, I write a lot, and it is very important to capture the ideas when they are hot and fresh. A small solar system provides plenty of power to be able to support a laptop charger, and an occasional print job on an inkjet printer. Why cut an adventure trip short just because you need to work for a day or two on the computer? Today’s laptops are better than the desktop computers of a few years ago, so a complete small office can fit will into a small Mobile Cabin. There are enough “road warriors” running their business out of laptop carrying cases that there are a lot of choices of small low power devices these days.
One of the most critical “needs” is the stereo. By installing a car stereo in the Mobile Cabin you can run it straight off the 12 vdc without having to run an inverter to produce 120 vac. The car stereo I had installed uses about 8 watts an hour, which is less than most small lights, so this can fit into the power budget for most small systems.
Plan for 2 power usage levels: a more frugal level to stretch a battery charge when necessary, and a more comfortable level when there is more power available. If it has been cloudy for a couple days, you might use fewer lights, no power tools, and avoid turning on unessential appliances like the stereo. What is the least you can get by on so you can stretch out the remaining charge in the batteries?
On the other hand, if you are in a period of long sunny summer days, and installed small and efficient appliances, you might be able to run them day and night without running the batteries down.
Something to keep in mind is that once the batteries are charged, if it is sunny, all that solar power is going to waste – you might as well turn on the stereo and all of your power tools and have fun while the sun shines.
As an example of the 2 level approach is I’m going to have a diode light over the stove and reading area for use during cloudy days, and a powerful overhead fluorescent light to light the whole area when there is enough battery charge.
Don’t forget the basics; having lots of large windows will reduce the need for artificial lighting during the day (and reduce the risk of cabin fevers caused by small cramped spaces).
Even if you are using 120 vac, you should still convert to very low wattage appliances. Efficient appliances cost more to produce, so they don’t sell well and aren’t very commonly available. Most companies selling solar equipment also sell the more efficient appliances needed to keep the solar systems in a reasonable price range. Even if you are on the grid, at today’s prices for electricity, it still pays off over the lifetime of the appliance to buy the more expensive appliances that are more efficient.
Picture yourself parked in that remote mountain meadow surrounded by deep silence, and then a bird calls from the far side of the meadow. The soothing silence and healing natural world start working wonders, and suddenly a fan comes on with a vibrating whining noise.
This section is actually about all of those little electric fans that keep electrical equipment from over heating. Not only do they use power that small systems can’t afford, but they bring you right back to the world you were trying to get away from. Nobody seems to know if their equipment has fans in them. This was pretty frustrating – you just look at the back side and if there is a fan grill, then there is a fan! Since most solar equipment is mail ordered, this can be a real problem to sort out.
Most of the smaller system components mount vertically on a wall so the warm air inside them will rise and pull in cooler air.
Some fans are “on demand” fans and turn on only when the temperature exceeds a set level, and others run constantly, even if it is freezing. Once again, better more expensive equipment will have a thermostat to control when the fans run.
Better designed and more expensive fans are quieter and put more of their energy into moving air instead of sounding like a poorly maintained airplane.
My laptop has 2 fans, but they are so quiet that it has to be very quiet before I can even tell if they are running, and they stop as soon as it gets cool enough.
I’m going to install a small inverter to provide 120 vac to run a laptop charge and a inkjet printer. It doesn’t have a fan, but the next size bigger does. This fits in well with the small Mobile Cabin concept.
This overlaps into many areas, and is a big enough issue, so it gets a category of its own. The phrase “space heating” means heating the space that people occupy. The 3 main types of space heaters are: forced air from a furnace, convection meaning hot air rises out of a heater, and radiant meaning heat radiates from something that is warm or hot.
A big issue is to make sure the space heater is vented, meaning the combustion gases are vented to the outside and fresh air is drawn in to replace the used air. Contrary to the advertising, natural gas and propane produce smoke; it just is colorless and has a faint odor. People who are having allergy problems are very sensitive to this smoke. There isn’t much to be done about the propane stove except to open the windows while cooking.
More critical than these issues is the fact that if an un-vented combustion device doesn’t get enough fresh air, and uses up too much of the oxygen, then it will produce carbon monoxide which is colorless and odorless and kills people each year. This is a very critical problem in small closed spaces like an RV.
RVs are notorious for having a furnace the size of a lunch box that roars like a plane revving for takeoff, uses lots of electricity, and doesn’t heat well. It might keep the RV 20 degrees Fahrenheit above ambient temperature, and cycle on and off every few minutes to do this. This is pretty unpleasant at 35 degrees – you are cold and can’t sleep from the intermittent noise.
The big problem is that usually a radiant or convection space heater that produces a decent amount of heat needs so much clearance that it doesn’t fit well into a small cramped RV. It needs the clearance to avoid overheating things and starting fires. A full dinette sells better than a quality furnace you can’t see, or an open wall with a space heater that cuts into dinette space.
I avoid sitting in chairs or at tables so I don’t care about the dinette. At home I use a twin bed and pillows for a couch. I have converted the couch area in the travel trailer into a permanent bed/lounge area, and am planning on cutting out part of the dinette to put in a wall heater.
The most important point is I travel in below freezing temperatures and need some serious heating ability. If you are getting 30 degree temperature swings and it only gets up to 60 degrees in the day, you are going to have some pretty chilly nights, so this isn’t just an issue in the winter.
Even if you don’t travel when it will be icy, if you get real cold and wet, it is almost impossible to warm up unless you can get into a heated space or sit in front of a toasty heater. Since I like being outside the year around, this will be an issue for me.
So my solution will be to cut out part of the dinette and install a vented propane wall heater that uses zero electricity. As long as there is propane in the tank, I won’t freeze or get sick from being cold and wet.
In any case, if you want to heat the Mobile Cabin and are going to be living off the solar power system, you won’t be able to do this very well with the furnace that came with it, and you will need to come up with some other plan.
Propane is generally available in North America, and all of the solutions discussed here are based on this fuel.
While propane does produce combustion gasses, it burns more completely and produces much less smoke than kerosene does. Any unburned propane gas dissipates quickly in the breeze. This isn’t true for Kerosene. You can buy more highly refined and more expensive kerosene, but it still has some smell and if spilled, will smell like kerosene for a long time. So if you will be traveling where propane is available, it is the preferred fuel. If you are traveling anywhere but North America, probably you will more reliably find kerosene to fuel your stove and space heater.
An issue that more and more people are thinking about is being able to quickly exit an area where people are harassing you, or to leave before anything starts if you spot very unpleasant people in the area. Having an RV where you can hop out of bed, hop into the driver’s seat, and take off is a real advantage here over trailers.
At the very least, you would have to exit the trailer, deal face to face with the people you want to avoid, and get into the tow vehicle before you can leave the area. This assumes that the tow vehicle is hooked up to the trailer, there aren’t any jacks or wheel blocks, and the tow vehicle is pointed in a direction where you can just drive out. Trailers are very awkward to backup in tight quarters. Even if I leave the trailer physically hooked up to the truck, I have to disconnect it electrically because the electric trailer brakes will discharge the truck battery if left connected for an extended period.
Also, if you are exploring the outback on a one lane road, and find the road blocked by a washout, what do you do with a trailer? Push it over the edge, drive out, and get another? This is a real issue because it could easily be impossible to back a trailer a mile up a narrow difficult road to find a turnaround big enough to do a U turn in.
Also, since so many people commute in an RV, people are used to them parked all over. However a tow vehicle and trailer stand out as a visible indication of a squatter and are much more likely to attract attention than an RV. The traditional biases against gypsies still apply to all transients living on the road. Weekend vacationers receive more tolerance than people living on the road.
So at this point I wish my Mobile Cabin was an RV instead of a travel trailer.
The more stuff you have strapped on outside, the more you will look like you are living on the road instead of vacationing. The RV community is generally real friendly, and usually goes to RV or scenic park campgrounds where they are likely to get a warm welcome instead of harassed.
If you are going off the pavement with a trailer, you should consider flipping the axles. Usually the springs are bolted to the underside of the axel to reduce the overhead clearance, provide a lower center of balance for stability, and make it easier to step in and out of the doorway.
All of those big gouge marks entering and exiting parking lots are an indication of the problem RVs have with even a little unevenness. The solution to this is to flip the axel so the springs are bolted to the top of the axle instead of underneath. This will provide several critical additional inches of clearance underneath the trailer.
So it is a real tossup, is a 4 wheel drive truck with a flipped axel on the trailer better for off-road driving than an RV that is close to the ground, but is easier to do U turns in? A well graded gravel road by definition is maintained, and is unlikely to be blocked, so in this case, either will work well. Even if the road isn’t blocked, an RV is likely to hang up on a rough road, so the trailer with a flipped axel has an edge here.
The real solution is to buy a heavy duty 4 wheel drive truck frame and cab, and build the Mobile Cabin onto the back of it. A runner-up to this solution is to buy a large 4 wheel drive pickup, and put a medium sized camper on the back so it isn’t very top heavy. This has the advantage of being able to drop the camper off for a base camp, or take it with you if vandalism of an unoccupied base camp is a concern.
An interesting concept for group travel is to pull a trailer behind an RV or pickup with a camper. Since both trailers and campers tend to put extra weight on the rear axel, you would need to be careful that the real axle of the tow vehicle isn’t overloaded and that there is enough weight on the front axel to steer without problems.
After the conversion of my travel trailer to solar and low electrical usage appliances, I will write up a complete description of what I used, and the design of the system. This won’t be happening very soon because I haven’t finished yet, there will be a lot to write up, and the system needs to be tested for a while before I share it with others.
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Last updated: August 27, 2002