DC-DC Conversion and Power Efficiency

Many of our electronic gizmos plug into a common 120 VAC outlet.  Most of the time, you don’t need to know anything else: things just work.  But, living in a van is not “most of the time.”

The typical electrical system in a van, RV, or boat is nominally 12 VDC (often closer to 13.8 VDC).  To get 120 VAC, you need an inverter.  This conversion process is anywhere from 50% – 95% efficient, depending on the quality of the inverter, input voltage, and load.  Take a look at the manual for an inverter to learn about efficiency.  You should find a curve showing efficiency vs. load.  Lazy manufacturers will just give you a single best-case efficiency number.  Cheap inverters often don’t list efficiency, and, in these cases, I assume the worst.  In any case, using an inverter wastes 5% – 50% of your precious, limited power.

To rub salt in this wound, consider that most small electronics, including computers, LCD monitors, and phones, internally operate off low voltage DC, often between 3.3 VDC – 24 VDC.  So, these gizmos contain an internal AC-DC converter or an external AC-DC “brick”, which are between 20% – 90% efficient.  You waste power converting from low-voltage DC to 120 VAC.  Then, you waste more power converting from 120 VAC to low-voltage DC.  And, you waste space for both the inverter and possibly one power brick for every gizmo.

What can you do?  How do you eliminate this waste?  In some cases, you can connect a gizmo directly to the 12 VDC system bypassing the inverter and power brick.  In other cases, you can connect your gizmo via a DC-DC converter, which regulates the voltage to whatever the gizmo requires.

  1. Make sure your gizmo has an external power brick.  If the 120 VAC power cord connects directly to the device, then you’re out of luck, short of hacking up the internals of the device.  Shop around for devices with external power bricks.
  2. Figure out the power requirements.  Typically, there’s a label on the device listing input voltage or voltage range, peak current, and AC or DC.  You might also see peak power instead of current.  Divide power by voltage to get current.  If you can’t find the power requirements printed on the device, look on the power brick or in the manual.  If your device requires AC rather than DC, it’s probably not worth the effort to eliminate the inverter and power brick.  I’ll assume the device requires DC power, which is the case for most small electronics.
  3. Understand that your vehicle’s 12 VDC system may range from 11 VDC – 15 VDC, depending on temperature and battery charging.  If you gizmo accepts this voltage range, your life is easy.  Along with a suitably sized fuse and wire, you may connect directly to your vehicle’s electrical system.
  4. If your device needs a voltage within this range but does not specify that the entire voltage range is acceptable or if your device needs a voltage outside this range, you cannot connect directly.  You need a DC-DC converter.  The Internet is your friend.  Jameco Electronics and Digikey sell a huge selection of DC-DC converters.  Shop for a converter that accepts 11 VDC – 15 VDC input (or greater range), an output voltage within the range printed on your gizmo, and an output current at least as large as that printed on your gizmo.

Here are a few tips to help with wiring:

  1. Always use wire rated for at least the maximum current.  Better still, make sure the gizmo’s maximum current is no more than 80% of the wire’s rated current, just as a safety margin.  There’s no harm in using larger wire than necessary, except for higher cost and more difficult routing.
  2. Always use a fuse or circuit breaker to match the wire’s current rating.
  3. Swipe the connector from the device’s original power brick.  Most gizmos have a small round power connector of various diameters.  Cut off and reuse the original plug.  Make sure you get the polarity correct when you rewire the connector.
  4. Use heat shrink tube and electrical tape to protect connections and bare wire.
  5. If you need part of your wiring harness to be removable, consider using Anderson PowerPole connectors.  They’re easy to assemble and durable.

Living in a small DC house, I find these changes to reduce wasted power and to reduce piles of power bricks worthwhile and relatively easy.  So far, I’ve converted a monitor and modem, and I’ve specifically chosen other items that run directly off 11-15 VDC input.

Counting down …

The van is scheduled to roll out of Jeff’s shop in about a week.  Here are week old pictures showing the progress.

  • Solar panels are installed but not yet secured.  The frame that Aluminess built isn’t wide enough to bolt the panels directly to the rack.  Jeff is going to fabricate clamps, four per panel, to hold the panels in place.
  • Cabinets are finished and bolted into the van.  The door faces are done, too, but won’t be installed until later.
  • The water tank (20 gallons), exterior water fill, water pump, and water filter are installed.
  • The slide out desk is partly built.  The last half of the desk surface is hinged and isn’t installed yet.
  • The toilet is mounted and will vent underneath the van to avoid cutting another hole in the side of the van.
  • The shower pan area is cut out but not installed yet.
  • The sink and cooktop have been test fit.  Unfortunately, the hoped for double sink won’t fit.  The counter is a few inches too shallow to fit a faucet with a double sink.  If we had known earlier, we would have made the cabinet a few inches deeper.
  • Exterior work lights and awning are installed.
  • The gray water tank is installed but not quite finished.
  • The propane is plumbed to the interior of the van.  RVI did a nice job mounting the regulator and the black iron gas line out of harm’s way.
  • The thermostat and battery monitor are mounted.
  • The bed foundation is built. Next, Jeff will weld the bed frame and hooks for the Metolius ladder.

Build Photos by Brent Haywood Photography

Here are some recent photos of the van build by photographer Brent Haywood. These pictures show the window cut outs (before the windows are installed), decor wall panels, fabric headliner, ceiling fan, LED ceiling lights, water heater, and subflooring. Thanks for the photos, Brent!


Furring and Wiring Complete

The “guts” of our walls are complete, and Jeff at RVI is ready to seal them up behind fabric and panelling.

Here are some photos of what the inside of van walls looked like as the furring and wiring were being built. The photos are of wiring and wood, prior to placement of insulation. In the pictures, the upstairs is mostly complete and the downstairs is being built. Insulation then gets layered on top, and then the whole thing gets sealed behind wood or fabric.

Build Progresses

Windows and Fan Installed

Visited the RV again today. The electrical wiring is nearly complete. The subfloor is in place and the upstairs furring is done. We reviewed options for the downstairs windows and confirmed that all the wiring is in place. It looks much more homey with the upstairs windows and fan installed.

In the next few days, Jeff at RVI will complete the wiring, insulation, and furring. By next week, we expect that the walls will be sealed up and the flooring will be in. It’s exciting to see progress on the van.


Build Update

We visited RVI today to check out the progress on the build.

Van outside RVI

The carpet is gone and the driving cabin is sealed off with plastic to keep out debris. The upstairs furring is complete and has just finished drying.  To avoid rattling and allow the vehicle to flex, Jeff screws the furring together, glues it to the fiberglass, and then glasses the furring to the top.

Jeff marked up the van interior with the locations for wiring. We reviewed these and added a few modifications. We also checked the parts that had arrived and labelled all the electrical components. We upgraded the wiring to 00 to allow for the capacity of the high-output alternator. Higher capacity wiring makes the electrical system more efficient. This is often used in solar systems, since bigger wire is cheaper than adding another solar panel.

We chose fabrics and accent lighting. We also narrowed the hallway to 24″ to allow for more water storage capacity in the galley. Over the next few days, Jeff will add the downstairs furring, subfloor, insulation, and electrical wiring. Next time we see the van, it will look very different.


O Power, Where Art Thou?

Power, of the electrical sort, seems so easy.  Just find an outlet and plug things in.  But, when living off-grid, whether in a remote cabin or a motorhome, you cannot take power for granted.

I need power in my van for typical motorhome appliances:

  • lights
  • fans
  • water pump

Since I’m living and working in my van full-time, I also want power for a few other items:

  • computers
  • cell phones
  • wireless Internet
  • network attached storage (for storing and backing up data needed for my work)
  • two way radio (for entertainment and emergency communication)
So, where does power to run all of this come from?  Many motorhomes throw a generator and RV park hookups at the problem.  This works but doesn’t match my desired lifestyle.  Generators are noisy and take up valuable space.  And, I don’t like RV parks.  I’d much rather call a nice meadow home for a week or more at a time.  After much reading, thinking, and wondering “how the heck is this gonna work”, here’s the process I’m following to address my power requirements:
  1. Estimate power requirements – Make a list of everything that requires power.  Don’t forget the little things like lights, water pumps, and furnace blowers.  Ideally, measure the power requirements with a multimeter or a device like the Kill A Watt.  In my case, I don’t yet own many of the devices I plan on adding.  For these, I check the manual or data sheet from the manufacturer and look for the power requirements.  The manufacturers usually specify the maximum power requirements.  Typical or average power requirements will be lower than the maximum, but there isn’t, unfortunately, a way to figure this out without measuring the device yourself.  Multiply the power draw for each device by the time you expect to use it each day.  Add it all up to get your energy requirements for the day.  This is a rough estimate, but it’s a useful place to start.  I recommend entering your estimates in a spreadsheet so you may adjust as you learn more.
  2. Use less power – This is a critical and difficult step.  After finishing your estimate, you’ll notice that small devices suck up a lot of energy.  For example, a 60 Watt light operating for 12 hours drains 720 Watt-hours.  That’s over 60 Amp-hours from a typical 12 VDC battery bank.  Ditch the items you don’t really need.  Do you really need a microwave, drip coffee pot, a blender, and a wide-screen television?  Maybe a propane cooktop is enough for warming food.  Make your coffee with a press pot instead (tastes better this way, too).  Consider leaving the blended margaritas at home and switch to beer.  Find lower power alternatives to the items you do need.  Get small, lower-power LED lights that just illuminate the area you’re working in.  Look for a more energy efficient refrigerator.  Switch computer parts out for more power efficient ones. Buy a nice down comforter and run the heat less.  Question everything.
  3. Get more power –  Alternators, solar panels, generators, grid hookups (“shore power”), fuel cells, wind generators, Mr. Fusion … it’s all a compromise between time, cost, and space.  Space is a huge constraint on my van, and I returned to steps 1 and 2 several times trimming ever more from my requirements.
That’s the high-level overview.  I’ll write about the details and share my decisions later, especially if you ask questions.

RVI Factory Visit

We visited RV Interiors, Inc (RVI) in Spring Valley, CA this week to plan our interior build. Owner Jeff Hickey does complete RV interior builds and also makes custom cabinetry for hotels and homes.

We learned about Jeff’s work through friends on the Sportsmobile Forum and Expedition Portal. We then met with both Jeff and one of his former clients in June, and were impressed with the quality of Jeff’s work.

Choosing a Builder

There are only a few semi-custom RV interior builders in the nation that we know of that work with class B (van chassis) motorhomes. These include Sportsmobile in California, Texas, and Indiana; Van Specialties in Oregon; and Outside Van in Oregon. If you know of others, please post a comment and we’ll add the data. The two fully custom builders that we know of are RVI in San Diego and Global Expedition Vehicles in Missouri. Companies selling fully fabricated expedition vehicles include TigerXP Camper, and EarthRoamer.

For our interior build, we interviewed Sportsmobile Texas and RVI, amongst others. Our choices were due to geographic proximity and apparent fit. If we had known about Global Expedition Vehicles when we started our build, we likely would have interviewed them as well.

A few factors that stood out as we made our decision were:

A) Handling of a Custom Build   Since we’re living and working out of 70 square feet, we have a lot of specific needs and creative designs. We were pleased that both Sportsmobile Texas and RVI were willing to consider customizing. However, the two businesses varied in degree of flexibility and the pricing structure.

Sportsmobile primarily sells semi-custom work characterized by set options. The experience is similar to designing an Ikea shelving system or kitchen. You assemble Galley A with Closet B plus Stove E to come up with a “custom” build. For most recreational part time RVers, this gives enough options without overwhelming the buyer with decisions. That is — for the average buyer, Sportsmobile customization works great, and Sportsmobile prudently serves the average buyer. Of the Sportsmobile locations, Texas is reputed to do the most customized interior work. President Paul Meyer was certainly affable and flexible about customizing things for us. As the design developed, we did feel like our build was stretching beyond Sportsmobile’s skill set, and that Paul was managing that by billing heavily for anything custom, even if it was simple and inexpensive to build. For example, Sportsmobile Texas quoted a grand to build a simple desk cabinet because the design wasn’t one of their standard cabinets. Jeff bills for his time and parts, and the cost is reasonably proportional to the effort involved. It’s also clear that custom work is at the heart of Jeff’s business model, and that he enjoys custom work. It’s much easier to plan a custom build when the architect is excited about the work.

B) Construction method and materials Sportsmobile uses a veneered marine plywood product held together by L brackets and sheet metal screws. Benefits of the marine plywood are ease of construction, lighter weight compared to hardwoods, and improved durability in comparison to fiberboard. The marine plywood is an improvement over the medium density fiberboard (MDF) that Sportsmobile used to use and that other builders continue to use. BadgerTrek discusses some of the problems that the MDF and L-bracket construction method poses when you camp in snowy or wet weather. The veneer on the marine plywood can chip with heavy use, then swell when exposed to moisture. Once swollen, it will not reform unless you deconstruct the cabinet and replace the boards. Fortunately, there are some RV builders using solid woods and even sustainable materials. We’ve heard good things about Bamboo construction at Outside Vans. When we visited one of Jeff’s RVs, we were impressed with the quality of the cabinet work as well as the ability to use solid wood and natural materials. We chose kumala wood, a semi-sustainably grown hardwood that resembles teak and that combines light weight with durability. We’ll also be able to use residential-quality materials for the flooring and counter.

C) Broad knowledge base. It takes many skills to build an RV Interior, as it would to construct a house. Electrical work, woodworking, flooring, and space design are some of the most heavily used skills in our build. The foreman at Sportsmobile Texas seemed very knowledgable with fiberboard work, flooring, plumbing, and counters. Conversations with their sales team revealed moderate knowledge of insulation, strong knowledge of standard electrical systems, and minimal knowledge of solar or custom electrical systems. Jeff’s passion is woodworking, though he is skilled in the other areas as well. Peter is knowledgable in electrical system design, and is working closely with Jeff on our electrical system.

D) Responsiveness and Professionalism  We value simple acts of professionalism like communicating clearly, sticking to timelines, and answering calls within a business week. Some businesses would put great effort in to work with us closely (and we with them — including a 10 hour drive to one factory), and then not return our call for 6 weeks with no explanation. The amount of our business that they lost would have more than paid for another secretary to respond to client calls. Sigh.

E) Ability to think about systems. In a small space it’s important to think about the interconnections of systems, locations, and uses.

In our post on “Becoming A Home”, we talked about the order of steps in a build. Since wiring and the headliner must be completed first, it’s important to think of roof and electrical add-ons at the beginning of the build. So, while external lights and an awning are low on our priority list, we’ve got to decide on them soon.

Another aspect relates to understanding how the customer will use the vehicle, and then catching problems before things are built. We’re planning on being reverse snowbirds — skiing and ice climbing in the snowy mountains that most RVers flee from. When we discussed our shower design with Jeff, he pointed out an important and obvious detail that no one had caught before — a standard recessed shower pan involves cutting a hole in the floor. The recessed location saves space and stubbed toes. However, this also means that we’d shower with only fiberglass to support our weight, and that the uninsulated shower pan would be very cold when snow camping. As an alternative, Jeff suggested building up a false floor in the bathroom area. The floor could be filled with extra insulation (always a bonus!), and the shower pan could be recessed into the false floor. We’re very impressed that he thought of that!

Designing the Build

Our meeting with Jeff this week focused on planning the upcoming build. We examined the van, reviewed the floor plan and design, examined the parts that have arrived, and made several design decisions.

Last time we met with Jeff, we didn’t have the van yet. Now that we have the vehicle, we were able to map out exact locations for tanks, antennas, cabinets, etc.

A few updates that arose:


The previous plan was to drop the van off and start the build immediately. However, at literally the last minute, Jeff bumped a VIP build ahead of us, pushing our build back by 3-4 weeks. Though we understand the business reasons for his decision, we’re also disappointed in him and in the circumstances. Jeff has otherwise treated us with the utmost professionalism, and this sort of behavior really isn’t like him.


We’ll be able to fit 3 8-D solar batteries and an at least 8 gallon propane tank under the vehicle; this is good news for maximizing our power resources.

The roof is more curved than expected, which may limit our solar panel carrying capacity. This was a big disappointment.

Background: Peter worked hard to get clear specifications for the fiberglass top before purchasing it. We corresponded with Fiberine, Sportsmobile West, and even two generous Sportsmobile owners to get top specifications. The SMB owners were most helpful. We were dismayed with the “architectural drawings” that Fiberine provided, which looked like a child’s scrawl and were nearly inscrutable. It amazes us that they’ve manufactured from the same mold for 30 years, but never produced a proper architectural drawing. After visiting and measuring two of Fiberine’s tops on built SMBs, we thought we understood what we were getting. However, these tops were finished inside and out, so some of the curves weren’t apparent.

After Aluminess and Jeff examined the Fiberine top, we learned that the top is curved front-to-back and is significantly narrower at the top than at the base. So, we may have to scale back our solar power system, which already takes up nearly all the available roof space. Peter is back to researching all the available panels, which vary in shape, wattage, and percent efficiency in ways that make the decision complex. Our finished solar system will likely be one of the largest (in Watts) ever built on a Class B RV, so understandably this build stretches everyone’s skills.

Chassis and Bumpers

The towing plugs need to be reoriented. The current design from Sportsmobile 4WD would make towing power cables a low point of the vehicle (at risk for catching on road debris) and also rubs the cables against a sharp metal bracket that risks eventually breaking through the wiring harness. We’re glad to catch this small problem early before it becomes a larger issue, and will reach out to Aluminess about rotating the bracket.

Living Space

Taller than expected counters. Our backs rejoice! We’ll be able to have 33″ tall counters while still putting a kitchen window above them. House counters are 35″ tall in kitchens and 33″ tall in bathrooms. Counters in most Sportsmobiles with a pop-up Penthouse top are (from what we remember) 27″, because of fire regulations relating to the position of the stove. Most Sportsmobile owners go on short trips and either grill outside or reheat premade meals, so short counters are tolerable for them. This is going to be a long term home, so cooking comfortably is important. Sportsmobile Texas was willing to build us a taller counter, but they weren’t sure about fitting a window on top of it because their window supplier had a limited selection of shapes. No kitchen window would make for a dark and dreary kitchen!

We selected our flooring, counter, and cabinet wood.

We discussed the possibility of a bypass “camper drain” for the shower, so that shower water could bypass the greywater tank and drain to the outside when environmentally appropriate. This would save capacity on our greywater tank, while still making it possible to contain all greywater when needed.

The Natures Head composting toilet package had arrived at Jeff’s workshop. Peter had reviewed a (new and unused) model before by visiting the company owner’s family in rural Phoenix. (When you will drive 100 miles to see a toilet, you know you’re a gear head!) This week was Katherine’s first introduction to the Nature’s Head. It’s such a neat technology for environmental responsibility, water conservation, mobility, and hygeine. Check out our detailed posts about the Nature’s Head on Sportsmobile Forum. We look forward to reviewing the product after it’s installed in our RV. This might seem like a weird topic to discuss. But if you’ve ever changed an RV porta pottie or dumped a black water tank, you’ll understand why we’re excited about a cleaner alternative.

Working Space

We discussed our workspace designs further. We’ll need to install the swivel seat bases before finishing the design. Katherine’s workspace will be fairly simple, with a swing out table on the passenger barn door. We may model the table after an airline seat table. Peter’s workspace is more intricate, possibly involving a secretary-desk- style cabinet with sturdy arms for computer equipment.

In an ordinary workspace, the work chair adjust extensively so the table doesn’t have to. We’re using the driving chairs as work chairs, with space savings and loss of adjustability. So, the depth, height, and location of work surfaces involves a lot more planning.

Next Steps

Our next steps are to: finalize design for roof rack and solar system, then get the roof rack installed; refine the floor plan and design documentation; create an electrical wiring diagram for the interior build; and order more parts and appliances. We expect to get the roof rack installed in 2-3 weeks, and to start the interior build in 3-4 weeks.

To friends we know and friend we haven’t met yet: Thanks for accompanying us on this journey. If you know a fellow nomad or gear head, please invite them to join us.


Becoming a Home

How does a van become a home?  What makes our van different than the one your plumber or favorite creepy guy drives?

  1. Start with a Ford E-350 Extended Body van.  A van is small; many of us have closets larger than the van interior.  But, the van is a sturdy foundation.  A larger, more conventional motor home is too large, flimsy, and close to the ground to visit the off-road destinations we covet.
  2. Bolt on four wheel drive. Depending on where you’re going, consider structural upgrades such as a full floating axle, air lockers, and stronger tires.
  3. Add a fiberglass top to give over 6 feet of standing room and space for a loft bed. Most Class B (small) motorhomes have a rear sofa that converts to a temporary bed. This saves space, but involves remaking the bed every night. For full time winter living, a permanent bed is highly desirable.
  4. Add exterior storage and strength. Aluminum bumpers and racks provide storage and protection for the solar power system, vehicle rescue equipment, and tools. These items are optional, but very useful. While the bumpers can be added later on, the roof rack needs to be installed before the interior build. The roof rack bolts permanently onto the fiberglass, and then is reinforced from the inside. It is preferable to install all attachments to the fiberglass prior to installing the wall panels and headliner.
  5. Build the interior into a house. Electric wiring, insulation, and wood furring are installed first. Then, add decorative wall panels, flooring, and the fabric headliner. Next, install cabinetry, appliances, and lighting. Basic residential RV features includes spaces for sleeping, cooking, eating, hygiene, and work. Small RVs often have multipurpose or convertible furnishings to make the most of a small space. Check out this classic Disney clip for Katherine’s early childhood inspiration in compact living, and see grown up versions here and here.