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.

Chillin’ Down

With the hot summer temperatures, I’ve become unsatisfied with my refrigerator temperatures.  Even after running all night without opening the door, my refrigerator runs about 12C-14C.  With normal use, the temperature rises upwards of 20C.  Your friendly FDA recommends a maximum temperature of 4C.

After a bit of investigation and discussion with a fellow van owner, I set out to improve matters.

  1. I measured about a 0.53 VDC voltage drop (3.9%) from the battery bus bar to the refrigerator.  The manufacturer of the refrigerator requires no more than a 0.5 VDC drop.  The requirements I gave my installer called for no more than a 2% drop, a common standard for solar-powered systems.  Excessive voltage drop wastes power and reduces the performance of the compressor.  I replaced the needlessly roundabout and undersized 14 gauge wiring (the refrigerator’s manufacturer requires 12 gauge and recommends 10 gauge) with a more direct 8 gauge run, and I now measure a 0.14 VDC (1.1%) drop.  In an ongoing effort to cut RFI inside the van, I added a mix 31 ferrite to the power leads, too.  But, I haven’t actually noticed any RFI from the refrigerator.
  2. I insulated the compartment containing the refrigerator.  Due to the size of the opening and clearance for the mounting flange I could not add insulation directly to the refrigerator.
  3. I added a fan to the rear of the compartment containing the refrigerator.  I manually control this fan with a switch.
  4. I added a fan inside the refrigerator to circulate cool air from the freezer area.  The fan runs only when the refrigerator is running — I spliced into the fan control output of the compressor electronics.

Time in hot climates will prove my efforts.  But, the initial results look good.  The temperature drops faster and stays below 10C.

Refrigerator temperatures before improvements

Refrigerator temperatures before improvements

Refrigerator temperatures after modifications

Refrigerator temperatures after modifications

 

Lift Off

The Yeti has rolled away from Jeff’s shop, RV Interiors and Custom Woodworks, and we’re living full time in the van.  It’s been a rough week learning the van and moving in, hence the lack of recent updates.  Overall, everything is going well.  I’ll have more details, photos, and comments over the coming weeks.  Until then, please enjoy the pictures.

Build Updates

Updates from the van. Here’s the galley. We like how the wood grain on the cumala turned out. Jeff put a nice walnut-toned stain on it. The stove, sink, and pantry are also visible here.

Galley

Here’s the skeleton of the bed frame:

Bed Frame Skeleton

While we were there, Mark added fabric toppers to the frame so that the mattress won’t slide.

Bed frames with fabric toppers and stacked

Here’s the indoor/outdoor shower:

Shower

The build is finishing up and we should be rolling soon.

Test Drive

Jeff Hickey at RVI took the Yeti out for its first test drive today.

The solar panels held solid and quiet at 70 MPH. Jeff had warned us that solar panels often make loud drum-like noises at speed, so we’re quite pleased with the results so far. Aluminess did a good job with positioning the roof rack a bit aft to decrease wind drag.

The rear bumper rattled quite a bit, but that should be fixable with tightening a few bolts. Aluminess had told us to expect bolts to loosen over the first few miles.

Jeff filled up the propane tank and is pressure testing it.

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.

Floors, cabinets, and more photos

Here are more photos of the build.  Work is moving along quickly now, and we’ll be driving off into the snowy sunset soon.

First up, we have the battery box housing four SunXTender Group 4D AGMs.  Jeff’s team designed and custom built this box, and I’m very happy with the result.  The box is bolted to the frame and won’t have any effect on departure angle.  Also, there’s enough room above and next to the box to access battery terminals without lowering the box.  The P-trap for the shower will drop down into the small empty space in the corner.

Inside, we have the cabinet face frames nearly complete, the flooring installed, and a box for the shower pan finished.  Also, the counter top material arrived.

 

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!

 

Well-wrapped Explosives

Propane Tank

The propane tank and solar battery box are installed. We’re really pleased with the install on these. The fit is tight to the vehicle so that neither propane nor batteries is the low point of the vehicle.

The battery box is custom welded and super sturdy. This is so much better than what the previous builder had offered — batteries suspended by a few angle arms of metal and some zip ties.

Battery Box

We encouraged Jeff to think about developing the battery box as a commercial product. For vehicles with a significant solar array, there’s certainly a need for a sturdy way to carry these heavy batteries.