Travels with Tucker

Travels with Tucker

Wednesday, June 18, 2014

On and off the grid—electric power for our rig

I have been asked to describe the solar power system I designed and built for our trailer, so this post sets out to do that, but to do so, I have to give some basic introduction to typical RV electrical systems.  If you are not interested in the technology, you should probably skip this post ;-)

 RV electrical systems are complicated, more so than residential home systems.  In additions to the 110 volt power system that a typical home has, an RV has a 12 volt system powered by batteries.   The two systems operate in tandem and interrelated to each other, as I will describe.

When an RV is parked in an RV park with electrical hookups, there is a big cable that brings 110 volt AC to the RV.  This 110 power operates the following in the RV:

  • Microwave
  • Air conditioner(s)
  • TV/stereo systems
  • Wall plugs for small appliances like coffee maker, blender, hair dryer, electric toothbrush, etc.
  • Battery charger for the 12 volt system below
  • The refrigerator can typically run on either 110 volt AC or propane gas.

The RV 12 volt DC system is powered by one or more deep cycle batteries, similar to a car battery but designed to discharge slowly over a long time.  Golf cart batteries are often used in RVs.  The 12 volt system operates the following in the RV:

  • Most or all of the room lights (I recently changed all our interior lights to LEDs)
  • Water pump to deliver fresh water from the storage tank to the faucets
  • Furnace blower fan (with the heat provided by propane)
  • Motors to operate the slide-out rooms
  • Motors to operate the jacks that level and stabilize the RV when parked
  • 12 volt plugs (like a cigarette lighter) to charge cell phones, tablets, etc.

You can see that the critical functions for living in an RV mostly run on 12 volt or propane and don’t need 110 volt AC—water, heat, refrigerator, lights.  The things that need 110 volt AC are more like luxuries—air conditioning, TV, microwave oven and small appliances.  This means that in a pinch you can live in an RV with only 12 volt power—off the grid, so to speak.  There is only one problem…eventually your batteries will go dead.  And eventually is not very long.  In our trailer, our batteries would lose about ¼ to ½ of their charge every 24 hours just running the lights, water pump and furnace.  Without some way of charging the batteries, we would not be able to boondock for very long.  And it is also nice to have some luxuries like TV or a blender or hair dryer sometimes.  So a way of charging the batteries is pretty important and there are two main ways to do this—a gasoline or propane-powered generator or a photovoltaic solar power system with solar panels.

We have a generator, a little Honda 1600 watt gasoline-powered thing that is a wonder of mechanical design.  It is small (40 pounds), runs about 5-10 hours on a gallon of gas and it is very quiet (compared to the obnoxious generators you sometimes hear at construction sites).  We lug it around in the back of our truck and have used it for a few 3-4 day camping trips.  I hate it.  I hate listening to it because as quiet as it is, in the woods or in the desert it is SO out of place. And it is inefficient.  It can theoretically push 1600 watts of power and it runs our microwave oven or a hair dryer just fine, but charging the batteries it only pushes from 120 to 350 watts of power and would take about 5 hours to recharge one days use of our batteries (about one kilowatt-hour, that your utility charges you about 20 cents for).  That’s about $4.00 worth of gasoline.

So I wanted a solar charging system and set out to design one.  I did a lot of research and shopping around for components, purchasing many of the building blocks on Ebay and Amazon.  The whole thing cost me about $600 and it works like a dream—effortlessly charging our batteries to full by about  1 or 2 PM every day. It is completely quiet, charges the batteries even when we are driving, doesn’t need any attention.  But it does need sunshine, so in case we have several cloudy days in a row or have to park in the shade, we have the Honda generator as a backup.

Oh, I almost forgot.  One of the cool things about having the solar charging system is that we now have enough battery capacity to have an inverter.  This box converts 12 volt DC to 110 volt AC, so we can run the TV/stereo, hand blender, laptop charger, electric bed warmers and other luxuries and still have our batteries top off every day. We still need the generator to run the hair dryer, which Lynnae REALLY likes every once in a while ;-)

For the techies reading this, here are the details of the solar system:

  • 2 ea. 140-watt solar panels
  • Morningstar 30 Amp solar charge controller
  • 2 ea. Trojan 6 volt batteries, wired in series to give 12 volts (225 Amp-hours total capacity)
  • Xantrex 1000-watt pure sine wave inverter
  • Bogert Engineering Trimetric Battery Meter
  • Various wires, cables and brackets

Batteries, meter and BIG cables

Charge controller (left) and inverter (right) and more big cables

Overall characteristics of the system:
Typical daily drawdown of the batteries is about 60 Amp-hours, or 30% of capacity.
Typical solar charging current is about 12 Amps, so 5 hours of sun per day will give a full charge.
When boondocking we run our lights as much as we like, the furnace might run 3-4 hours at night, we might watch a few hours of TV and use the bed warmers for about an hour before retiring, we charge all our phones, tablets, laptop, toothbrushes and internet hotspot and we run the hand blender for breakfast smoothies. I could add a third solar panel to the existing system if needed, but so far we have almost never been at less than 100% charge by the end of each day. We live quite luxuriously off the grid anywhere there is sunshine!

The second- best part is that, based on the cost of generator power, the system will pay for itself in less than a year.

The best part is that I rarely have to fire up the generator and break the silence while camping!

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