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CAMPSITE POWER Chapter 4 – Converting Battery Power to Household Power

December 9, 2010 by · 4 Comments 

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We have been exploring different ways to generate renewable power for a camper’s 12 volt batteries.  But, we have not devoted any time explaining how to  change the power we have generated and stored in our 12 volt batteries  into 120 volt household power.  Reading Chapter 4 will give you the basics on the inversion process and the equipment needed.

I am sure most everyone is aware that a camper’s 12 volt system is direct current.  This means the electrons flowing in a wire only go in one direction – from negative to positive.

120 volt household power is alternating current.  This means the electrons change their direction of flow sixty times per second.

A Typical 400 Watt Portable Inverter

Appliances designed to work on 12 volts DC will not operate on 120 volts AC.  So, we must do something to change the electrical power stored in a 12 volt battery to 120 volts AC.  We can accomplish this with a device called an inverter.  Don’t confuse this with the converter that is standard equipment on your camper.  The converter changes 120 volt household power to 12 volts DC to recharge your battery and power 12 volt appliances. A converter brings voltage down, an inverter brings voltage up.

100 Watt "Pocket Inverter" includes USB Charging Port

Inverters are available as little pocket units that plug into a vehicle’s 12 volt power socket (cigarette lighter) that provides up to 100 watts of household power for low current devices like laptop computers and electric shavers.

Inverters that can provide power over 100 watts are best directly connected to a battery.  These inverters can range all the way up to a whopping 5,000 watts.

In Chapter two I explained the relationship between voltage and current (amperage) and how they are related to power.  I also told you that energy could neither be created nor destroyed – It can only be converted.  This fact is what makes the use of inverters somewhat tricky.

The television in our camper is rated to use 200 watts of power at 120 volts.  The 200 watts is equal to 1.66 amperes at 120 volts.  If I want to convert the voltage from 12 volts DC to 120 volts AC the power will stay the same – 200 watts.  But, 200 watts from a 12-volt battery is equal to 16.66 amperes of current!  That’s right, to get the same power the current (amperage) is increased by at least a factor of 10 (just like the ratio of 120 volts to 12 volts).  I can power our TV from a 500 watt inverter connected to four 50 amp-hour sealed AGM batteries in a near-by cabinet for about 10 hours.  Less if I use the satellite receiver or DVD player.  That translates to about five nights at 2 hours per night.  Actually, not all that bad!

If you are lost at this point, do not worry.  Just remember that when inverting battery power to household power the amps taken from the battery will increase by 10 times the 120-volt amperage.

A Typical 1,200 watt inverter. This size inverter must be hard wired to the batteries.

Inverters in campers are rarely used to power air conditioners.  Both the size of the inverter and the needed battery bank would be tremendous.  But, inverters are sometimes called upon to power a small microwave or hand held hair dryer.

When plugging a camper’s power cord into the outlet of an inverter it is important to be sure the camper’s refrigerator is set to GAS, the converter is turned off or unplugged and the electric heating element (if you have one) in your water heater is turned off.  In this set-up 12 volt appliances and lights will continue to draw from the batteries and their current must be added to any life-span computations for the attached inverter.

Use of inverters for coffee makers, electric heaters, and toasters is possible, but extremely inefficient.  Again, the size of the battery bank to sustain power for any reasonable time would have to be extremely large.  Large translates to 600 or more pounds of added battery weight, which often causes an overloaded camper.

Inverters over 2,000 watts may require twin large diameter battery cables.

Selecting the size of an inverter is not just a matter of picking out one with a high number.  The more power an inverter supplies the larger the battery cables will need to be.  A 2,000 watt inverter will use up to four copper cables the diameter of your thumb.  That’s some pretty big (and expensive) wire!

Since a 2,000 watt inverter can draw up to 167 amps from a battery bank your battery life will be very short.  A 100 amp hour battery will probably provide no more than ½ hour of power.  It will take a 45 watt solar panel array like we explored last week about 44 hours of direct sunlight to replace the energy used in that ½ hour.  So, I hope you can see what I mean when I say taking that much power from an inverter is inefficient!

My advice is to NOT plan on powering a microwave oven from an inverter unless it is rated under 700 watts and you have at a minimum a 1,200 watt inverter and four golf cart batteries capable of delivering 240 amp hours of power.  Even then, your total cooking time will be limited to approximately 2 hours at the most.  It will all depend on the battery temperature and starting state of charge.

Inverters over 1,500 watts are usually only found in large motor homes that have sufficient space and extra weight capacity for batteries.

A Modified Sine Wave is a Stepped-Switched Square Wave Signal.

Lastly, you will find most inverters sold as “Modified Sine Wave” and others as “Pure Sine Wave”.  The inverters using a modified sine wave output are less expensive and may cause synchronous motors like those found in fans to hum or buzz.  Television sets that do not have adequate line filtering may also have a buzz in the audio.  Microwave ovens will produce less cooking power from a modified sine wave inverter.

The pure sine wave inverters are closer to the household power your appliances are designed to use and will operate more efficiently from a pure sine wave.  While they are 3 to 4 times more expensive, their use will translate to longer battery life and better appliance operation.

A True Sine Wave is a Smooth Time-Amplitude Signal.

Still, a modified sine wave inverter will work satisfactorily in a budget installation.  No appliances or equipment should be harmed by plugging them into this type of inverter.

For those that may be curious, we carry 34 sealed absorbed glass mat batteries supplying a total of 2,500 amp hours of power.  This allows us to run our modified 6,000 BTU bedroom air conditioner from a 1,500 watt inverter for at least three nights before recharging.  I hate to sleep on sweaty sheets in 90 plus degree weather!  But, you must keep in mind that our batteries have a combined weigh of over 2,500 pounds.  That’s when an ex-semi tractor as a RV hauler is really nice.  The extra battery load just makes the truck ride smoother.  We have 3 inverters; a 500 watt to power the entertainment center, a 3,000 watt for all of the camper’s electrical outlets and a 1,500 watt for the bedroom air conditioner.

Twenty (20) AGM Batteries Fill a Compartment in the Bed of Our Hauler for Powering Inverters.

Next week we will begin to explore portable generators.  We will look at both inverter and synchronous designs and weigh the pros and cons of each.

Until then, Happy Camping Trails to Everyone!

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Comments

4 Responses to “CAMPSITE POWER Chapter 4 – Converting Battery Power to Household Power”
  1. Don & Irene says:

    Question; we have a 1500 watt pure sine wave inverter. When we plug in our main power cord it runs through the charger and wants to act as if we were plugged into shore power. Therefore we are able to unplug the built in charger and do so, assuming this is correct? otherwise it is drawing battery power to recharge them.
    I can only assume the solar panels will do their job of recharging or at least maintaining my 4 6volt batteries.
    I bought the unit used so had no instructions only trying to use common sense.
    One thing I noticed in the “Excellent Article” you said put the fridge on Gas why is this does it draw to much? the reason I am asking is we were on the Alaska Ferry last year and all propane must be shut off and as the trip was more than 48 hours we had to put all food in ice packed coolers, seemed to work but maybe could have used fridge on inverter power?

  2. Professor95 says:

    Don,

    You are correct that when you plug your shore power cord into your inverter your converter will operate and use the inverter to recharge your batteries. This is an extremely inefficient loop and wastes a significant amount of battery power and will reduce the usable output from your inverter.
    This is why I said to unplug your converter or turn off the 120 volt circuit breaker to the converter “IF” you plug your camper shore power cord directly into your inverter. You may need to install a switch (like a light switch) in the power cord to your converter so it can be turned off if the above methods do not work with your camper. In any event, when using an inverter, get the 12 volt converter/battery charger out of the loop.

    Your refrigerator has a “boiler” that must be heated so it will cool. I know it sounds crazy, but heat is needed to make the fridge work. When the fridge is on AC it will use “about” 400 watts of AC power to run an electric heating element to replace the gas burner. Of course, this wattage will vary depending on the size of the fridge. This is why I said to put the fridge on gas. In the gas mode it will not take power from the inverter, but still draws a small amount of power from the battery. Running the fridge off of the inverter will quickly deplete the batteries. But, if you are connected to your tow vehicle or motor home alternator, which you should be, it may be possible to run the fridge from the inverter while traveling since the camper batteries will receive recharge power.

    Many folks put frozen bottles of water in their fridge to keep the contents cool while traveling with the fridge off. To keep food frozen in the freezer section you will need some “Dry Ice” in the freezer. For the freezer in our 10 cubic foot fridge 3 pounds will easily work for 48 hours. Our local Kroger supermarket sells dry ice.

    I hope this helps and answers your questions. If not, I am willing to try again!

    Randy

    We have been ab

  3. Don & Irene says:

    Thank you Randy for the clarification. When the after market unit was installed the built in battery charging unit was isolated by installation of a regular household plug, so by unplugging it disables the charger Just have to put post it notes here and there to remind one to plug it back in when the inverter is no longer needed.
    After studying it for awhile I could re do ity with a switch alongside the inverter.
    Cant say enough about the solar and inverter setup though this is our third motorhome and the first with solar setup and would never buy another unit without it. It simply opens up the whole camping experience.
    We live in Northern British Columbia Canada so when we are not heading south to warmer climates we have to boondock a lot.
    Thanks again for the excellent articles Randy

  4. Ron says:

    http://blog.woodalls.com/2010/12/campsite-power-chapter-4-converting-battery-power-to-household-power/

    Appliances designed to work on 12 volts DC will not operate on 120 volts AC. So, we must do something to change the electrical power stored in a 12 volt battery to 120 volts AC.

    These 2 sentences DO NOT belong together! Or the first has reversed contents. Or maybe I just don’t understand?

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