The Complete Book of Boondock RVing

by Bill Moeller

  The unit immediately delivered 46. 7 amps. The voltage of the batteries was 13. 5 volts.

  After 10 minutes, the amperage dropped to 29. 6 amps as the voltage rose to 14 volts.

  After 1 hour, the amperage was down to 22. 2 amps and the voltage remained at 14 volts. The amp-hour reading on the meter was −20. 6 Ah.

  After 2 hours, the amperage was 10. 9 amps, the voltage was 14. 2, and the amp-hour reading was −7. 2 Ah.

  After 3 hours, 21 minutes, the amperage was 3. 2 amps, voltage was 14. 2, and the amp-hour reading was 00. 0. This was with a tapered-type charger that is far superior to most other chargers.

  Table 11-1. Test of Iota 45 Amp Tapered Charger

  Test 2: Truecharge 40 Amp

  Multistage Charger

  The second test was with a Truecharge 40 amp multistage charger also powered through a campground outlet. We discharged the batteries to −47.5 amp-hours (29.6% DOD). This test showed some interesting results:

  When we turned on the charger, it immediately produced 43.7 amps and held that output for 10 minutes while the battery voltage rose rapidly from 12. 2 volts to 13.9 volts.

  This rapid rise in voltage caused the charger to trip to the second stage, the acceptance stage, and the amperage tapered off to 25.3 amps.

  After 1 hour, the amperage was down to 22.9 amps, the amp-hour meter showed -23.5 Ah, and the voltage was still 13.9 volts. During the first hour the charger repeatedly recycled to a higher amperage output (this is normal for this type charger), which helped effect a faster charge.

  After 1½ hours, the voltage was at 14 volts.

  The amp-hour meter registered 00.0 after 3 hours and 47 minutes. The Truecharge fully charged the batteries 48 minutes faster than the Iota, and this was with a slightly larger (by 3.7 amp-hours) DOD.

  We called the manufacturer about how quickly the charger switched to the acceptance stage. We were told that the charger was rated at a higher amperage than our battery bank needed (the battery bank was 160 amp-hours). This resulted in a rapid increase in the voltage, which caused the switchover to the acceptance stage. In spite of this glitch, the multistage charger did charge faster than the tapered charger, restoring 24 amp-hours in the first hour and 15.3 amp-hours in the second. However, the tapered charger did give a good showing.

  Testing our multistage Truecharge 40 amp charger.

  We concluded that either of these chargers could put a fast partial charge into your batteries in an hour or so on a cloudy day.

  Table 11-2. Charging Test of Truecharge 40 Amp Multistage Charger

  * * *

  Calculating Charging Time for Multistage Chargers

  We recently obtained a formula for calculating the charging time for multistage chargers. It’s quite simple:

  (CAP × DOD) ÷ (CC × 80) = charging time

  where CAP = the amp-hour capacity of the battery bank,

  DOD = depth of discharge as a percentage,

  CC = the maximum charge current of the charger, and

  80 = a constant

  As an example, let’s plug in the test parameters: battery bank capacity is 160 amp-hours, DOD is 25%, and the charger is rated at 40 amps:

  (160 × 25) ÷ (40 × 80) = 1.25 hours

  Note that this formula does not reflect the elapsed time of Test 2, probably because of the rapid tripping to the second stage. But we think—if used judiciously—the formula can give you a rough estimate of battery recharging time with a multistage charger. If the charger is powered by a generator (see Test 3), the formula will also give you an idea of how long the generator will need to run to do the job.

  * * *

  Test 3: Truecharge 40 Amp Multistage

  Charger with a Generator

  We did another test with the multistage charger, only this time we plugged it into our 1,850-watt portable generator. The generator worked well and did a good job of powering the charger. We measured the amperage output of the generator during start-up and found that it delivered 30 amps momentarily. The batteries were only discharged to a level of-18. 5 amp-hours, so it was a short test. After the test began and the initial amperage dropped, we ran a coffeemaker during the charging period with no problems. The batteries were fully charged after 2 hours, 7 minutes.

  Table 11-3. Test of Truecharge Multistage Charger (powered by an 1,850-watt Coleman portable generator)

  While we know that −18. 5 amp-hours is not a large DOD for a battery, the test still shows how fast a good charger can work even when powered by a portable generator. This generator was able to handle the load of the charger because of its rating of 1,850 watts.

  INVERTERS

  Inverters produce 120-volt AC power from 12-volt DC batteries, providing the same type of electrical power as a generator. So if you already have a generator, why do you need an inverter? One good reason is convenience. With an inverter, you won’t have to start a generator, burn expensive fuel, and create a lot of noise, fumes, or smoke for just a few minutes’ use. This is the great advantage of inverters: they can provide power for small purposes quickly and quietly. Inverters are at their best running small loads for short periods of time, while generators are most efficient running big loads for long periods of time.

  Inverters are a great addition to any rig for boondocking, providing AC power while dry camping or underway. It is nice to be able to turn on the inverter and use the microwave to heat up a sandwich, brew a cup of coffee, or heat a bowl of soup while stopping at a rest area or along the side of the road. But this level of convenience has a price: you need a large battery bank and the ability to recharge it.

  You can also use inverters for battery charging. Many larger inverters (more than 600 watts) have the option of a built-in battery charger. These inverters usually have a multistage or at least a two-stage charger with suitable amperage output for the inverter’s size. These chargers work well with a suitably sized generator for providing a rapid recharge.

  To understand inverters, we need to explore AC current a bit more. Depending upon how it is generated, AC voltage is expressed as either a sine wave, a square wave, or a modified square wave. As you can see from the illustrations on page 122, a true sine wave is smooth and continuous. It is equivalent to output produced by a generator or an electric company. A square wave is the least efficient form of AC voltage, and a modified square wave is an approximation of a pure sine wave. The electronic switching within an inverter that creates the "steps" of a modified sine wave is really nothing more than the power turning on and off as gets "stepped up" to AC voltage potentials.

  A true sine wave and a square wave.

  A modified sine wave superimposed over a pure sine wave.

  Types of Inverters

  Square-Wave Inverters

  The basic inverter produces voltage in the form of a square wave. Square-wave inverters have been around for years (Bill carried one around Italy in 1958 to run a tape recorder for recording background sounds for a movie he was working on). They are suitable for use with lights, small tools, and resistive loads such as a soldering iron. Usually inexpensive and very heavy, many square-wave inverters do not have any frequency control other than a rheostat knob. The rheostat allows the operator to adjust the voltage, which in turn adjusts the frequency. They are not suitable for use with many electronic appliances such as TVs, VCRs, and microwaves (and it was even risky running a tape recorder with one, as well). These are now obsolete, but we discuss them here for purposes of illustration.

  Modified-Sine-Wave Inverters

  Modified-sine-wave inverters produce, as mentioned above, a modified sine wave that can power almost all electronic equipment, appliances, and most types of motors. Most good inverters today, from the smallest pocket portable inverters to the largest, permanently mounted units in Class A motorhomes and large trailers, are modified-sine-wave inverters. They are available in sizes from 75 to 2,500 watts and range in price from $25 to several thousand dollars.

  Our 600-watt, modified-sine-wave Trace (a
company now owned by Xantrex) inverter with its 25 amp tapered charger provided excellent service for eighteen years on two different trailers. The only things we couldn’t run on it were a heater, a coffeemaker, a 450-watt microwave (unless it was only for a couple of minutes), and, of course, the air conditioner. We did run our computers, printers, disk drives, color TV, satellite receiver, VCP, and a variety of tools, and never had a problem. We recently acquired a 1,000-watt inverter so we will be able to run things we couldn’t before.

  The Xantrex Freedom 458 modified-sine-wave inverter has a built-in 100 amp multistage charger. (Xantrex)

  One of the MS Series pure-sine-wave inverter/chargers from Magnum Energy. (Magnum Energy)

  Pure-Sine-Wave Inverters

  Pure-sine-wave inverters have recently become available for RV use, although most RVers find them too expensive. They can power certain types of equipment that modified-sine-wave inverters can’t. They are very expensive and usually only available in larger wattage sizes.

  What Size Inverter Should You Buy?

  Inverters are rated in wattage, just like generators. They have a surge rating and a continuous or constant rating. The surge rating applies to equipment and appliances that require a high amount of start-up power, such as an electric motor. It is the maximum amount of power the inverter can supply, but only for a short period of time (several seconds to a few minutes). The constant rating is the amount of power the inverter can supply on a steady, ongoing basis to operate appliances and tools.

  Choosing the appropriate size for your RV, be it a built-in inverter or a portable one, is quite simple. First determine the total wattage of the items you would most likely power at one time. Then select an inverter that has a constant wattage rating at least 25% higher than the total of these items. This calculation works for all sizes of inverters.

  Built-In Battery Chargers

  Most inverters over 1,000 watts have a built-in battery charger, usually a multistage charger. You can use this charger instead of the converter/charger supplied in most new RVs, and it will probably do a better job of charging your house battery bank. You can also use the main converter/charger as backup equipment.

  Relays and Transfer Switches

  Most good-quality inverters—usually those more than 1,000 watts—will have a built-in transfer switch. A transfer switch consists mainly of a double-pole, double-throw relay, which is an electromagnetic solenoid that can handle two circuits. One is the positive wire and the other is the negative one. Think of it as a railroad switch that switches between two tracks, with one rail being the positive circuit and the other, the negative circuit. While a transfer switch can do many things in your electrical system, its primary purpose is to automatically switch the RV’s electrical system to campground power from inverter power the moment you plug in. When you unplug from campground power, the relay will automatically switch back to inverter power.

  If you have an inverter with a built-in transfer switch, it may be worth the cost of having a good technician wire it for you. It can be very complicated since, by necessity, it must be wired in ahead of the fuse and distribution panel. If your rig has several main circuits and 50 amp service, it could be very involved.

  If you have an inverter and 30 amp service that you would like to simply wire in a relay, you can buy just the relay, and if you have the knowledge, do it yourself. You can purchase relays from any Grainger store or online (www.grainger.com). The relay is Dayton model #5X847 and the dust cover is #4A079.

  When installing a relay, you must make the field connection on the incoming positive wire from the land-power cable. This is so the main circuit will open to provide the RV park’s electricity. When the cable is unplugged, the relay trips back to the inverter side for its service. You must use stranded three-wire cable, so the system will have a grounding green wire. We recommend using triplex boat cable that meets Coast Guard specifications, which has a higher ampacity than any wire used in your RV. Do not use Romex cable in your RV—it is brittle and can break, which can cause a short.

  A single-pole, double-throw relay. One half of a double-pole relay is shown. (Dayton Electric Manufacturing Co.)

  The Xantrex XPower Micro Inverter 175 produces 140 watts of continuous AC power and will run a laptop computer with an attached printer.

  Small Portable Inverters

  Many good, inexpensive portable inverters in the 300- to 1,000- watt range can provide enough wattage to run a small color TV, a radio, or even a computer. Most of these portable inverters have a cord with a cigarette lighter plug on the end so you can plug it into a lighter socket.

  We know many RVers who use small pocket inverters, only 150 to 300 watts, to power TVs with satellite dishes and computers with printers. All these pocket or notebook inverters also have a cigarette lighter plug so you can plug it in anywhere you have a lighter socket. They really deliver a lot of power for their size and are easily stored.

  However, we really don’t hold cigarette lighter sockets in high regard. Manufacturers first made use of them many years ago when some 12-volt appliances, such as small coffeemakers, came on the market for use in your automobile. Manufacturers needed some place to plug them in, and the only available source was the cigarette lighter socket, which we think is a very poor choice.

  Our dislike is based on experience. In the 1970s, we owned a small, black-and-white, 12-volt DC TV It had a strong spring on the center contact of the plug that would pop out of the lighter socket all by itself, disconnecting the TV right when we wanted to watch something.

  To their credit, many of today’s manufacturers have put an adjustment on the plug that increases the side pressure of the negative contacts on the plug against the walls of the socket.

  However, we recently purchased a 140-watt inverter to use for running our computer, and after plugging it into the cigarette lighter socket, we couldn’t pull the plug back out. Since we were boondocking at the time, we couldn’t leave the inverter plugged in when we were not using it because it ran a small built-in cooling fan, which then became a phantom load. Fortunately, one final, strong pull separated the inverter from the socket.

  One of the nice features of these small inverter units is that you select only the appliance you wish to use, no more or less. Whereas if you have a large inverter wired into the RV’s AC system, other electrical phantom loads may be inadvertently turned on at the same time you are running your smaller load (see Chapter 8), which can account for a lot of wasted amp-hours of battery capacity.

  When we hooked up our new 1,000-watt inverter, we found we were drawing 3 to 4 amps of various phantom loads over and above the loads we were trying to run. The refrigerator was probably part of this, since it drew almost 1 amp of power, but finding the rest of them took some detective work, which involved a lot of turning many circuits and appliances on and off until we found the problems.

  Our three-way lighter socket, which is mounted under a cabinet, allows us to plug in a small inverter plus two other pieces of 12-volt equipment.

  Wiring

  When considering purchasing an inverter, or upgrading one, make sure the wiring in your trailer or motorhome is adequate for the job. We recently installed a three-socket unit over our computer table for boondocking use. The unit has 12-gauge twin wires for hooking up to your system. (It was really designed to be mounted under the dashboard of your car).

  The wiring you will be fastening it to will probably be only 14-gauge wire, which is common in RV use. The 140-watt inverter can draw 11. 66 amps if it is run at full capacity for 1 hour (140 watts ÷ 12 volts = 11. 6666 amps). The ampacity of 14-gauge wire is 20 amps, which has a voltage drop per foot of 0. 05231 volt at 20 amps. Witharunof 40 feet (the total of both the positive and negative wires to the battery), the inverter would deliver only a theoretical 11. 4 volts. So you can see why it is necessary to use the heaviest-gauge wire as much as possible over the shortest distance possible.

  Inverter Amperage Draw

  While we
are doing math, let’s go over how to determine the amperage draw (in DC amp-hours) of an inverter. The formula is:

  wattage of the appliance to be used ÷ 12 volts ×

  1. 1 (an efficiency factor) × hours of operation = amp-hours

  For example, if you run a 200-watt appliance for 9.6 minutes, the equation would be:

  (200 watts ÷ 12) × 1. 1 × 0. 16 hour = 2. 93 amp-hours

  If you have the appliance’s amperage instead of the wattage, then you can use that and not do the voltage division.

  If you want to determine amperage draw for an AC appliance, remember that 1 amp of AC equates to 10 amps of DC. So, for example, 5 amps of AC becomes 50 amps of DC when it comes from the batteries via an inverter.

  Battery Bank Size

  Next let’s consider the proper size of the battery bank needed to safely operate a large inverter. There are two ways to calculate this size. Going back to our Rule of Twenty-Fives (Chapter 9), the first rule was never apply a load greater than 25% of amp-hour battery capacity.

  Applying this rule, we can calculate the minimum size of a battery bank for an inverter:

  inverter wattage 4 ÷ 12 × 4 = minimum battery bank size in amp-hours