The Complete Book of Boondock RVing

by Bill Moeller

  P = V × I

  V = P ÷ I

  I = P ÷ V

  If you know any two components, you can calculate the third. For example, let’s say you want to know how many amps (I) a 120-volt (V), 1,000-watt (W) microwave will draw. The equation would look this:

  I = 1,000 watts ÷ 120 volts = 8.3 amps

  While the formulas are simple, some of us may find it easier to remember visuals. The triangle gives a graphical perspective of the equations. If you place your finger over any one of the components, you’ll see how to calculate that value.

  * * *

  TV: 54 watts × 3 hours = 162 watt-hours Receiver: 34 watts × 3 hours = 102 watt-hours

  Total watt-hours: 162 watt-hours + 102 watt-hours = 264 watt-hours

  Total amp-hours: 264 watt-hours ÷ 12 volts = 22 amp-hours

  Determining Storage Capacity

  Once you’ve estimated your daily amp-hour usage, you can estimate the size of the battery bank you’ll need. Because we advocate not discharging batteries by more than 25% of their amp-hour capacity (see Chapter 9), we’ll use 4 as our multiplier. The total will be a good estimate of the amount of storage capacity you’ll need. To supply the 37.83 amp-hours in Table 8-5, you’d need a battery bank capacity of at least 150 Ah, which you could get from two Group 24 12-volt batteries wired in parallel or (more comfortably) from two 6-volt batteries wired in series. If you wanted to supply that same demand for two or three days, you’d either need a very large battery bank or you’d need to do some recharging as discussed in Chapters 10 through 12.

  Wiring batteries in series and parallel.

  Wiring Batteries in Series

  and Parallel

  All batteries are classed by their voltage, with 12-volt batteries being the standard because that is the voltage used in most RVs. When 6-volt batteries are used, they are wired together in series to make a bank of 12 volts.

  To wire batteries in series, connect the positive terminal of one battery to the negative terminal of the other. When wired this way, voltage is additive and capacity remains the same. Two 6-volt, 220 amp-hour batteries wired in series will yield 12 volts and a 220 amp-hour capacity.

  A word of caution: If you have a 12-volt system in your RV, never wire two 12-volt batteries in series. You’d end up with a 24-volt battery bank, which would damage your 12-volt appliances and equipment.

  When wiring batteries in parallel, connect positive terminals to positive terminals and negative terminals to negative terminals. Voltage remains the same, but capacity (amp-hours) is additive. For example, if you wire two Group 24 12-volt batteries, each with a 73 amp-hour capacity, in parallel, you’ll have a 12-volt battery bank with a capacity of 146 amp-hours.

  Understanding these two wiring options gives you flexibility. If you want to use 6-volt batteries for your battery bank, you can join multiple 6-volt batteries to get the storage capacity you need. First wire the batteries in pairs in series to make 12 volts, and then wire each pair in parallel to double the amp-hour capacity.

  For example, say you have four 6-volt batteries, and each battery has a capacity of 220 Ah. Wiring each pair in series yields 12 volts, with a 220 Ah capacity. Wiring both pairs in parallel yields a bank of 12 volts with a 440 Ah capacity. Eight similarly wired batteries would yield a 12-volt bank of 880 Ah. Multiple banks of 12-volt batteries in series and parallel are used for the 24-volt house systems common to many large bus conversions.

  * * *

  6-Volt vs. 12-Volt Batteries

  There is a bit of controversy over whether 6-volt or 12-volt batteries are better in a battery bank. Two of the arguments for using 6-volt batteries are (1) there are fewer cables involved in series wiring, so there are fewer connections to corrode; and (2) in 12-volt parallel wiring, one of the batteries in a two-battery bank will receive most of the load and most of the charge, and therefore will fail faster than the other.

  The first argument has some validity as there are fewer cables in series wiring, so there is less corrosion. The second argument is not necessarily true, if you wire the bank as shown above. If a battery goes bad in a 12-volt bank, you can just disconnect it and use the remaining one. You’ll still be getting 12 volts. With a 6-volt bank, however, one bad battery means the loss of the whole two-battery bank.

  * * *

  The number of batteries you can have often depends on the available space in your RV. Our RV was designed to hold two Group 24 batteries. When we decided we wanted to use two Group 27 gel-cell batteries, we had to lengthen the battery compartment to hold them. And if you decide you want to use golf-cart batteries, you’ll have to accommodate their height.

  PHANTOM LOADS

  One issue that can be a problem when boondocking is that of phantom loads, also called parasitic loads, idling current, or standby current. Phantom loads are caused by any device that consumes amperage whether it is on or off.

  For example, our refrigerator is the typical AC/propane type used in most RVs. What is not commonly known is that these refrigerators use 12-volt DC power whether the main power source is 120-voltAC or propane. Altogether, when our refrigerator is operating on propane it draws 0.9 amp.

  Over a 24-hour period, this level of consumption has the potential for depleting the batteries by a whopping 21.6 amp-hours. We say "potential" because the amperage draw will fluctuate as the propane is turned on and off, and we do not use the high-humidity setting. However, the point is still valid—small amperage draws can add up and may catch you unawares. Here are the numbers:

  storage switch in the On position draws 0.2 amp

  automatic ignition system (for lighting the propane) draws 0.5 amp

  high-humidity setting draws 0.2 amp

  Other examples of appliances and equipment that have phantom loads are listed below. You can see that the list includes both 12-volt DC and 120-voltAC items.

  TVs with circuits that maintain channel-memory settings and provide for fast start-ups

  satellite receivers with live circuits that maintain programming information

  microwave ovens with timers and clocks

  clocks, stereos, and radios

  pilot lights

  switches with LEDs that indicate a device is on

  VCRs and VCPs

  RV refrigerators

  propane and carbon monoxide detectors

  security systems

  automatic-ignition propane water heaters

  An RV can have several or all of these items installed, and the phantom-load effect is cumulative. Even milliamps of current draw can be significant. A draw of 500 milliamps equals 0.5 amp, and over 24 hours, would amount to a 12 amp-hour depletion. Several low-milliamp devices added together could cause quite a drain on your batteries. Car stereos are often used in RVs as the entertainment center to provide AM/FM radio and either cassette or CD music. These units all have phantom loads due to the push-button station selection memory and built-in clocks.

  If you are using a large inverter to power your AC appliances, eliminating phantom loads will reduce the load drawn by the inverter. Why draw 20 amps when all you need is 10 or 15 amps for the job? Remember that for every amp of 120-volt AC current used, you are drawing approximately 10 amps at 12 volts from your batteries (1 AC amp = 10 DC amps).

  It was customary some years ago for some RV manufacturers, particularly those who made motorhomes, to install an inexpensive 13-inch AC-only color TV. They then supplied a small, cheap 100-watt inverter to power it. This practice provided a constant phantom load since the inverter was always on and the TV, even though it may have been turned off, was still drawing a small amount of current.

  If you have this type of installation on your rig, install a switch on the DC circuit to the inverter. Use an in-line switch and install it on the inverter cord. When the TV is not being used, you can turn it off at the switch.

  More generally, you can eliminate phantom loads by installing in-line switches on all of your devices and appliances and turning them off whe
n you’re not using the items. For example, we have a power strip with a switch on it that we use to power the TV and the satellite receiver. When we run anything else on the inverter, we first turn off the strip and then the refrigerator so that we can be sure these phantom loads are removed from the inverter before we turn on the other appliances. Jan says Bill is "switch happy" because he puts switches on everything, but he believes it is the best insurance against phantom loads. This applies to both 12-volt DC and 120-volt AC devices, with the latter becoming ever more prominent in RVs.

  Another solution is even simpler. Just switch off or unplug any appliance not being used so it will not operate inadvertently when on either generator or inverter power.

  WET-CELL BATTERY SAFETY

  Most of us are rather cavalier in the handling of our automotive and RV wet-cell batteries. After all, they’re part of our everyday RV life. They usually give us good service and the life expectancy we think they should, so it’s rather natural for us to take them for granted. And when a battery fails to start the engine and needs to be recharged, we matter of factly connect it to someone else’s battery for a jump start. We rarely even consider the possible danger.

  Lurking in a wet-cell battery however, are two hazardous and potentially dangerous ingredients: sulfuric acid and hydrogen gas. Sulfuric acid is corrosive and can severely burn skin and eyes on contact. If you get it on your clothes, it will eat away at the cloth.

  Bill has had some personal experience with the effects of sulfuric acid. When still in high school, he got a good-paying summer job working at a battery factory. After two weeks, and in spite of wearing heavy gloves and a rubber apron, he had acid sores on his hands from handling the batteries. One day he was walking down an aisle carrying a battery and the entire front of his fairly new jeans disintegrated from their brief exposure to sulfuric acid. That’s when he decided to quit—and the boss had a good laugh.

  Hydrogen gas can be even more deadly because it is explosive. Please take this gas seriously. Batteries have been known to explode because of hydrogen gas at the terminals, usually during charging and with poor ventilation, and an exploded battery is not a pretty sight.

  We don’t mean to scare you into avoiding batteries altogether, but you should be aware of the hazards and use caution when handling batteries.

  Safety Practices

  Here is a basic list of safety practices for working around wet-cell lead-acid batteries. It is not exhaustive. You should also check any manufacturer’s instructions or documentation that came with your battery.

  Do not smoke, because of the possible presence of hydrogen gas and the risk of an explosion.

  Wear goggles, or even better, a full-face shield to protect your eyes and face from acid. Sulfuric acid in your eyes can cause blindness.

  Always wear heavy-duty rubber gloves. The best gloves are ones like those worn by commercial fishermen.

  Wear a rubber or plastic apron to protect your clothes and body from acid spills.

  Do not charge your batteries with the vent caps off. If the caps are off when the full gassing stage is reached, the acid can bubble up and splatter over you and maybe even reach your face and eyes.

  Keep a jar of baking soda nearby. Pour it on any spills or on your hands if they come in contact with acid.

  Have a bucket of clean water handy to wash away acid in case of major spills and accidents. If an item becomes contaminated with acid, plunge it into the water. Immerse acid-contaminated clothing in the bucket to stop possible damage. But do not use this water to flush your eyes if you get acid in them. If anything has been rinsed off in the bucket, the water will contain acid.

  Keep a large bottle of water close by in case acid gets into your eyes. Pour water into your eyes to help stop further damage, then quickly get to a faucet. Flush your eyes with running water for 15 minutes. Get immediate medical attention.

  WET-CELL BATTERY MAINTENANCE

  Battery maintenance varies with the type of battery you have. Sealed batteries (gel-cell and AGM batteries) usually require only an occasional cleaning of the top of the batteries. Wet-cell batteries, however, need a consistent maintenance program to keep them operating to their full capacity. Basic maintenance tasks are outlined below.

  You’ll need the following tools and supplies in addition to the safety items mentioned above:

  hydrometer

  wire brush

  distilled water

  lubricating spray (e.g.,WD-40)

  dielectric grease

  pump pliers

  two ½-inch, open-end wrenches

  baking soda

  water

  paper towels

  mirror

  flashlight

  Inspect and Clean the Battery Case

  Before doing anything, check the battery case itself for cracks, corrosion, or other obvious signs of damage. Look for any fluid on the case, which may indicate leaks in the battery case. Inspect connections, looking for broken or frayed cables or damaged parts.

  Always clean the top of the battery when you do maintenance work. Wet or greasy dirt on the top can create pathways for current to leak between the posts, which will discharge the battery. Regular cleaning is especially important for batteries mounted on the tongue of a conventional trailer, where they have less protection from dirt, and for those on diesel-fueled vehicles as the soot produced from diesel fuel is a particularly bad contaminant for the electrolyte.

  1. Be sure the cell caps are tightened before you begin cleaning. Any dirt or cleaning solution that gets into the electrolyte will shorten the life of the battery.

  2. Remove dirt from around the cell caps using a damp paper towel or wire brush. If the dirt is greasy, use a dry paper towel.

  3. Clean the case with a solution of baking soda and water; use 1 teaspoon of baking soda in 4 ounces of water. This solution will dissolve the crud that can build up on the terminals of the batteries. Pour it on the terminals and wipe up with paper towels. We usually use paper cups with one side pinched together to form a pouring spout for the soda solution.

  4. Clean the terminals and inside the cable clamps with a wire brush. You can use the baking soda solution here as well, if needed.

  5. Coat the terminals with Vaseline, or even better, dielectric grease (you can buy this at any automotive parts store). Dielectric grease will ensure all connections to the terminal have a good solid electrical contact with no resistance. It will also prevent the battery cables from corroding. (Note: Corrosion only occurs on wet-cell battery terminals and is caused by the hydrogen and oxygen gases that escape through the vent caps during charging. Corrosion can eat away battery cable connections until only the terminals are left.)

  Check the Specific Gravity

  Specific gravity is the ratio of the density of a substance to the density of water. The electrolyte in a lead-acid battery is a solution of sulfuric acid and water. The sulfuric acid is more dense than water so as the battery discharges, the acid becomes less dense. Thus the specific gravity of the electrolyte is a measure of the battery’s state of charge. Specific gravity is measured with a battery hydrometer calibrated for the range of electrolyte densities normally found in a battery, 1.000 to 1.3000. (Note:There are different kinds of hydrometers; make sure you get one for use with batteries.)

  When an electrolyte sample is drawn into the hydrometer, a float indicates the specific gravity. However, the indicated reading must be corrected to what it would be at a standard temperature, often 77°F, but use whatever temperature your hydrometer calls for.

  As we all know, battery compartments in an RV are usually cramped. You may have to remove the battery for this test if there isn’t enough room above the battery to maneuver the hydrometer. Remember, wear heavy-duty rubber gloves to protect your hands.

  1. Do not add water first.

  2. Insert the hydrometer into a cell. Fill and drain the tube two to four times before taking a sample to read.

  3. For the sample draw, be sure to f
ill the tube completely so the float is floating.

  4. Record the reading and return the electrolyte to the cell.

  5. Repeat these steps for each cell.

  6. Replace the cell caps and wipe off any spilled electrolyte.

  Calculate the State of Charge

  To calculate the state of charge, you need to convert your specific gravity readings to volts:

  1. Compare the readings of the cells. If the reading of any individual cell varies by more than 50 points from the other cells, the cell may be bad; you should replace the battery.

  2. Otherwise average the readings and add 0.84 (a constant) to this number.

  3. Multiply this figure by the number of cells in the battery.

  4. The resulting voltage is the state of charge.

  Here is an example using a 12-volt wet-cell battery with six cells. If the average specific gravity equals 1.265:

  1.265 (specific gravity) + 0.84 (constant) = 2.105

  2.105 × 6 (number of cells) = 12.63 V (state of charge)

  You can also use a table, such as Table 8-6 on page 88.

  Check the Electrolyte Level

  It is important to check the level of the electrolyte acid at least once a month, and more often during hot weather. Never allow the electrolyte level to fall below the top of the plates in the cells. This will cause the plates to overheat and buckle, ruining the battery.

  1. Charge the batteries before adding water to the cells. Electrolyte expands during the charging process. Filling the cells first can result in overfilling, and excess electrolyte will bubble out during charging. The only exception to this is if the electrolyte is below the top of the plates. If you encounter this, add just enough water to cover the plates and then charge the battery.