The Value Of Inverters

By Mark Quasius, F333630

August 2015

An inverter is one of a motorhome’s handiest accessories. The device transforms 12-volt-DC battery power into 120-volt-AC power, which means that coach owners can operate 120-volt electrical equipment when driving or dry camping, without using a generator.

An inverter essentially is the opposite of a converter or battery charger. A converter converts 120-volt-AC power to 12-volt-DC power to operate a coach’s 12-volt accessories and to charge batteries; an inverter inverts 12-volt-DC battery power into 120-volt-AC power. It happens electronically, so there is no exhaust or noise, which makes inverters great for campgrounds with quiet-time periods. It also can save fuel, because there’s no need to run the generator to produce AC power when driving or camping.

Inverters do have their limits, however. Because of the battery power required to create the AC output, inverters do not work well for high-current loads, such as air conditioners, water heaters, or any item with a sizable electric heating element. In such cases, the battery bank’s charge level depletes quickly. Inverters, therefore, typically are used for short-term, heavy loads, such as powering a microwave oven, or for smaller loads that may run for a longer duration, such as running a TV or DVD player on an entertainment system.

Inverter-equipped coaches usually have larger battery banks to provide plenty of inverter runtime. Eventually, the battery bank must be recharged, but this can be accomplished by running the generator for a few hours at a convenient time, or by driving the coach.

Small inverters connect to the battery bank via wires rated to handle the current that creates the inverter’s AC power output. That power output can be hardwired to a remote electrical receptacle in the coach, or a receptacle may be mounted in the inverter itself.

The output from larger inverters generally is fed into a breaker subpanel in order to power multiple branch circuits. Such units are called inverter/chargers, because they can operate as an inverter or a converter/charger.

Inverter/chargers have an input connection for 120-volt-AC power. When AC power is present at this input, it passes through the inverter’s transfer switch to the output circuits. At the same time, some of that incoming AC power operates the inverter’s battery charger mode to recharge the batteries and provide DC power to operate the coach’s lights, fans, and other 12-volt accessories, eliminating the need for a separate converter.

When the incoming AC power is no longer present, the inverter draws power from the batteries, inverts it as 120-volt-AC power, and sends it to the inverter’s output circuits. Many inverter/chargers also have the ability to equalize wet-cell batteries to remove sulfation from the plates and restore battery capacity, if the batteries are in good condition.

AC power is transferred via a waveform called a sine wave. All electrical components are designed to run on what’s called a true sine wave, which is what is provided by normal utility power or a generator. Sine waves are created by the rotating fields within a generator. But an inverter has no moving parts, so it creates its output electronically.

Most early inverter models created what’s called a modified sine wave, which makes for a less expensive inverter. Basic loads — such as toasters, lights, and heating elements — operated fine, because RV electrical components were fairly simple. But as technology has improved, high-tech electronics have become common in everything from refrigerators to entertainment systems. And in some sensitive devices, a modified sine wave can create issues such as overheating and early failure rates. Fortunately, thanks to advancements in inverter technology, it’s now possible to buy an inverter with true sine wave output at a reasonable price.

Electrical equipment is designed to operate on 120 volts RMS voltage. RMS stands for Root Mean Square, which is a means of measuring the effective voltage of an AC wave. A true sine wave, however, rises and falls on a curve, peaking at 170 volts. A modified sine wave results in a square wave, with peaks at 145 volts. Many of the inverter/chargers used to power modern motorhomes use true sine wave technology.

Energy cannot be created, but it can be transformed from one form to another. An inverter, as we’ve noted, transforms 12-volt-DC battery power to 120-volt-AC power. Running more accessories or running them for longer periods requires a fairly large battery bank. Drawing 10 amps of 120-volt power for one hour requires 1,200 watts of power for that hour, which is equivalent to 100 amp-hours at 12 volts from the batteries.

Inverters, however, are not 100 percent efficient, so the efficiency rating should be checked to verify a unit’s true output. For example, an inverter with a 90 percent efficiency rating will need approximately 110 amp-hours from the batteries. Battery banks should never be drawn down below 50 percent charge; doing so will drastically shorten the batteries’ life. Therefore, a 220-amp-hour battery bank will be needed to power that load for one hour, which is about the rating for one pair of six-volt batteries. After running that load for one hour, the battery bank will be depleted and will need to be recharged.

Manufacturers generally supply inverter-equipped coaches with four to eight batteries. A typical bank of four six-volt, golf-cart-style batteries will provide 440 amp-hours at 12 volts. Applying the 50 percent rule, 220 amp-hours are available. Additional batteries will increase the potential runtime, which is why inverters typically are used for low-power, long-duration loads or medium-power, short-duration loads. A 3,000-watt inverter that must pull only 3,000 watts for a few minutes will put less of a strain on the batteries than a 2,000-watt inverter running at half capacity for an hour.

By the way, residential refrigerators, which are becoming more popular in motorhomes, are widely misunderstood to be energy hogs. They consume medium power, but their efficiency is so good that the actual runtime is minimal, and the electric consumption isn’t as much as one would think.

A modified sine wave inverter can be replaced with a true sine wave inverter of the same size, or with an inverter that has a larger output capacity. Larger inverters require more battery current to achieve higher output levels, so it might be necessary to upgrade with heavier-gauge inverter-to-battery cables. Consult the inverter’s manual for the correct size. If you keep the same size inverter, the existing cables should be fine, but verify that with a quick check of the manual. Also check the size of fuse that must be installed in the battery cable to protect the inverter and DC cables in case of a short circuit. The existing AC circuits should be sufficient unless a fairly small inverter is replaced by one with a much larger output. In that case, treat it as a new installation.

Upgrading an inverter generally means replacing its remote display panel with a new one. Often it’s possible to use the existing communications cable from the inverter to the control panel, but some cases might call for running a new cable. Again, refer to the owners manual or call the manufacturer.

Adding an inverter to a coach that doesn’t have one will require some research and planning. First, consider the inverter’s mounting location. Inverters generate heat, so they must be located in a large basement area or in a smaller compartment that is dry and can be vented.

Placing the inverter close to the batteries helps to reduce the voltage drop when large amounts of current pass through the battery cables. If the inverter must be placed farther away from the batteries, the gauge of the battery cable may need to be increased. However, never place the inverter in the same compartment as the batteries; the inverter could ignite explosive battery gases.

Once the inverter location and cable requirements are established, plan how to connect AC devices to the inverter. A small inverter may be used to power a television and a few entertainment system accessories, such as a DVD player and a satellite receiver. If the inverter has a built-in receptacle, you can locate it close to the entertainment system, making certain to run adequate DC wiring to the batteries. A small, 450-watt inverter may require only 8-gauge or 6-gauge wiring, depending on the length of the cable run.

If the inverter has a fused, hardwired output, you may choose to place it in a basement area closer to the batteries to minimize the voltage drop, and then run longer AC wiring to a receptacle near the entertainment system.

Some inverters do not have pass-through AC power, which means devices must be unplugged from their normal outlet and then plugged in to the inverter. Certain inverters, such as the Magnum Energy model MM612, do have pass-through power and can be located anywhere in the AC line and controlled via a remote display panel.

Larger inverter/chargers are capable of powering many circuits. Typically, the main breaker panel will feed these inverter/chargers with 120-volt power to use as bypass power and to charge the batteries when generator or shore power is present. The inverter/charger’s output is sent to a subpanel; any circuits that are fed by the inverter — such as the microwave, various receptacles, and a residential refrigerator — are connected to the subpanel. If a motorhome does not currently have an inverter, a subpanel will have to be added, although in some cases it’s possible to replace the main breaker panel with a combination breaker panel that has a subpanel built into the box.

An inverter can perform only as long as it has adequate battery power. Unless a small inverter is being added to power an entertainment system, more battery capacity likely will be needed. The size of the battery bank must be proportionate to the intended load and how long it will run. Once it’s determined how many batteries are needed, make sure there is enough space for them. Wet-cell batteries give off hydrogen, so they must be located in a vented area. AGM batteries do not give off significant amounts of hydrogen, so there is more tolerance in locating them; however, they cost more.

The most common option is a remote display panel that controls various inverter functions and setup parameters. Without it, most of the inverter’s features cannot be accessed. The panel is connected to the inverter by a communications data cable, most likely a four-conductor, twisted-pair cable or a category 5 (cat 5) cable similar to that used in a computer network. Often, other accessories also can be controlled from the panel. Magnum’s Battery Monitor Kit displays exact state-of-charge information for the battery bank. Automatic generator start (AGS) modules can be used to monitor battery voltage and automatically start the generator should the battery voltage drop below a set level. The modules also can start the generator should shore power fail while the air-conditioner thermostat is calling for cooling. This is an invaluable feature for people who leave their pets unattended in the motorhome.

Inverters also have the ability to communicate with an energy management system (EMS). The Power Control System (PCS) by Precision Circuits can communicate with and control a Magnum MS or MSH (Hybrid) series inverter to provide load assist in addition to load shedding. When power demands exceed the available power, a basic EMS will temporarily shed circuits to prevent tripping breakers. The load assist feature of the PCS will first switch on the inverter to provide power for any inverter-supplied circuits, reducing the load on the normal incoming AC power so that circuits such as those involving air conditioners do not necessarily have to be shed.

Most inverter/chargers contain an internal transfer switch to toggle between incoming generator or shore power and power created by the inverter and battery bank. It’s an either/or situation, switching back and forth between the two sources. Magnum’s MSH series of inverter/chargers are hybrid in that the inverter can boost and supply extra power in addition to any incoming power. So an 8,000-watt generator, for example, can meet the same needs as a 10,000-watt generator by allowing the inverter to provide the extra 2,000 watts. This reduces the generator’s fuel consumption and also allows the manufacturer to install a smaller unit that weighs less. The operation is controlled by the Precision Circuits EMS programming.

An inverter makes sense for a motorhome. It eliminates the need to run the generator for small tasks. It provides quiet, emission-free power for light loads over a long period or for heavy loads for a short duration. The communication with AGS modules and energy management systems helps to keep RVing simple and enjoyable.

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