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Wednesday, January 17, 2001

Is Solar Battery Charging in Your RVing Future?

By Gary Bunzer

One of the newest forms of advanced battery charging technology to come upon the RV scene in recent years, and certainly one that will likely become more prevalent as time goes on, is the science of photovoltaics (PV) or solar energy. Free electricity from the sun is becoming more integrated into the RV industry each year. In recent years, a valid interest has developed at the manufacturing level as well as in the aftermarket. The primary use of solar power in recreation vehicles is for battery charging.

The terms “solar power” and “RV” have certainly been mentioned sporadically in the same paragraph since the 1970s, however, not many people in the RV industry gave solar power much credence back then. Except by a relatively small group of believers, the value of solar power was not truly embraced by all during its earliest forays into the realm of RVs.

Recently, however, advances in technology have legitimized the viability of adding solar power configurations to the 12-volt DC system found in RVs. Today, more and more RV owners, especially full-timers and those who like to travel off the beaten path and dry camp, are capturing and utilizing that free energy from the sun.

The science behind photovoltaics and the refinement of solar cell technology is quite interesting and warrants additional study should you feel so inclined. For our purposes here though, the instruction will be kept relatively brief. The accompanying diagram illustrates the process of converting light into electrical energy. Keep in mind, improvements continue to occur rapidly within the science of photovoltaics.

The making of solar cells begins with the process of melting purified crystalline silicon and inducing the growth of silicon ingots in a cylindrical mold of sorts. (For an analogy, this is similar to early grade-school experiments of making simple rock candy). The ingots are then sliced longitudinally and sawed into thin wafers that are given their electrical characteristics through a diffusion process of phosphorus-doping the wafers that had been previously doped with boron. Okay, now even I'm confused!

Some wafers are cut as thin as 200-microns. Silver paste is then applied to each side of the cell, and electrical circuit contacts are formed by the subsequent screen-printing process. The cells are electrically connected together, laminated in an embedding medium, sandwiched between glass and a weatherproof rear panel, secured and framed. The larger the cell, the more sunlight it can capture. The bottom line is; the more sunlight captured, the more current is produced. The more current produced, the better we can manage and direct that energy to a purposeful end.

Major Components of an RV Solar System

Solar Modules and Panels
Mounted on the roof, (preferably an inch or two above the actual roof surface), solar panels collect the sunlight and convert that light energy into DC electrical current. Connecting individual panels into a multiple configuration will increase the current-producing capacity. The specific need of the RV (and the RVer), will dictate how many solar panels will be necessary in a given system.

Much like other electrical components such as generators, inverters, alternators, switches and relays, etc., solar battery charging components must be sized to its electrical requirements. Don’t fall prey to the common mistake of simply buying the panel that happens to be on sale or the one in stock that the salesperson is pushing. True forethought is required when designing a solar charging system for RV use.

Each solar panel consists of an arrangement of individual solar cells. Each cell produces approximately 0.5 volt DC. Therefore, a typical 36-cell panel will have a nominal voltage output of approximately 18 volts DC. Though RV solar systems are used solely to charge a 12-volt battery bank, a higher voltage output is necessary to offset the effects of degradation and voltage loss, among other factors.

Solar panels today are routinely rated and marketed by their output wattage but this number is easily converted to amp/hours by applying the Power Law portion of Ohm’s Law that states; power (watts) equals the current (amperage) times the voltage (volts). Each cell in a panel is connected in series. In systems in which more than one panel is required, (an array), the panels are connected together, typically in parallel, but sometimes in series.

The variables concerning the total output of solar panels are determined by four major factors; the number of cells in the panel, the degree of light intensity absorbed by the cells, the cleanliness of the panel and the actual temperature of the cells. Shade has a reducing effect on a panel’s ability to produce anywhere near its rated output. The time of year also plays an important role in properly sizing a system for the RV. Dry camping in winter, for example, will produce less overall current than the same scenario played out in the summer months in warmer climates. The accepted “average” efficiency of most solar panels lies somewhere between 75-80% of the rated output.

Types of Cell Technology
The three primary types of solar panels currently available include a thin film type (amorphous), a multiple crystalline type (poly-crystalline), and a single crystalline type (mono-crystalline). Physically similar; each type contains no moving parts and as long as sunlight reaches the cells, electricity will be produced. The differences are found in how efficient they are in an RV application. Keep in mind, solar panels on an RV are quite different than a system mounted on a residential cabin or home, but very little maintenance is required. Usually just keeping them clean and checking the electrical connections periodically is all that is necessary.

The amorphous panels of today have great potential, but some amorphous designs in the past suffered the negative effects of delamination and low output efficiency. Time will tell if the newer amorphous type of cell technology advances to the current level of popularity as the crystalline panels. Most of the crystalline panels, on the other hand, are built to last. Some even offer a 25-year warranty and can be effective for upwards of 35 years! Obviously, the best type to consider for an RV installation would contain a crystalline-type cell construction.

Speaking of Efficiency
What percentage of the free energy harvested will be transformed into effective and usable charging voltage and current? A few things to consider:

·       Screen-printed mono-crystalline panels retain an efficiency rating between 15 and 17%
·       Screen-printed poly-crystalline panels are approximately 14 to 16% efficient
·       Amorphous panels are only about 6 to 8% efficient

It’s probably obvious which type of technology comes with the higher cost, right? The amorphous panels may cost less per watt, but will take up about twice as much space on the roof in order to produce the same amount of voltage and wattage necessary. Serious RVers should consider one of the crystalline-type panels at a slightly higher cost. Less real estate is needed and they are considerably more efficient overall.

Efficiency-reducing factors affecting of all types of panels include minimized sun exposure and high cell temperatures. Just by being installed flat on the roof of an RV precludes all panels will enjoy only a limited amount of full sun exposure during daylight hours. That, and elevated cell temperatures both contribute heavily to voltage drop. That’s why a higher voltage output panel is always preferred.

Probing Deeper
Voltage and current loss afflicting RV solar systems is attributed, but not limited to:

  • Light Intensity. The brighter the sun shines, the more power is produced. As an example, if an installed panel receives 1000 watts per square meter of sunlight, (a Standard Test Condition*), it will produce its fully rated output wattage. If the sun produces 500 watts per square meter of light intensity, then the panel can only pump out half its rating. 100% of a panel’s output rating is basically only attainable at high noon and a few hours on either side. 
            *  Panel manufacturers apply what is called “Standard Test Conditions” (STC), to calibrate panels
                   during the manufacturing process. Solar panels are subjected to a “flash test” to calibrate 
                   the panel to deliver the equivalency of 1000 watts per square meter of sunlight intensity while
                   maintaining a cell temperature of 77-degrees F. The flash test determines a panel’s STC rating.
                   It is akin to the EPA mileage ratings for new automobiles. Not really reality, but something 
                   constant for the sake of comparison.

  • Shade. RV applications are always susceptible to periodic shading of solar panels. Antennas, satellite domes, storage pods, air conditioners, campground trees, local topography, etc., can all occasionally cover a portion of the cells on solar panels, thereby reducing its relative output rating. Tests have shown that only a partial shade covering just a few of the individual cells can reduce the panel output as much as 50%. 
    • Cell Temperature. Plainly put, the hotter each cell in the panel, the more voltage drop is experienced through that cell and subsequently through that panel. Always opt for a panel that can produce an operating voltage of 17.0 volts or more; the higher, the better.  

    • Sunlight Angle. Unless equipped with a motorized mount that can track the sun across the sky and keep the panels squarely pointed at it throughout the day, some of the sunlight will be reflected off the surface of the panel, resulting in some power reduction. Tilt mounts exist that can minimize this loss, but personally I feel it’s better to simply have enough panels up there to offset losses attributed to the angle of the sun’s rays.

    • Poor Electrical Connections. All electrical connections have an integral resistance that leads to voltage drop and reduced effectiveness and the poor connections have even more resistance. Correct conductor size and the proper types of connectors used are important considerations with any RV solar installation. Clean, dry and tight are appropriate attributes to strive for concerning all electrical connections. 

    • Parasitic Drains. Most battery display monitors and charge controllers are predestined to consume some of the electricity produced by the solar system they are monitoring and controlling. Such “self consumption” is typical for powering the LED display, internal logic calculations, etc.; current use that will never reach the battery bank, but must be factored in when “sizing” a solar charging system nonetheless.

    Charge Controller
    A crucial component in an effective RV solar charging system is the abovementioned charge controller. In some inexpensive prepackaged systems, this item may be listed as an optional “voltage regulator.” In reality, it is a must in order to protect the battery bank from overcharging. If photovoltaics are a consideration to augment dry camping potential, insist on a quality charge controller. A good rule of thumb is if the peak charging current of a given solar panel is greater than 1.5% of the total amp-hour capacity of the battery bank, it necessitates a quality charge controller.

    The charge controller is the device that monitors the voltage and current being passed to the battery bank. It is an important component in a conscientiously planned RV solar system. Without it, current from the solar panels will flow uncontrolled into the battery system. It is obvious what the result would be. Overcharging is one of the most detrimental forms of 12-volt abuse administered to battery banks.

    Three major types of solar charge controllers can be found in RV applications. They include:

    ·       Shunt-Type
    ·       Dual-Stage
    ·       Multi-Stage, MPPT

    Shunt-Type Controller. The shunt-type controller represents the most basic of types and is considered simply an on/off switch at best. It is the least expensive type and simply allows the current generated from the solar module to charge the battery at whatever voltage is available. The controller monitors the voltage and switches the charging current from the battery through a low-resistance transistor to ground when the battery voltage has reached a predetermined, non-adjustable value.

    Allowing the charging current to be dissipated and wasted through such a controller creates an inordinate amount of heat that requires heavy heat sinks to dispel. There also are strict limitations on how much current this type of controller can handle.

    Dual-Stage Controller. The dual-stage charge controller is more useful, as it eliminates the need to waste abundant solar energy as a heat by-product. This type monitors the battery voltage and applies direct charging capability from the solar system to the battery bank. However, when the full charge limit has been reached (typically at only about 95%), the controller switches its circuitry to a trickle mode that slowly tops off the battery. Though energy is seldom wasted, efficiency and battery optimizing are still lacking.

    Multi-Stage, Maximum Power Point Tracking Controller (MPPT). The multi-stage MPPT charge controller is by far the most sophisticated and most highly recommended. MPPT is a complex, processor-controlled method of battery charging that will maintain a battery bank at its highest state of charge at all times. This type of charge controller locates and maintains the peak power point of the total array output. In some cases, excess voltage from the panels can actually be used to increase or “boost” the charging current; an added plus.

    MPPT technology also protects against solar overloads, short circuits, high solar input voltages and low battery voltages. Other features of these state of the art controllers include reverse polarity protection, high temperature protection, an LCD digital readout display for battery voltage, output charging current and current logging of what is being consumed by the RV systems.

    Some controller models incorporate full-time 45-amp charging capability with temperature compensation and battery equalization. (The equalization charge is basically a very slight overcharge at regular intervals to help prevent battery plate sulfation and to allow all cells in each battery to reach the proverbial full charge). Advanced charge controllers employ user-defined parameters for battery voltage, size and type. Typically, the four charging stages found in the top of the line controllers are: bulk charge, absorption charge, float charge and the previously mentioned, equalization charge.

    Sizing a Solar System
    When sizing a system it is important to remember two things: First, the solar components must be sized to the requirements of that particular RV; second, the battery bank must be adequately sized to accommodate the current produced by the solar system. Avoid the situation in which an RV owner installs an undersized solar panel without taking into consideration the size of the battery bank and ends up with too much storage capacity and wonders why the batteries never seem to become fully charged, or worse yet, a woefully undersized battery or bank is ruined by too much charging current produced by the solar panels. In all cases, the sizing must be closely coordinated. Both the number of panels and the battery capacity will be determined by the actual consumption of electricity as predicated by the user’s habits, RVing lifestyle and the equipment found on the RV.

    Another factor to remember is that some electrical components have current ratings listed in amps, while others are rated in watts. To accurately determine the solar requirements, all consumption loads must be ascertained using the same common denominator. Since the typical solar setup is designed to charge the battery bank, it’s my recommendation to use amp/hours as that common denominator. All solar brochures will list the common requirements, but in truth, it will ultimately be necessary to go through the RV and calculate exactly what the daily individual requirements truly are.

    Calculate all the DC loads and determine exactly how many hours each day those loads will be used. If the coach is equipped with an inverter, calculate its AC loads, again determining how many hours each day those loads will be activated. List only those AC loads that will be powered by the inverter. The loads powered by an AC power-producing generator need not be factored.

    It’s also prudent to add 30% to the average daily DC requirements to cover such parasitic battery circuits such as radio and clock memories, the carbon monoxide (CO) alarm, the propane leak detector and inverter/converter simmer or stand-by circuits. Additionally, add another 40% to the AC requirements to accommodate those system losses and other deration factors such as local weather patterns and seasonal changes. Finally, add the total usage requirements together to come up with a figure that represents all loads per day. Keep in mind that every RV will be different. There is no stock answer to the question of how much and how many loads are average.

    How many solar modules would any particular RV need? By dividing the daily requirement by the wattage values of the panels, the number of panels required can be determined, but some common sense rules exist that may be helpful in determining how many panels should be considered.

    ·       If simple battery maintenance is the only consideration, perhaps just a single 100-watt panel is all that will be necessary. For most motorhomes, however, even for simplistic battery charging duties, it is actually recommended to have 200 watts of solar capability.

    ·       For RVers wanting to dry camp for the weekend in addition to charging a battery bank, perhaps three, 100-watt panels will suffice.

    ·       For gamers and TV addicts as well as music buffs (playing lots of CDs or DVDs), it might be best to bump up the solar system to a total of 400 or 600 watts.

    ·       And for die-hard dry campers, I would suggest you not leave home without 800 to 1000 watts of solar-collecting capability. Especially if you wish to power computers, printers, smart phones, tablets and other office-type equipment.

    More panels can always be added after the fact. Since they are usually wired in parallel, adding panels at any time is quite doable. The system can grow as the requirements escalate. Sophisticated “combiner boxes” are available that make the addition of more panels seamless, safe and effective.

    Storage Batteries
    Although most solar panel installations are typically wired directly into the existing auxiliary 12-volt battery system, it is important to note that not all batteries are compatible with photovoltaics. Until a few years ago, one of the most neglected aspects of RV solar systems was the lack of capacity to adequately store that free electricity extracted from the sun. It may be necessary to upgrade the battery bank in order to maximize the potential of an RV solar application, but today, many battery makers produce batteries compatible with solar applications. AGM batteries remain high on the list of recommended battery types for solar applications.

    How many batteries are required? Once the electrical loads and the number of panels have been determined, the next step is to figure out exactly how many batteries will be needed to store the necessary current for the system. Battery autonomy, or reserve factor, should also be included in the formula. The reserve factor is the number of days stored current will last without the benefit of having solar charging capability when dry camping. Usually a factor of two days is the average, but it also factors in where you travel, the time of year, the localized weather conditions, etc. In addition to the reserve factor, an additional 30% of the total is recommended as a safety buffer. Taking the daily power or amp/hour requirement and multiplying it by the reserve factor, then adding that 30% safety factor, will determine the total battery bank capacity needed. It has always been my recommendation to carry and store as much battery amp/hours as your coach can safely stow and your wallet can endure. Be alert to weight distribution! Commercial grade, deep cycle batteries, especially a bunch of them, can weigh a lot!

    The next step is to consider the type of batteries for the solar array. Realize that not all batteries are recommended for solar charging; especially automotive start batteries. True deep cycle batteries will suffice, however, RVers seriously considering delving into harnessing that free energy from the sun, should consider a properly sized AGM battery bank. Also, keep in mind that AGM batteries do not require equalization charges like flooded, lead-acid batteries. The sophisticated multi-stage charge controllers, by the way, will come equipped with charging algorhythms for all types of batteries; gel, sealed, flooded and AGM.

    Though highly sophisticated RV solar charging systems can be somewhat expensive, startup costs can be quickly offset, especially for the full-time RVer, by maximizing the entire DC electrical system, automatically. The age of harnessing the free energy from the sun is apparently upon us. Just remember, RVing is more than a hobby, it’s a lifestyle!


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