Sun Power

Bill Estes
February 5, 2013
Filed under Feature Stories

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While motorhomes are reasonably well equipped for dry camping — boondocking, as it’s often called — many come up short of 12-volt DC reserve power except in limited use. The result may be extended use of an AC generator for battery recharging, which can be annoying.

Solar panels

Solar panels, consisting of photovoltaic cells in a rigid frame, are a popular option for providing 12-volt DC power.

A popular remedy comes from the sun in the form of solar panels. No noise, no fuel consumption and the sun’s rays keep battery banks charged to provide all or part of our daily 12-volt DC power needs. Arrays of silicon cells in each panel convert sunlight into electrical power, the amount of which is limited only by the number of panels and the efficiency of associated hardware.

Of course, there is a catch: Solar panels and hardware are relatively expensive, although the prices have come down in recent years. Also, your battery bank must be large enough to store enough power to get you through the dark hours.

A solar system consists of two or more panels, a charge controller, a monitor panel and associated wiring. The need for 12-volt DC power can vary widely depending on size of the coach, the time of year and on personal choices, such as how much time is spent in front of a TV during dark hours.

The panels, consisting of photovoltaic cells in a rigid frame, are available in various physical sizes and output ratings, the most popular being those ranging from 50 to 150 watts. The wattage numbers don’t take panel efficiency into account, and the realistic output rates vary from one panel to another, necessitating comparison when shopping.

One high-efficiency example, the GS100 panel offered by AM Solar (www.amsolar.com), is listed at 100 watts and 5.8 amps maximum output (full sun) at 17.7 volts. The actual battery charge voltage is regulated downward to levels varying from 13.2 to 14.8 depending on battery type and charging conditions.

The Solar Boost 2512iX charge controller

The Solar Boost 2512iX charge controller can increase charge current up to 30 percent or more.

Sophisticated controllers are designed not only to create multistage charging — bulk (fastest rate), acceptance (reaching full charge) and float (DC voltage is reduced after batteries are fully charged) — but are capable of boosting the charge rate by operating the solar panels at their optimum voltage output. They also take battery temperature into account.

Size (and cost) of the system must be determined by individual needs, which can be based on actual use or by calculation using a vendor website as a guide. An actual use situation involving two minimum-size Group 24 batteries (85 amp hours each) could consist of dry camping for two days. Batteries should not be depleted beyond 80 percent of their rated capacity, so the available amp-hour total is 136 (85 x 2 = 170 x .8 = 136).

The motorhome owner would record how long it takes to draw the batteries down approximately 80 percent (11.8 volts, measuring with no load on the batteries). If it takes two days, your usage is about 68 amp hours per day — maxed out in two days, or less. Limiting depletion to 50 percent (12.1 volts, no load) extends battery life.  

Blue Sky displays battery voltage

This monitor from Blue Sky displays battery voltage, output current and charge control system status.

Assuming that each 100-watt panel will deliver about 30 amp hours per day, three panels would be required to provide all the power (except on cloudy days) needed in this example. Larger battery banks may range upward to 1,000 amp hours, calling for quite an array of solar panels.

For motorhomers who have limited funds and/or roof space, fewer solar panels can be used, providing only part of the power for battery charging, while an efficient multistage converter powered by an AC generator provides the remainder. 

Check specs to make sure your converter truly is multistage, rated for at least 45 amps output, or extended generator running time will be needed.

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