By Arlen Harbaugh and Jim Norcutt
See Photos Below
Loudoun County Aeromodelers Association (LCAA) is a club of approximately 150 members located in Northern Virginia. We fly in a nature preserve, and there is no AC power at the flying site. As more and more flyers choose electric planes, we’ve seen a big increase of members bringing 12-volt storage batteries to charge their flight packs. They also charge directly from their car batteries, a practice we wanted to discourage for safety reasons. A field charging station would be a great convenience for the electric flyers.
The idea of using solar power emerged in discussions with the preserve manager about how the club could provide charging power at the field. Another option is a portable gas-powered generator, but this has the disadvantages of fumes and noise, although it is true that there are some fairly quiet generators. Considering that the preserve manager felt that the solar approach was the most appropriate in the spirit of the nature preserve, we set out to evaluate the feasibility of a solar charging station.
We searched the Internet for information about other flying clubs that have made solar charging stations, and found nothing other than a couple of sites that were using very small systems, primarily for charging radio system batteries. We did find a lot of information about practical solar power, and it turns out that Recreational Vehicle (RV) users make a lot of use of 12-volt solar power systems similar to the system we were envisioning.
Our fundamental approach is to use solar cells to charge a large 12-volt storage battery at the field. The storage battery is then used to power the chargers for plane batteries. To start, we needed estimates of required power for charging our plane batteries. Power is measured in watt-hours. Power requirements were estimated from observations of the planes we typically saw at the field. We came up with several scenarios for the sizes and quantities of batteries that might be expected to be charged on a Saturday or Sunday, which are our high-use days. The power required from the storage battery depends directly on the number of cells to be charged and their amp-hour (Ah) capacity. The power to charge the plane batteries also depends on the degree of discharge of the batteries, but we assumed the batteries to be charged were empty.
For example, to charge a 3S 2Ah LiPo battery requires approximately 2Ah at approximately 12 volts. The charging voltage varies from around 11.7 volts at the beginning of the charge up to 12.6 volts at the end of the charge. This is 24 watt-hours of power, which is 2Ah from the 12-volt storage battery. Similarly, a 6S 4Ah LiPo battery requires approximately 96 watt-hours from the storage battery (4Ah at approximately 24 volts), which is 8Ah from the 12-volt storage battery. The LiPo charger raises the voltage supplied by the storage battery to the level needed by the LiPo battery; however, the current from the storage battery must be increased to supply the higher voltage. Thus, twice the current from the storage battery is required to charge a 6S LiPo battery as is required to charge a 3S battery of the same amp-hour capacity.
Throughout the design process, there was debate among club members about the required capacity of the storage battery. Although we never obtained consensus on what we might need, we all agreed that the demand would grow. Questions like, “What happens if more big electric users start using the charging station?” and “Will the storage batteries recharge fast enough if we have a heavy usage weekend?” suggested that there was considerable potential for growth in demand. We took the approach of starting with a moderate-sized system that could be readily expanded. We decided to design a system that could supply 200Ah to users in a week.
We chose to use a storage battery with a capacity of 400Ah based on the desire to:
Supply the estimated power needs for one week without the need for any charging of the storage battery. This is a very conservative objective because (as discussed in the next section) we do not expect that we will ever go a full week without some charging from the solar cells. Avoid discharging the battery more than half its capacity for longer life.
Although Absorbed Glass Mat (AGM) sealed batteries are frequently used in solar applications because of long life and no maintenance, they are twice the cost of flooded cell batteries. Several of our members use the 12V AGM batteries for powering their chargers and have had very good service from them. We decided to make our storage battery from some low-cost flooded cell golf cart batteries. Like the decision to design for 200Ah in a week, the storage battery decision was debated quite a bit.
The selected golf cart batteries supply 6 volts and have a capacity of 220Ah each; therefore, we purchased four batteries and connected them in a series and parallel combination. Using LiPo terminology, we connected the golf cart batteries as 2S2P. These flooded batteries require the electrolyte level to be checked monthly. We purchased these batteries from Sam’s Club.
A single solar cell, also known as a photovoltaic cell, produces only 0.5 volts and not much current. They are commonly provided in panels that combine cells in series and parallel to produce higher voltage and current. A typical solar panel for charging 12 volt batteries is designed to produce around 17.5 volts at room temperature. The required charging voltage at the battery is around 14 volts. This leaves a margin of 3.5 volts to compensate for voltage losses due to resistance of the interconnecting cable, the charge controller, and temperature (the output voltage of solar cells decreases with temperature).
Given the design goal of 200Ah in a week, we looked for a solar panel that could provide at least 30Ah a day into the 12-volt storage battery. Solar panels are rated based on the current they can supply in full sun. To know the necessary full-sun current, we needed to know how many hours of sun we could expect. We found information about the amount of solar radiation on the Internet. Solar radiation is measured in units of kWh/m2/d (kilowatt-hour per square meter per day); however, 1 kWh/m2/d equates very closely to one hour of full sun at Earth’s surface. So, the value of kWhh/m2/d value can be viewed as the equivalent number of hours of full sun per day. This number can be multiplied by the full-sun current of the solar panel to get average expected daily Ah. For example, if there are five equivalent hours of full sun in a day (5 hr/d), it doesn’t matter if the five hours of equivalent sun occurs as 10 hours of half sun or five hours of full sun—the daily output would be the same.
The National Renewable Energy Laboratory has data from actual solar stations around the country. There is a station in Sterling, VA, that’s very close to our flying site. This station showed average spring and fall values around 5 hr/d and a summer value of 5.6 hr/d for the period of 1961-1990. A more general table provided by some other solar sites showed that Washington, D.C., receives about 4.7 hr/d in summer and 4.2 hr/d in spring and fall. We used these more conservative values to calculate that we required the full sun output of the solar cells to be 7.2 amps to obtain the required 30Ah per day during the 4.2 hours of full sun that we could expect in spring and fall. We plan to take the system down in winter.
We chose a Kyocera KC130TM 130 watt solar panel that can supply approximately 7.5 amps at 17.5 volts in full sun. This results in about 220Ah in a wee
k in spring and fall, and 247Ah in summer. The panel measures 56 inches x 26 inches and requires a firm support, so we built a structure using angle iron from an old bed frame. The panel can be removed for winter storage.
The solar panel cannot be directly connected to the storage battery because it could overcharge the battery. Solar systems use charge controllers that are similar to our LiPo battery chargers. When the storage battery is close to being fully charged, it switches to Pulse Width Modulation (PWM) to reduce the total current being added to the battery. It also disconnects the solar panel when no current is being produced. Solar charge controllers typically have settings for use with the different storage battery types such as the flooded cells of our golf cart batteries or sealed AGM batteries. We purchased a MorningStar ProStar 30, which can handle up to 30 amps. This will allow us to add more panels without replacing the charge controller.
Connection between the charge controller and the solar panel
We chose a location for the solar panel in the field across from our runway that receives full sun. The preserve staff mows this field every three years, and it is typically grown up with weeds about knee-high. The goal is to have the panel be invisible to onlookers. The location is about 160 feet from the charging station where the storage battery is located, and it required us to dig a trench across the runway to bury the cable. Given the length of the cable, voltage loss would be a problem if the gauge of the cable were too small. We considered a 2 volt voltage drop to be acceptable. Just as we chose a 30A controller anticipating additional solar panels as demand grows, we chose to install a cable that could handle additional current to avoid having to bury more cable later.
We decided to use a cable that would drop no more than 2 volts with a current of 15 amps, which means the resistance should be 0.13 ohm. The distance the current must flow is actually 320 feet because the current must go to and from the solar panel, so the 0.13 ohm translates to 0.0004 ohm/ft.
6-gauge wire has a resistance of .0004 ohm/ft, but the cost came out less by using multiple 12-gauge wires in parallel. 12-gauge house wire has a resistance of .0016 ohm/ft. We used six in parallel for the positive and six for the negative. Six wires in parallel have a resistance of .00027 ohm/ft—better than our 0.0004 ohm/ft objective. We installed four 12-gauge, underground house cables. Each cable has three 12-gauge wires (black, white and bare) giving us the desired 12 wires.
We built the charging station at home where tools and AC power were more convenient. The station includes a large table, a power strip for connecting the chargers, and a shelter for the storage battery underneath. The materials are wood, screws, bolts, shingles, and roofing nails. We used treated lumber that is environmentally friendly and satisfies the “green” requirements of the preserve manager.
Much design consideration was given to four factors: the convenience of connecting the chargers, the amount of charging space for each user, the physical stability of the structure and the security from curious eyes and hands for our controller and storage battery investment.
We built a 33-inch by 8-foot tabletop using 1×6 boards, with a 2×4 frame and 4×4 legs. The legs are sunk about 6 inches to further stabilize the structure. A “doghouse” enclosure, complete with a shingled roof, was mounted on 2×6 rails between the legs and under the table top to support the 250 pounds of storage batteries. The end result was a 350-pound structure (including the batteries) that is very stable. Additional space for future expansion was included in the final size of the enclosure.
The side of the “doghouse” opens down to the ground, forming a ramp for easier access to each 62-pound storage battery. The end door is secured with a lock, and provides for regular access to the controller, our testing equipment, and the side ramp latches. The floor and sides have enough small gaps to allow ventilation for gassing from the flooded batteries.
The size of the tabletop provides room for four or five work stations on either side, with the power strip running lengthwise down the middle. We added a rail lip around the outside edge to contain errant batteries and chargers from falling. The 1×6 lumber top allows small gaps for rain drainage (the shingle roof underneath protects the batteries and controller).
Photo 1 shows a close-up of the power strip. The power strip was constructed from two boards that were slotted lengthwise to sandwich the two 6-gauge copper bus wires. Holes were bored through the boards to allow battery clip connections to each bus wire from either side. Brass tubes were inserted with through holes and soldered to each bus to provide banana jack connections for a second method of attaching the chargers. A craft wood-burning iron served to label the bus and jack connections with the appropriate plus and minus voltage indicators. The design of recessing the bus wire and the jacks provides a safety barrier from accidental shorting. A cap board on top of the power strip provides some shelter from the elements.
We brought the charging station to the field in two pieces, the shelter and the top. Installation required digging four holes for the 4×4 posts, and screwing on the top. Installation was quick and easy (Photo 2).
Installation of the solar panel required digging four holes for the 4×4 posts and attaching the iron frame to the posts (Photo 3). We used some care to align the panel so that it pointed toward the sun at midday. The front of the panel is mounted a few inches above the ground. The vertical angle can be adjusted to match the sun angle as the season changes by using different holes in the rear mounting arms and rear post; however, information on the Internet indicated that seasonal adjustment is not very important.
We rented a power trenching machine, and dug an approximately 6-inch-deep trench across the runway for the cable (Photo 4). The width of the trench is only 4 inches or so. The digging was not as destructive to the runway as we feared, but it was a lot of work. Digging, laying the cable, and filling the trench required 6 hours of work by three people.
The solar panel produces slightly more than it is rated for—8 amps in full sun. We have power meters monitoring the load from the system and the current from the solar panel. The meters are made by Medusa Research, and they can accumulate up to 999Ah, which is important so that we can go a long time between readings. We also installed a 50 amp fuse between the storage battery and power strip to protect against a short circuit.
Photo 5 shows the completed charging station in use. Club members are very happy to have the charging station, and it is being used a great deal. The only “complaint” so far is that we need to add a shade because the station is exposed to the heat of the day. During the first five weeks of use (late August through September), the weekly load supplied by the storage battery averaged 134Ah. One week we used 224Ah, so we may need to add another solar panel early in the next season.
The golf cart batteries have more voltage drop under load than we expected. This limits the output to approximately 40 amps before the battery voltage drops low enough (less than 12 volts) to cause some chargers to cut off due to low voltage. It would be useful to make it possible for users to see the meter that displays current from the storage battery. This would allow users to know if the system is being pushed beyond its limits.
Solar Panel: $600
Solar Charge Controller:
Cable from solar panel to batteries: $360
Golf cart batteries (including series/parallel interconnection cables and fuse): $465
Other materials (much of the wood was donated): $235
Trencher rental: $90
Total Cost: $1930
Solar radiation data from National Renewable Energy Laboratory: rredc.nrel.gov/solar/old_data/nsrdb/
Kyocera Solar, Inc: kyocerasolar.com/products/
Morningstar Corp.: morningstarcorp.com/en/products
Photo captions for “Solar Powered Charging Station for Electric Planes” by Arlen Harbaugh and Jim Norcutt
The power strip accommodates chargers with banana plugs and alligator clips.
The completed charging station is shown with the battery shelter doors open. The solar charge controller is mounted on the end door. This door also holds a meter for monitoring the solar AH. The yellow LiPo battery is needed to maintain the meter memory when the solar panel voltage drops to zero at night.
The solar panel is mounted in a metal frame at an angle facing the sun.
A power ditch digger was rented to dig the trench for the cable going from the solar panel to the storage battery.
The solar panel can barely be seen on the far side of the field across from the charging station.