by Bill Moeller
60 amp-hours ÷ 4 hours = 15 amps
Here two 120-watt, 7. 5 amp panels would most likely do the job for you.
SOLAR PANEL TESTS
We conducted three separate tests with our solar panels: two tests to determine the efficiency of our solar panels, and the third test to check their battery-charging capabilities.
Test 1: Solar Efficiency
The object of this test was to determine the actual maximum amperage that our solar panels were capable of delivering on a bright, sunny day.
Conditions
Our solar panel array consisted of two 40-watt panels and one 48-watt panel, which had a combined total of 8. 12 amps maximum output. The panels were installed on the sloped roof of our fifth-wheel trailer.
We ran the first test at a park near Carlsbad, New Mexico, on March 10,1998. Since this is about the halfway point between the winter solstice and the summer solstice, we thought this time of year and location would give us an accurate average output of our panel array. The sun rose at 6:22 a.m. Mountain Time and set at 6:10 p.m., for a total of almost 12 hours of sunlight.
We started our test at 7:00 a.m. and ended at 5 p.m.—10 hours of charging time. For this test, we didn’t charge our batteries. Instead, we turned on several lights to create a good load and force the panels to deliver their full amperage.
Results
Table 12-2 and the graph show the output we recorded each hour throughout the day. In spite of the small amount of charging per hour, the total of 47. 1 amp-hours was impressive, translating to an average hourly output of 4. 71 amps and an efficiency of 58%.
Table 12-2. Solar Panel Output for a Typical Day
You may notice, either from the table or the graph, that the panels never produced their maximum output. There is a reason for this. Although the panels were mounted on the 15-degree roof slope of our fifth-wheel trailer, the rear of the trailer was pointed to the northwest, so the panels didn’t have a good sun angle. As you can see, this cost us 1. 2 amps at maximum output. In spite of this, the panels still produced a good output.
A graph of the typical solar output of our solar panels.
Test 2: Solar Efficiency
The object of our second test was to further determine the panels’ maximum amperage output.
Conditions
We conducted this test a month later in Midland, Texas, this time ensuring that the rear of our trailer pointed to the south so the panels were at the proper angle to the sun. We used the same panels: two 40-watt panels and one 48-watt panel.
The sky was clear with no clouds when we began the test, although a light cloud cover appeared later. The sun rose at 7:02 a.m. Central Time and set at 7:56 p.m., for a total of almost 13 hours of sunlight. We began recording readings at 8 a.m. and ended at 7 p.m., for a total of 11 hours of testing time.
Results
During this test, our amperage output total was 71. 2 amp-hours, with an hourly average of 5. 93 amps and a 73% efficiency. Between noon and 3 p.m. our panels delivered over their maximum rated amperage output of 8. 12 amps. This happens when there is a light cloud cover but the sun still shines through brightly. The sunlight bounces between the ground and the clouds, which can increase the intensity of the sunlight.
Table 12-3. Maximum Solar Panel Output for a Typical Day
The results of these two tests show the high efficiency of solar panels, and their capability to recharge batteries quickly during the charge period if you monitor discharges to match the solar output.
Test 3: Charging Batteries
On June 28,1998, while in Albany, Oregon, we ran a test to determine the charging capability of our solar panels.
Conditions
We were still using the same solar panels, two 40-watt panels and one 48-watt panel, with a maximum amperage output of 8. 12 amps and a total wattage rating of 128 volts. We also used an early shunt-diode regulator.
Our batteries were discharged to -48. 9 amp-hours according to the amp-hour meter. The trailer was oriented west-northwest so the panels were pointing more or less toward the morning sun. Sunrise was at 5:24 a.m. Pacific Time and sunset was at 6:10 p.m. The sky was clear with bright sun and no clouds came up during the day.
We turned on the solar panels at 8 a.m. and recorded until 5:50 p.m., when the batteries were completely charged. The total time was 9 hours and 50 minutes. This may seem like a long time, but the batteries were discharged to 30% of capacity, and we were in Oregon, a poor area for maximum sunlight because of the low angle of the sun.
Results
Our average hourly output for this test was 6. 27 amps, with an average hourly efficiency of 77. 2%. It was interesting that during the peak hours of charging from 10 a.m. to 2:30 p.m., the charging amps ranged between 7. 1 and 8. 1 amps, close to the maximum panel output. Consequently, 27 amp-hours (55% of total charging) were restored to the batteries during this period.
There were two items of interest during this test. First, there was a slight drop in amperage at 12:30 p.m. As there was no apparent change in the sunlight at the time, we believe this was due to the panels getting too hot, which can cause voltage drop. A good breeze came up shortly thereafter, which cooled the panels and restored the efficiency. The second item was the drop in charging that occurred at 4 p.m., when without thinking someone turned on four interior cabin lights. Note the large drops in charging (1. 2 amps per light) and voltage because of the load.
We think this test stretched the limits of charging with the panels we were using then and the large discharge of 30%. It would have been better for us to limit our DOD to around 20%.
SOLAR CONTROLLERS
A controller, or regulator, controls the amount of voltage produced by the panels and holds it to the proper level to effectively and efficiently charge the batteries. The controller also prevents a reverse flow of current at night when the panels are not producing (otherwise, the batteries would discharge as the current flowed back into the panels). There are two basic types of controllers—series-pass controllers and shunt-type controllers—and they come in a variety of amperage outputs, ranging from several amps to 40 amps.
Table 12-4. Charging Test with Three Solar Panels
Series−Pass Controllers
A series-pass controller is wired into the charge circuit, on either the positive or negative lead, in series with the battery. It turns off current flow when the voltage reaches a certain set value, when the batteries are charged. When the batteries are discharged, and the voltage drops to yet another set voltage value, the controller turns the current back on. Usually a series-pass controller contains a relay, which switches the current flow off and on. Some series-pass controllers use MOSFETs (metal oxide semiconductor field effect transistors) or SCRs (silicon-controlled rectifiers) instead of a relay to switch circuits on and off in response to a control signal.
Shunt Controllers
A shunt controller is a solid-state device installed across the positive and negative leads from the panels. It diverts the current flow back to the panels once the set voltage is reached and the batteries are charged. In effect, a shunt controller works by short-circuiting the panels. Shunt controllers also employ a diode in the positive lead to prevent reverse current flow at night and when other charging devices are used (e.g., an engine alternator, converter/charger, or portable charger).
Selecting a Controller
When selecting a controller, look for one with a high enough amperage output to handle (1) your present needs, (2) any future additions to your panel array, and (3) those times when your panels may deliver more than their rated output. A good rule of thumb in choosing a controller is that the controller should be able to handle current at least 25% higher than the panel array amperage output.
Although there are two basic types of controllers, you have several options to choose from.
Multistage Charging
Multistage charging is available with both series-pass and shunt controllers. It uses the same charging stages we
’ve become familiar with: bulk stage, absorption stage, float stage, and equalization stage. Not all controllers utilize all four stages, and some offer the equalizing stage as automatic or manual, or both.
Pulse-Width Modulation
A technology available in many regular and multistage controllers is pulse-width modulation (PWM). PWM maintains the full flow of current (instead of tapering off as the battery becomes charged), but it varies the length of time the full current is applied using pulses. These pulses are created by switching the current on and off, and varying the lengths of time, from longer On periods to longer Off ones. Switching and pulsing occurs very rapidly, usually in microseconds. The advantage is the batteries have a higher charge acceptance rate with pulse charging because each pulse is at the highest current rate the panels can provide. With PWM, the current from a solar array varies according to the battery’s condition and recharging needs.
Two-Stage Charging
Several controllers now offer two-stage charging with a bulk stage and an absorption stage for the best possible method to charge your batteries.
MPPT
A few controllers have incorporated a new technology called maximum power point tracking (MPPT). MPPT converts excessive voltage available from solar panels into additional current for battery charging. This current "boost" can be 30% or more.
MPPT technology is similar in purpose to an automatic car transmission, which shifts to keep the engine at the best rpm. During the day, as lighting conditions change and affect the maximum power available, the MPPT controller adjusts many times a minute to keep the charging rate at the highest amperage output possible. In other words, it is tracking the maximum power point, thus the term MPPT. These current increases are highly variable, and depend on the intensity of the light, the panel temperature, and the discharge level of the voltage of the batteries. The colder the panel temperature and the greater the difference between the panel voltage and the battery voltage, the greater the boost, or gain, will be of the charging amperage. The current increases tend to be greatest in cooler conditions when days are short, the sun is low on the horizon, and batteries may be more highly discharged.
To illustrate how this feature works, we’ll use a 75-watt panel with a rated peak power voltage of 17 volts and a peak power amperage of 4. 4 amps. The batteries have been discharged to 12. 2 volts, a DOD of about 50%. Because of the difference between the rated output voltage of the panel and the battery voltage, 21. 12 watts are, in effect, being wasted:
17 volts (peak power) − 12. 2 volts discharged = 4. 8 volts
4. 8 volts × 4. 4 amps (peak power) = 21. 12 watts
We did a lot of our early tests with MPPT on the Solar-Boost 2000E, made by Blue Sky Energy (www.blueskyenergyinc.com), which is a very good controller. It has a multistage PWM charging system, and was one of the first to have this sophisticated means of control. Most recently, we tested a new controller, the Heliotrope HPV-22B (www.heliotrope-pv.com). It is also a very good controller, and we are still using it.
Both controllers have LCD panel readouts that show charging voltage, the amperage coming from the panel array, and the total boosted amperage. The Heliotrope controller also has a three-stage (PWM) charging system with bulk, absorption, and float stages. It is also designed to work with a converter/charger when plugged into shore power. Both controllers offer temperature sensors that control the charge voltage.
The HPV-22B has two new features that improve on its predecessor, the HPV-220: an On/Off switch and a Dry Camp/Shore Power switch. The On/Off switch means you don’t need to install a separate switch in the positive lead from the panels to turn them on or off. There are times when you don’t want the panels to operate, especially when you want to rest your fully charged batteries after a recent charge.
The Dry Camp/Shore Power switch lets you engage or disengage the float mode. When set to the Dry Camp position, the controller charge set point is 14. 3 to 14. 4 volts to allow full charging of the batteries from the panels. When in the Shore Power position, the float mode is engaged and holds the voltage at 13. 2 volts. If a small load (or loads) comes online, the controller allows the required amperage to flow, as long as the float voltage can be maintained. The important thing is that heavier loads can sometimes trip most controllers to return to the high bulk stage voltage. If after being fully charged the batteries are subjected to 14. 4 voltage for long, they can be overcharged, causing the plates to dry out.
The Heliotrope HPV-22B solar panel controller.
We installed an HPV-22B in our rig for testing. It controls our two 85-watt and one 100-watt panels. Recently while driving down I-5, and with fully charged batteries, we left the switch set to Shore Power. Periodic monitoring showed that the controller was holding the batteries to a constant 13. 2 volts with no amperage output. Yet when we turned on several lights and the water pump, the controller delivered the amps necessary to handle the loads. Pretty good for a solar panel controller.
Table 12-5. Heliotrope HPV-30 Evaluation1
1. Date of test: 9/20/05; Location: Springfield, Oregon.
2. Insolation readings based on 1,000 watts per square meter.
3. We changed the interval between readings so that we could determine the exact time of changeover from bulk to absorption and from absorption to float.
While we were testing the HPV-22B, we also tested Heliotrope’s new HPV-30. This controller can handle panels up to a total of 30 amps, and in a motorhome, can also charge the engine battery (the SLI battery) as well as the house batteries.
During the tests we experienced a remarkable MPPT boost of 71.4%. We attributed this large increase to the 100-watt panel’s high peak power voltage of 21.5 volts and the atmospheric conditions that morning.
Our Experience
We used a shunt controller during the early years and were reasonably happy with it. However, after we installed an amp-hour meter, we discovered the panels would not charge the batteries fully enough to "zero" the meter (see Chapter 9). The problem was that when the battery voltage reached the upper set limit, the panels shut off until the battery voltage had dropped to the lowest set voltage. The charging process wasn’t held at the set-point voltage long enough to completely charge the batteries, and the amperage wasn’t allowed to taper off. The result was that the batteries were only getting about 75% of their total charge. We eventually solved the problem with a multistage controller.
INSTALLING YOUR SOLAR POWER SYSTEM
Properly installing your solar power system takes careful thought and planning. Before you climb on the roof, solar panel in hand, you’ll need to make some decisions.
Placement and Orientation
How and where do you want to place the solar panels on the roof? If you are only installing a two-panel system, the answer is fairly easy, but if you are planning on four, six, or even more panels, then it takes more planning. You have limited space, and there are other things on the roof to take into consideration. Do you have a satellite dish, for example, or an air conditioner or swamp cooler? Shadows from any of these items will affect solar panel performance. Once during a test, Bill left a pen lying on the panel while he was taking readings. Removing the pen from the panel caused an almost 2 amp jump in output. One of the most important points is not locating panels too close to other structures on the roof.
You also have to decide whether your panels should go across the width of the RV or along its length. On our first panel-equipped RV, we installed the panels lengthwise because the roof had a 15-degree angle as it rose from the rear to the front. (It was a fifth-wheel trailer, and the slope provided the necessary headroom in the bedroom). So the roof of the front area was higher than the roof along the rest of the trailer; in effect, the roof had a permanent tilt. When camping, we always tried to find a site that ran north and south so we could back the trailer in facing south.
Three 100-watt solar panels installed lengthwise on the roof of a friend’s motorhome.
Tilt
&n
bsp; Should you tilt your panels and follow the sun? In the winter it might be a good idea to elevate one side of the panels between 15 to 25 degrees so they face toward the south. This orientation will make your panels more efficient, but we don’t see the need in the summer. We have seen rigs with their panels mounted on racks that allow them to swing around to track the sun. Frankly, this just seems like too much work to us, plus we don’t really think it’s necessary. Also, when panels are tilted up, they can be more easily damaged by the high winds that occur during the winter months, particularly in desert areas.
A problem we experienced in Alaska is that solar panels there do not deliver their full power at any time if they are mounted flat on the roof. Although the summer sun shines for many hours, in some areas as many as 20 hours a day, it is very low in the sky most of the time, similar to the early and late hours of the day in the lower forty-eight states. Tilting and tracking the panels will help a lot, although the amps delivered are still reduced because of the dense atmosphere. If you tilt your panels, you can adjust them for your latitude to make them more efficient, which is good for long-term stays.