Saturday, December 17, 2011

RV Solar Electrical Systems

by Linley Gumm (ver1.1 December 2011)

Introduction: Solar panels look simple but have some subtle twists that sometimes makes understanding them a bit difficult. This article hopefully will allow those without an electrical background to gain enough understanding to understand a RV solar panel system.

Some Basics About Electrical Measures:
First of all you must understand a bit about electrical energy. Luckily, DC (direct current) systems can be readily explained by comparing them to a water system. Pressure, in a water system is measured in PSI. In an electrical system it’s Volts. The RV’s DC system is nominally said to be a 12 Volt system but actually operates in the range from 12 to 15 Volts or so.

Flow in a water system is measured in units like gallons per minute or gallons per hour. Electrical current flow is measured in Amperes or Amps for short. If you consult a physics textbook you will find that an Ampere is the flow of one Coulomb of charge per second … but forget I mentioned it.

Now for a quantity that water systems don’t use much: Power and Energy. Power is the rate that energy is either created or consumed. It’s measured in Watts. Energy is … well you know what energy is but a classical textbook definition is that energy is a measure of the capacity for doing work . Energy is really hard to define without saying Energy is … energy.

Power being created or consumed by a DC electrical system is simply the electrical pressure times the electrical flow (i.e. Volts * Amps = Watts). The Energy thus created or consumed is the power times time (i.e., Watts x time). Time can be in seconds and the answer will be in Watt-Seconds. Time can be in hours and the answer will be in Watt-hours. When the values become large, you will get to kilowatt-hours like the numbers on your electric bill.

The primary reason for solar panel(s) on a RV is to charge the house battery. Therefore some pertinent information about charging batteries is in order. There are three phases or stages when charging either flooded or AGM batteries. (An example of a flooded battery is your standard Trojan 105 battery. AGM stands for an absorbed glass mat battery.)

The first stage is bulk charging where large amounts of charging current are sent into the battery. This is intended to charge the battery to perhaps ¾ of its ampere-hour capacity. Large amounts of current can be applied but there are limits. For the flooded batteries in most LDs, Trojan says to limit the charging current for this stage to 13% of the batteries ampere-hour capacity. That figures out to be about 29 Amps for LDs. The maximum current for an AGM battery is 20% of the ampere-hour capacity or about 44 Amps. Having too much charging current is rarely a problem but if too much is supplied, it can damage the battery.
While bulk charging proceeds, the battery’s voltage increases. The second phase or stage of charging is reached when the battery’s terminal voltage reaches 14.8 Volts for flooded batteries or between 13.8 and 14.4 Volts AGM batteries (for an 80°F battery temp). When this voltage is reached, the charging source should shift from supplying lots of current to holding the battery’s voltage constant. With the voltage held constant, the amount of current that flows into the battery slowly falls. Batteries are very slow to take the last part of their charge so this stage may take several hours and cannot be hurried.

The third charging phase or stage occurs when the amount of current the battery draws in the second stage falls to a small value; 3% of the A-H value for flooded batteries (i.e., 7 Amps for LD batteries) or 5% for AGM batteries (11 Amps). At this point the charger should reset its constant output voltage to the batteries “float” voltage. This is the voltage which should be used for long term charge maintenance. It is 13.2 Volts for flooded and 13.5 Volts for AGM (again for an 80°F battery temp).

When one reads the fine print for other temperatures one should adjust these voltages by -0.017 Volt per °F away from 80°F. (The negative sign means the voltage falls with increasing temperature.) This results in the following set of values (rounded to 0.1 Volts):

Temp. °F
Flooded Battery
AGM Battery

Charge Volts
Float Volts
Charge Volts
Float Volts

Solar Panels:Table 1 Battery charging voltages for various temperatures

When you look at a solar panel, you will see that it is a regular array of black areas. Each of those areas is called a cell. The cells may be rectangle or round or square with rounded corners depending on choices made by the manufacturer.

When bright sunlight falls on a cell it creates a current flow. The cells in a RV panel are connected in series, just like miniature Christmas tree lights. When bright light falls on all of the cells, the current that is created in each individual cell is passed from cell to cell and becomes available at the panel’s terminals. If a large leaf falls on the panel and shades just one of those cells, that cell will not make any current. In doing so, it will not pass the current the other cells causing the entire panel’s output to fail. This is just like removing one lamp in a string of miniature Christmas lights; the whole string goes out. Therefore: do not let any portion of the solar panel be shaded; if any portion is shaded, the whole panel will cease to make electricity.

Figure 1 Volt-Ampere characteristic of a typical solar panel in full sunlight.

The Volt-Ampere characteristic of a typical panel in full (1 kW/m2) sunlight is shown in Figure 1. This figure may be confusing. It shows two curves. The solid curve at the top is the amount of current the panel will output into a load at a given voltage. For instance, if I connect it to a 14 V RV battery, the panel will put out 5.1 in bright sunlight. If I attempt to charge a 22V battery with the panel, it will output only about 0.8 amperes.

Solar panels typically have three key specifications: the short circuit current, the open circuit voltage and the total electoral power that the panel can create at what is known as the max power point. If I short circuit the panel there will be zero volts across its terminals. Figure 1 shows in full sunlight (the top trace) that the panel will output 5.1 Amps with zero volts cross its terminals. If I open circuit the panel there will be zero current at the panel’s output terminals. Figure 1 shows that in full sunlight that there will be a bit more than 22 Volts across the panel’s open terminals.

The dotted curve shows the amount of power the panel will create for a given terminal voltage. It’s odd shape is caused by the fact that that output power is the product of the panel’s Voltage and Amperage. When the terminals are shorted and the panel sends 5.1 Amps into a short circuit, the output power because the Voltage is zero. Likewise the power created with an open circuit (i.e., 0 Amps) is zero. The panel creates it maximum power where the output Voltage and currents are near their maximums.

Solar panels, like batteries are also affected by temperature. Figure 2 shows this effect. When the panels are cold, they are able to operate to a higher voltage before the panel’s current output starts to decrease. Conversely, when the panel is hot, its output current starts falling at a much lower voltage. The panel, being intensely black, will be notably hotter than the ambient air temperature; thus the choice of 150°F for Figure 2.
Figure 2
Figure 2 Temperature effects in solar panels. The solid line is for temp = 75° F. The dashed line (lower voltage curve) is for panel temp of about 150 °F. The dash and dot line (higher voltage curve) is for temp of about 32°F.

The curves and specifications used as an example here are for a 36 cell, 85 Watt panel. The reason why the output current goes to zero at high voltage is that when the voltage across each cell goes too high, a diode across the cell, a diode inherent in the design of the cell, will, in essence, short the cell out. There is an equal voltage across each cell. The panel’s 22V open circuit specification divided by 36 cells says that there is about 0.61 Volts across each cell when the panel is in full sunlight with its terminals open at 75° F. The highest output Voltage before the current starts to decrease at 75° F is about 16 Volts. This falls to about 14 Volts at 150° F. This works out to roughly 0.44 Volts per cell for 75° F and about .39 Volts at 150° F.
Panels are made with other cell counts; for instance AM Solar sells panels with 32, 36 or 44 cells. Using these rules of thumb, a 32 cell panel will feature an open circuit voltage spec of about 19.5 Volts. At high temperatures, the current capability of a 32 cell panel will start falling when its terminal voltage goes above 12.5 Volts. Compare this with the charging voltages required in Table 1.

Solar Panel Controllers:
When one compares the requirements for charging a RV battery with the characteristics of a solar panel it is pretty obvious that disaster will result if the solar panel is hard wired to the battery. All will be well when the charging cycle begins and the battery’s voltage is low. After the battery’s voltage rises to the Charge Voltage, the panel will not hold the voltage constant. It will continue to charge the battery at full current until … it explodes. Not good.

To successfully use the solar panel, some form of control unit must be used to keep the solar panels from overcharging the battery. There are two basic types of controllers. The first is some form of a switch that appropriately connects and disconnects the solar panels directly to the battery. The second, is a unit that attempts to maximize the charging current available from the panels by transforming the energy from one voltage to another (MPPT technology).

Figure 3
Figure 3 Switch mode solar panel controller

The block diagram of the first or switch mode controller is shown in Figure 3. The contents of the switch logic box can be very simple or quite complex. The simplest units of this type employ a mechanical relay that physically opens the path between the solar panel and the battery when the voltage gets too large and recloses later when it gets small.

A 36 cell panel is pretty much required when a switch mode controller is used. On a warm day, say 90° F air temperature the voltage required to charge the battery is lower than nominal but is still about 14.6 Volts. Bright sunlight on the panel will drive its temperature higher than air temperature, perhaps to 150° F. Figure 2 shows that at that high panel temperature the panel’s output current will just start to decline at 14.6 Volts. So, as long as the voltage drop in the switch and wiring is small the panel will effectively charge the battery on very hot days. As mentioned above a 32 cell panel will not provide full current output on hot days as the battery approaches full charge. Cold days does not present a similar problem.

The Heliotrope RV-30S controller used in earlier LD coaches is a switch mode unit. But, here the switch is electronic and the logic complex. This controller features essentially a two stage charging system. When the battery voltage is below the charge voltage, all the current the panel(s) can create is channeled directly to the battery. When the battery’s voltage rises to the charge voltage, the electronic switch is used to control how much of the panel’s current can reach the battery. The switch is closed and opened many thousand times per second with the length of the closed period vs. the length of the open periods adjusted so that on average the correct amount of the panel’s current gets through to the battery to just keep its voltage at the Charge Voltage. One of the RV-30S’ nice feature is a temperature compensation system that adjusts the Charge Voltage based on the battery’s temperature. All in all, the RV-30S is a very good controller of its type.
The issue with sophisticated switch mode controllers is not that they do a good job, it is that there is an opportunity to get more out of your panels. The issue has so many if’s or buts that the basic concept is best illustrated with a detailed example. Suppose it’s mid winter. You have escaped the snow and ice and are dry camping somewhere in the south at 32°N. It’s 9 AM and it is a clear, bright, sunny day, unlike yesterday when it was cloudy. It’s nearly freezing outside.

Your battery is getting low, down perhaps 60 amp-hours because the sun didn’t shine much yesterday. As a result the battery voltage is a bit low. I’m making this up but suppose the battery voltage is about 12.3 Volts. You have two solar panels with the specifications of those above (85 Watt) mounted flat on the roof and you are hoping that the solar panels will charge the battery up.

At 9 AM in bright sunlight at 32°N in mid winter with flat panels, you will get about 25% of the short circuit current out of the panels. The short circuit current of two panels is ~10 Amps; 25% of that is 2.5 Amps. At that rate it will take 60/2.5 = 24 hours to charge the battery. Yes, the current will go up as the sun rises higher in the sky but you can expect to harvest at total of about 29 amp-hours for the day (minus what you use during the day).[1]

When the switch mode controller connects the panel to the battery, the panel is operated at the voltage the battery’s voltage; in this case 12.3 volts. That is, the two 85 Watt panels are actually creating (2.5 * 12.3) = 30.7 Watts of power! Looking at Figure 2 and comparing it with the power curve in Figure 1, it can be seen that the cold panels could make as much power as it is capable of if they could be operated at, about 22.5 Volts instead of 12.3. At the panel’s maximum power point the output current will be about 0.9 of the short circuit current or 2.26 Amps. Thus, in this situation, if the panels could be operated at 22.5 Volts the total output power would be 22.5 * 2.26 = 51 Watts. Thus, under some conditions (primarily when it’s cold) much more power is available from the solar panels if, somehow, one could but operate the panel at a higher voltage. Enter the MPPT controller.

Figure 4
Figure 4 Solar panel with MPPT controller

The maximum power point tracking (MPPT) controller makes use of an electronic power conversion system to operate the solar panel at the voltage that maximizes its power output while converting its energy (minus a loss) to the battery’s voltage. By this strategy the amount of charging current can be maximized for any given situation.

The big issue with MPPT controllers is efficiency. It is difficult to build electronic power conversion systems that have a loss less than 7% to 10%. And, the loss can quickly eat up the MPPT’s gain. For instance in our example, the panel voltage is 22.5 Volts and the panel current is 2.26 Amps for a total power input to the MPPT controller of 22.5 * 2.26 = 51 Watts. If the energy conversion process has a 10% loss then 51Watts * 0.9 = 46 Watts comes out. The battery voltage is 12.3 so the MPPT unit’s output current is 46/12.3 = 3.7 Amps. In this example, there is a current boost of 3.7/2.5 ~ 1.5:1.

In our example, as the day wears on and the panels get warmer and as the battery voltage increases with charge the amount of current will decline. When the panel voltage falls to 1/0.9 = 1.11 times the battery voltage, the MPPT’s current gain will be unity. At this point, one could hook the solar panel directly to the battery for better performance. In fact many MPPT units sense when there would be no current gain and return to a switched mode operation.

The efficiency of the HPV-25 controller sold by LD is not specified for energy conversion efficiency. AM Solar has apparently discontinued that controller (which they built themselves) in favor of the Solar Boost 2000 MPPT controller. This unit is specified to have a conversion efficiency of 95% at 15 amps of output. There is no specification for lower output currents but one must assume the efficiency will be lower.

[1] If you park the coach so that 45° tipped up panels face exactly south, you will get about 56 Amp-hours of charge.