Any time the sun is shining, a PV array is ready to produce electricity. All that’s needed to get the electrons in those wires moving is a complete circuit. But in off-grid battery-based systems, overcharging will occur if the batteries are full and if more energy is being generated than is being consumed by loads. This can permanently damage the batteries.
That’s where controllers come in—to protect the batteries. The simplest PV controller is a manual switch that’s flipped to disconnect the PV array when the battery bank is full. However, since battery damage can happen quickly, an automated solution is needed. Modern PV charge controllers can:
The earliest PV charge controllers were voltage-controlled switches that disconnected the PV array when a certain voltage was reached. When voltage fell to another, lower point, it reconnected the array. These controllers protected the batteries from overcharging, but were unable to provide the three-stage charging regime recommended by battery manufacturers for longest battery life, since the controller output was either “on” or “off.”
The advent of pulse-width-modulated (PWM) controllers made efficient three-stage charging from a PV array possible. This technology charges batteries with high-frequency electrical pulses, and can continuously change the amperage being delivered by varying the length of the pulses. When the batteries are discharged, the PWM controller senses this from the battery bank voltage and stays on to deliver the full current available, called the “bulk” stage of charging.
The next stage of charging is “absorption” and occurs as the batteries approach a full state of charge (SOC). The controller holds battery bank voltage constant for a period of time, and the “off” time of the pulses is increased to gradually reduce current as the bank is topped off.
The “float” charging stage occurs when the batteries are full, and is sometimes referred to as “trickle charging.” The battery bank receives just enough current to hold it at a constant voltage, but below the point where excessive gassing occurs. The gas produced is explosive hydrogen and gassing drops the electrolyte level in a battery, so it's critical to carefully control the float stage of charging. If the electrolyte level drops even slightly below the tops of the lead plates inside at any time, the battery will be damaged.
Controller set points for all three charging stages must be adjusted to the proper specifications for the battery bank type and voltage, and all of the controllers listed in the ”Comparison” table allow some type of adjustment.
Flooded lead-acid batteries require regular “equalization”—a timed, controlled overcharge that extends battery life by stirring up stratified electrolyte, knocking sulfate deposits loose from the battery plates and bringing all cells in the bank to an equal SOC (Sealed batteries can be permanently damaged by equalization.) Some of the controllers listed allow users to program equalization on a time schedule. Be sure to check the specifications from your battery manufacturer and properly set your charge controller.
Some charge controllers have a “diversion mode,” where the controller is connected directly to the battery bank and sends a varying rate of amperage from the batteries to heating elements or other “dump loads.” This is primarily used when wind or hydro turbines are connected to the system. They must always have a full load connected or they will over-speed, resulting in turbine damage. Air and water heaters are common dump loads, and DC water heater elements are available for all common battery bank voltages. Diversion mode is rarely used with PV systems, as the modules don't need to be always loaded.
Some controllers have a “load control” mode, which is sometimes used as a “low-voltage disconnect” (LVD). When battery voltage drops below a certain point, the load controller disconnects DC loads from the system. This mode is used to prevent battery damage only in smaller DC systems, usually at remote, unattended installations, such as lighted billboards and communications repeaters. Some load controllers can also be used for automated backup generator starting and stopping.
Most controllers can't perform the functions of all these “modes” at one time—the controller must be set for PV control, diversion control, or load control—and that's all it can do.
A newer innovation in PV charge controllers is called maximum power point tracking (MPPT). Computer-driven circuitry in the controller scans both battery bank and PV array voltages at regular intervals, and calculates the optimum match between the array and battery bank. MPPT is able to trade volts for amps and vice versa (see the MPPT sidebar). Under some conditions, such as cold weather, overcast days, or low-horizon sunlight, energy gains of up to 35% are possible.
MPPT controllers have another big advantage of allowing higher-voltage PV arrays, which operate more efficiently and require smaller wire sizes from the array to the controller. In some cases, especially when the PV array must be located a long distance from the battery bank, the extra cost of MPPT is offset by the savings from being able to use smaller-gauge wire.
Beware of some caveats when choosing between MPPT and non-MPPT controllers. First, all the PV modules (or series strings of modules) feeding an MPPT controller should be identical. Mixing modules from different manufacturers; using different PV technologies (monocrystalline, polycrystalline, or amorphous); or using modules with different voltages and different power ratings should be avoided, as MPPT gains can be compromised and mismatched modules could be damaged.
Be careful when sizing series strings of PV modules that feed an MPPT controller. The “maximum open-circuit voltage” listed in the comparison table is just that—if you exceed this voltage, the controller can be permanently damaged. Remember that PV modules produce higher voltages in cold weather, so an array of three 45 V modules in series for 135 V might be just fine during the summer for an MPPT controller with a 150 V maximum rating—but could damage the controller when it is cold and sunny.
Extra caution is needed when designing and installing PV systems higher than 48 V. All combiner boxes and circuit breakers must be rated for the maximum system DC voltage, as DC electrical arcs are difficult to extinguish and can cause a fire. Accidental electrical shocks during installation that are merely unpleasant at less than 48 V can be lethal at higher voltages. Hire a professional if you have even the slightest doubt of your ability to install the system safely.
First, limit your choices to controllers that work with your battery bank voltage, which will usually be 12, 24, or 48 V. Then, calculate the approximate maximum amperage your controller will need to handle. Divide the PV array watts by the system voltage to get amperage, then add a 25% safety margin to account for higher irradiance conditions. For example, a 40 A rated controller could possibly handle 480 W of PV into a 12 V battery bank; 960 W into a 24 V bank; and 1,920 W into a 48 V bank. After factoring in the additional 25%, those maximum ratings become 384 W, 768 W, and 1,536 W, respectively.
Be sure to carefully check the charge controller manufacturer's specifications for the maximum recommended array wattage for different battery bank voltages. In the comparison table, maximum amperage is shown for 12 V battery bank charging, as some controllers have different maximum amperage ratings at different system voltages.
Next, decide on a PV array voltage. If you go with a non-MPPT controller, the nominal array voltage must match your nominal battery voltage. If you use a MPPT controller, many manufacturers have a spreadsheet or online calculator pre-loaded with the specifications for many popular PV modules. The calculator will also consider the temperature versus voltage performance of your modules, needed to protect the controller from elevated voltage during cold weather.
If there isn’t a string-sizing calculator available, you’ll need to use the PV module manufacturer’s specifications sheet (along with your area’s record low temperature) to determine how high the module voltage could go during cold weather (see this issue’s Methods). Use this figure to calculate how many modules you can safely place in series. In the comparison table, the maximum open-circuit voltage (Voc) beyond which controller damage can occur is shown. The PV array voltage recommendations for PWM controllers are shown with an “N” for “nominal” after the voltage—the actual Voc of a “12 V” PV module (for example) will be substantially higher than 12 V. See the “MPPT” sidebar for more details.
Digital display. It can be handy to look at the controller to see how the PV array is performing, what stage of charging the controller is using, and what the battery bank voltage is. Many charge controllers suitable for the typical home PV array have digital displays, but charge controllers intended for smaller PV systems might not.
Remote metering. Instead of trekking to the controller location to check performance, some have optional remote displays that can be mounted anywhere that’s convenient, using inexpensive cable to connect to the controller.
Remote temperature sensor. Batteries vary in their charging performance depending on temperature data, so a controller can use battery temperature to adjust charging. But because a controller should never be mounted inside a battery enclosure, remote temperature sensors are available for some controllers. These sensors are required for each controller if multiple charge controllers are used.
Communications options. Many RE system owners like to collect and display performance data from their systems. The ease of data logging and use varies widely between manufacturers and models. Some controllers are LAN-ready and can be monitored via your home network, while others can communicate only to other balance-of-system components (for example, inverters and remote meter panels) made by the same manufacturer. Some give you only RS-232 raw data communications, leaving you to interface the controller with your computer.
Control relays. A charge controller that can perform other tasks based on battery voltage can be useful. It can control relays for battery bank vent fans that turn on only when the batteries are gassing, and automatic generator starting. The control relays will be able to handle only very small loads—larger loads may require a separate, higher-power external relay triggered by the small internal one.
Equalization. If your batteries require regular equalization, some PV controllers allow you to equalize using solar electricity on a sunny day, instead of burning fuel in an engine generator. Scheduled automatic equalizations are available with some controllers. But these should be enabled only with caution—each battery cell’s electrolyte level must be checked before starting an equalization cycle. And if your batteries can be damaged by equalization (for example, as many sealed lead-acid batteries can), check and double-check that this feature is disabled. Also, be aware that some PV arrays cannot deliver the amps necessary to fully equalize a battery bank, so generator or grid charging may still be necessary.
Author and educator Dan Fink has lived 11 miles off the grid in the northern Colorado mountains since 1991. He teaches about off-grid systems and small wind power, and is the executive director of Buckville Energy Consulting, a NABCEP/IREC/ISPQ-accredited continuing education provider. Dan is the coauthor of Homebrew Wind Power.