Charge controllers are vital components in battery-based PV systems, keeping the PV array from overcharging the batteries and optimizing charge current. These smart little boxes are placed between the PV array and the battery bank, and allow maximum energy in when the batteries are at a low state of charge (SOC) and reduce incoming energy from the array as the batteries approach full. When the battery reaches 100% SOC, the charge controller reduces the energy flow to a small maintenance (float) charge. Without a charge controller, a battery can be overcharged, resulting in excessive battery gassing and, in chronic cases, damage to the battery, shortening its life span.
Matching the appropriate charge controller to a particular PV system is important for proper system function and for maximizing the amount of solar energy harvested. This guide lists charge controllers 40 amps or above commonly used for whole-house PV systems.
Nominal Battery Voltages—The nominal, or “name plate” voltage of a battery bank depends on the nominal voltage of the individual batteries and how they are wired together. Most residential-scale battery banks are 12, 24, or 48 volts, with 24 and 48 V most common. Some less common battery banks are 36, 60, or, rarely, 120 V nominal. Many newer charge controllers can be field-adjusted to accommodate any of several different battery bank voltages.
Rated Output Current—This is the maximum amperage the charge controller is designed to put out continuously at the rated operating temperature. Above these temperatures, the output will decrease and energy will be wasted. These temperatures may seem high for most climates, but the charge controllers and other associated equipment also produce heat during operation.
Rated output current is used to determine the size and quantity of controllers needed for an array. For non-MPPT controllers, the output rating of the controller(s) should be at least 125% of the array short-circuit current to allow for higher irradiance conditions. For MPPT controllers, the output current is based on the total array power (in watts) divided by the expected lowest battery voltage (usually the low battery cutout voltage set for the inverter). For matching an MPPT charge controller to a particular array, see the “Array & MPPT Charge Controller Sizing Example” on page 73, as well as controller installation manuals for specifics, which vary by manufacturer.
Maximum PV Open-Circuit Voltage—Charge controllers are designed to withstand a specific maximum input voltage. Exceeding this can damage the charge controller and possibly void its warranty. PV array voltage increases with decreasing temperature, i.e. as temperatures drop below 25°C (77°F), module voltage rises.
The controller’s maximum open-circuit voltage must exceed the array’s Voc. For example, if the array’s Voc is 130 VDC, then a charge controller specified for a maximum of 130 VDC should be used. To calculate an array’s Voc, use the module’s temperature coefficient in conjunction with the site’s historic low temperature (see Code Corner in this issue).
Maximum Power Point Tracking (MPPT)—PV arrays operate most efficiently at voltages higher than battery bank voltage. But because PV array voltage is affected by the load attached to it, batteries can cause the array voltage to drop below the ideal power production point to just above battery voltage, causing some loss of PV array energy. MPPT charge controllers manipulate incoming PV array power by converting excess PV array voltage (when available) to extra amps for filling the batteries.
The overall gain from an MPPT controller depends on temperatures at the site and battery SOC. More voltage is available during cold conditions and less during warm conditions. Additional power is only advantageous if we can absorb or use it. If the charge controller is doing its primary job and actually stopping current from flowing into the batteries because they are already full (high SOC), then this extra power is not harvested. MPPT charge controllers can really shine during the winter months when cold temperatures increase available PV voltage, and fewer sun-hours lead to increased loads, while less array output tends to keep the battery SOC low. MPPT charge controllers are especially advantageous in battery-based grid-tied systems—when the batteries are full, the extra MPPT-recovered energy can still be sent to the grid.
Array Voltage Step-Down Options—MPPT-type charge controllers allow the connection of an array with a higher nominal voltage than the battery bank. Given this step-down feature, nearly any module type or size can be configured into an array to charge any size battery bank, and fewer parallel strings of modules are required for the same power output. This spec gives you the step-down ranges for each controller. For more details on the voltage step-down feature, see “Input Voltage & Controller Efficiency” sidebar.
Built-in Battery SOC Meter—This meter reports how full the battery is, eliminating the need to purchase a separate battery state of charge meter. As of this writing, only Apollo charge controllers include this feature. Some manufacturers offer a separate product for this purpose.
Terminals’ Wire Size Range—Charge controllers have terminals for the wires coming from the PV array and those going out to the battery bank. These terminals accept a range of wire gauges. Because conductors often need to be up-sized due to voltage drop constraints, having a controller with terminals that can accommodate a wide range of wire sizes can be advantageous.
Battery Temperature Sensor—All the charge controllers in this guide include temperature compensation functionality, which adjusts charge voltage set points based on battery temperature. Some charge controllers include the battery temperature sensor and others offer it as an option.
Temperature Compensation —The internal resistance of a battery fluctuates with battery temperature, so charge controllers are most effective if they adjust their charge termination (voltage) set points to accommodate this changing internal resistance. Understanding how temperature compensation works requires Ohm’s law:
Voltage (V) = Current (I) x Resistance (R)
When a battery is cold, its internal resistance increases, which causes the voltage to rise (assuming a constant current). Charge controllers use voltage to determine the shutdown point for when the battery is full. Without temperature compensation, a false high-voltage reading means that charging would get shut off too soon, resulting in an undercharged battery. Conversely, high temperature causes a battery’s internal resistance to drop. This causes a false low-voltage reading, and thus charging gets terminated too late, causing the battery to be overcharged. Temperature compensation allows a charge controller to increase the charge termination set point for a cold battery and decrease this set point for a warm battery—resulting in an appropriate charging regimen.
Warranty—Charge controller warranties range from two to five years. Although having a long warranty to back you up is great, there are other factors to consider, including down-time and service-related expenses. A product with a long history made by a reputable company may save you money (and time) in the long run, regardless of the warranty offered. All the controllers shown are produced by reputable companies that are known to stand by their products.
Manufacturer Suggested Retail Price (MSRP)—Each manufacturer lists a suggested retail price. This is typically the highest price distributors will list; some may choose to sell at a lower price.
All the units listed in this article include at least basic displays and metering. (If the meters are optional, the prices shown reflect their inclusion.) The displays provide basic voltage and amperage readings necessary for system checks during installation and, later, for monitoring the array for wiring, module, or even shading problems.
All the units listed display battery voltage, and some models (like Apollo) employ a separate wire to take this reading right at the battery bank. MPPT controllers show array voltage and current, as well as output (battery) voltage and current.
Some controllers show array power production in watts, and many have data-logging functions, which can be especially useful for off-grid systems. For example, data logging that includes “time in float mode” helps system owners know the batteries reached full charge for each day the controller dropped to float mode. A number of controllers can be connected to a computer for real-time monitoring and data collection.
Justine Sanchez is a NABCEP-certified PV installer, Home Power Technical Editor, and Solar Energy International instructor. Justine lives, works, and teaches from an on-grid, PV-powered home in Paonia, Colorado.
Brad Burritt is a NABCEP-certified PV and wind installer, and renewable energy consultant. Brad lives on an off-grid PV- and wind-powered farm near Hotchkiss, Colorado.