Blue Sky Energy's Solar Boost 50 controller (right) and remote display (below).

Blue Sky Energy's Solar Boost 50 controller (right) and remote display (below).

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

Apollo Solar's T-80 charge controller and wireless remote.

Input Voltage & Controller Efficiency

PV arrays are commonly wired for the highest voltage that the controller can handle, as this allows the smallest—and cheapest and easiest—wire size that can be used. Maximum power point tracking (MPPT) charge controllers allow the flexibility to configure a PV array for a range of output voltages.

Having fewer parallel strings in a PV array simplifies the installation and raises the voltage, compared to fewer modules in each string. This reduces costs, as fewer overcurrent protection devices—breakers or fuses—are needed, and a smaller combiner box can often be used. Having smaller and fewer wires speeds up the installation process, reducing costs even more.

But there are trade-offs involved with using higher PV array voltages. Higher array voltage results in a slightly lower operating efficiency for the MPPT controller. Controller efficiency decreases as the step-down ratio (PV array voltage to battery voltage) increases. For example, a system with a 4:1 ratio (96 VDC PV array and 24 VDC battery) will operate at a 0.5% lower efficiency than with a MPPT controller operating at a 3:1 ratio (72 VDC PV array and 24 VDC battery).

Some MPPT controller manufacturers provide information, such as graphs that show the impact on efficiency at various PV array voltages and battery voltages.

On systems with longer wire runs between the PV array and the MPPT controller, the savings achieved by being able to use a smaller wire size can be substantial. Sizing wires somewhat larger than required can reduce a typical array-to-controller wire loss of 2% to 1.5%, for instance. This can make up for the slightly lower conversion efficiency of a greater step-down ratio.

However, on systems with short wire runs, the savings from using a smaller wire size may not be enough to offset the lower MPPT controller efficiency. In these cases, having a lower PV array voltage configuration would be a better choice.

—Christopher Freitas 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:

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

Typical Charge Controller Efficiency at Different Array Voltages s a

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