Charge Controllers

There are 2 primary functions of a charge controller:

• Prevent excessive battery charging or overcharging the battery
• Prevent the battery from over discharging

A charge controller can also provide load control functions if DC loads are connected to it. A charge controller senses the battery voltage and when the batteries are fully charged the charge controller will stop the current flowing from the PV array into the battery bank. When the batteries are being discharged to a low level a charge controller will shut off the current flowing from the batteries to the load. For more information on availability and pricing visit my partners Charge Controllers section.

There are four different types of charge controllers, based on the controller algorithm implemented:

• Shunt controls
• Single stage controls
• Multi stage controls
• Pulse width modulation

Shunt controllers are designed for very small systems. They prevent overcharging by bypassing the batteries when they are fully charged into a power transistor. This acts like a resistor and converts the excess power into heat. Shunt controllers have heat sinks with fins that help to dissipate the heat. These controllers also have a blocking diode to prevent current from draining back from the batteries through the solar array at night.

Single stage controllers: Single stage controllers prevent battery overcharging by switching the current off when the battery voltage reaches a pre-set value called the charge termination set point (CTSP). The array and the battery are reconnected when the battery reaches a lower present called the charge resumption set point (CRSP). Single stage controllers use a sensor to break the circuit and prevent reverse current flow at night.

Multi-stage controllers: These devices automatically establish different charging currents depending on the battery's state of charge. The full array current is allowed to flow when the battery is at a low state of charge. As the battery bank approaches full charge, the controller dissipates some of the array power so that less current flows in the batteries.

The "trickle" charge tapers off as the battery bank approaches a fully charged state. This increases battery life. These controllers also generally have a relay type switch that prevents reverse leakage at night.

Pulse width Modulation controllers: These controllers provide a 'topping off" charge by rapidly switching the current on-and-off when the battery voltage reaches a fully charged state.

The controllers also provide over discharge protection by turning off DC loads (if present) when the batteries reach a preset voltage called the low voltage disconnect (LVD).

Another important feature to look for in Charge controllers is the "Maximum Peak Power Tracking" option. This allows the charge controllers to maximize Power output of the PV array by ensuring that it operates at its Peak power point.

Charge controllers that have the "Equalization" capability should be preferred if you have a flooded lead-acid battery pack (see section of batteries for more details). Equalization allows the controller to optimize the performance of flooded lead-acid batteries. An example of manufacturer specifications (Outback MX60) is shown below: