Charge Controllers

The primary function of a solar charge controller is to protect your battery bank from being over-charged by your solar array. Charge controllers continually monitor battery voltage, solar array voltage and output current and ensure your batteries get correctly charged. Moreover, most charge controllers will manage 3, 4 or 5 different stages of battery charging cycles to properly charge your batteries at different times and battery voltages. The most common battery charging cycles are:

Bulk Charge - This first stage of battery charging is used whenever your batteries are low on energy. Most charge controllers automatically begin bulk charging as soon as power is drawn from the battery bank. In this stage, current is sent to the batteries at the maximum safe rate they will accept until voltage rises to near (80-90%) full charge level.

Absorption Charge: This 2nd stage of battery charging is used to safely bring your battery bank up to 100%. Voltage remains constant and current gradually tapers off as internal resistance increases during charging.

Float Charge: The 3rd stage of battery charging is a "maintenance" charge cycle. After your batteries have reached full charge, charging voltage is reduced to a lower level (typically 12.8 to 13.2) to reduce gassing and prolong battery life. It's main purpose is to keep an already charged battery from discharging. PWM, or "pulse width modulation" accomplishes the same thing. In PWM, the controller or charger senses tiny voltage drops in the battery and sends very short charging cycles (pulses) to the battery. This may occur several hundred times per minute. It is called "pulse width" because the width of the pulses may vary from a few microseconds to several seconds.

The following illustration shows how a typical three stage charge controller works (illustration courtesy of Xantrex Technologies):

Equalization Charge: Some charge controllers are capable of a manually engaged equalization charge on wet lead acid batteries. Equalizing refers to an overcharge performed on flooded lead-acid batteries after they have been fully charged. This maintenance step helps eliminate stratification and sulfation. This charge cycle should always be done while you are present to monitor your batteries, as they can release excessive amounts of gas, or become overheated during this process. Make sure to carefully follow all battery, and charge controller manufacturer instructions.

Types of Charge Controllers:

PWM: Pulse Width Modulated, or PWM charge controllers offer a relatively inexpensive and effective way to regulate battery voltage. They are commonly found in mobile, marine, telecommunications and smaller off-grid battery charging environments. The use of PWM controllers in medium to large off-grid battery charging systems is waning due to the fact that these controllers need the solar array nameplate voltage to match the battery voltage being charged. Many people prefer wiring their solar array at a voltage higher than battery voltage to reduce voltage drop on the home wire run and also to take advantage of cost per watt savings available on larger "grid-tie" solar modules which do not have standard battery charging voltages. These newer solar arrays must use an MPPT controller. Please see the next section for more on MPPT controllers.

MPPT: Maximum Power Point Tracking, or MPPT charge controllers (in general) offer the user the ability to operate a solar array voltage at higher than battery voltage. These controllers (in general) are able to convert the higher input voltage into a lower voltage for battery charging, and convert that loss in voltage to an increase in charging current to the batteries (Ohms Law). These newer charge controllers are generally used on medium to large scale off-grid solar systems. They are much more expensive than traditional PWM controllers, and care must be taken to properly wire your array to prevent mismatch or overly high solar voltages.