Solar Knowledge: Solar Charge Controller
The primary function of charge controller is to prevent the battery bank being overcharged by PV array. It also protects battery from being overly discharged by DC load. It also serves as the junction point for solar module input, battery connection and load terminal output. Additional features of charge controller are:
- Low voltage load disconnect to protect the battery from being over-discharged.
- Metering indicator
- over-current protection
- Adjustment settings
- Temperature compensation
- Maximum power point tracking to optimize array output
- Shunt regulator
- Series regulator
- PWM regulator
- MPPT controller
Shunt RegulatorShunt Regulators are designed for very small system. They prevent battery from overcharging by bypassing the batteries when they become fully charged. The main components are power transistor between array positive and negative lines, and blocking diode between battery positive and array positive. During normal charging current flows from array to the battery. When battery voltage reaches the array disconnect setting, transistor is activated shorting the PV array. This converts excess power into heat. Shunt regulator are provided with heat sinks with fins to dissipate heat. Battery is prevented from being shorted by blocking diode. It also prevents current flowing back from battery to the array during night time. When the battery voltage falls back to array reconnect setting, transistor is released and current starts flowing to battery.
Figure: Shunt Regulator
Shunt regulators are simply designed and inexpensive. Their disadvantages are their limited load handling capacity and need of ventilation for cooling. Additionally voltage drop occurs across blocking diode which drops the available charging voltage.
Series RegulatorBasic series regulator consists of relay between battery positive and array positive, and voltage comparator. When battery voltage reaches the array disconnect setting, relay is opened disconnecting PV array from battery there by stopping further charging of battery. When battery voltage falls back to the array reconnect setting, relay is closed allowing current to flow again to the battery. These controllers are inexpensive and eliminate the need for bulky heat sinks as they don't product much heat. They have greater load bearing capacity compared to shunt regulators. Another advantage is they generally do not require significant ventilation.
Figure: Series Regulator
PWM RegulatorsPWM Regulator is a series regulator with solid state switch instead of a relay. By replacing relay with solid-state switches, flow of current from array to battery can be switched at very high speed which helps in controlling battery charge voltage more accurately. Instead of varying the voltage to control battery charging, the PWM regulator varies the amount of the time the solid-state switch is open or closed by modulating the width of the pulse as shown in fig.
Figure: PWM Regulator
MPPT ControllersIt is a variation of PWM charge controllers. MPPT charger controller adjusts the PWM to vary PV array voltage from battery voltage. By varying array input voltage while maintaining the battery charge voltage, maximum power output from PV array can be achieved.
Figure: MPPT Regulator
MPPT controller has many advantages over other types of charger controllers. In addition to getting more charging current from PV array, some MPPT controller allow PV array to operate at much higher voltage than the battery. This feature is useful in reducing the wire size and voltage drop from PV array to the controller. Although MPPT controller can increase output from PV array, they typically have greater losses than other types of controllers.