Charge controllers are available in several different styles. The two most popular today are the PWM controller and the MPPT controller.
Low-Cost Pulse Width Modulated (PWM) Controllers
The Pulse Width Modulated (PWM) controller turns an internal transistor or switch on and off at a very high speed and with a varying duty-cycle. To allow more current to flow from the solar panel to the battery, the duty-cycle is increased towards 100%. Conversely, to reduce the flow of current into the battery, the duty-cycle is reduced towards 0%. Riverside uses PWM controllers from Morningstar, Xantrex, Blue Sky, and other leading manufacturers.
Maximum Power Point Tracking (MPPT) Controllers
The Maximum Power Point Tracking controller is a more advanced and more expensive design that increases charging efficiency by causing the impedance of the charging panels to match closely the impedance of the battery. This can improve charging efficiency by up to 30-40% overall under best-case conditions. Even in less than optimum conditions (battery is charged, panels are hot), you can still expect a 10-15% increase in efficiency.
MPPT should not be confused with Array Tracking where an array of panels are physically moved to track the path of the sun over the course of the day. MPPT is an electronic process that maximizes the power transfer from a PV panel to a battery.
Advantage Of MPPT Controllers
To illustrate the advantage of MPPT, let’s use a 12-volt battery and a 12-volt PV panel. The PV panel for this illustration is rated at 140 watts. Looking at the data sheet, it is determined that the panel produces 140w when the output voltage is 17.7 volts and the current is 7.91 amps. This voltage is called Vmp (voltage at maximum power) and this current is called Imp (current at maximum power).
Standard Charge Controller
If this panel is connected to a partially discharged battery through a standard charge controller, the Imp (7.91 amps) will flow into the battery at the battery voltage. For a partially discharged battery, the battery voltage could be less than 12 volts. If we choose 11.5 volts for this example, the amount of power transferred from the panel is 11.5 volts * 7.91 amps = 91 watts. The efficiency of this system under these conditions can be expressed as the ratio of Power In over Power Out (91w/140w * 100%) which equals 65%.
MPPT Charge Controller
A MPPT controller in this situation would determine the power available at its input terminals and convert the voltage and current to the optimum values necessary to charge the battery connected to its output terminals. Some losses do occur but modern MPPT controllers are typically 94% efficient or better. The controller output current would be increased to approximately 95% of 12.2 amps (140w = 11.5V * 12.2A) or about 11.5 amps. The battery is being charged with considerabley more current with MPPT than with a standard charge controller. MPPT has the most benefit when any of the following charging conditions are met:
- the battery is discharged
- it is a cloudy day
- the temperature of the panels is cold