To demonstrate an off-the-grid charge controller, there are four major components needed. First, solar panels are used as the energy source. This power is fed to the charge controller which is output into a battery which allows for energy storage. On the output of the battery is an inverter which provides outlets for the user to access the stored energy. The solar panel, battery, and inverter were bought as off-the-shelf parts, while the MPPT charge controller was designed and built by Solar Knights. In addition to providing efficient power storage, the charge controller features a LCD screen for live data capture, and a radio frequency (RF) module for wireless data logging. The charge controller consists of a DC-to-DC Buck-Boost converter, 5V and 3.3V switching regulators, 10A fuses, reverse protection diodes, microcontroller, LCD, wireless transceiver, temperature sensors, voltage sensors, current sensors, and an irradiance sensor.
The MPPT charge controller maximizes efficiency by controlling the output voltage. Since a battery's voltage stays at a near constant value (in this case 12V), one wants to maximize current being delivered to the battery. The way this is achieved in PMC2 is by utilizing the perturb and observe MPPT method. This method increases or decreases the voltage on the battery once a second, and checks the present panel power versus the previous panel power. If the power is greater now than it was one second ago, change the output voltage in the same direction again. If the power went down, reverse direction of the output voltage. Another way to track the maximum power point is by checking the current into the battery at this instant versus one second ago. This is actually the methodology chosen for PMC2 because only one sensor value needed to be checked, namely battery current. The output voltage is varied by utilizing pulse width modulation (PWM) from the microcontroller to the MOSFET drivers. The temperature sensors and the irradiance sensors are not used in the MPPT algorithm but provide useful data to a user. The LCD and wireless capability are additional features which make the Solar Knight's charge controller superior to all other charge controllers.
- Parts List
- Solar Panel: Siemens SP75
- Battery: Sun Xtender PVX-420T
- Inverter: Cobra CPI 880
- Microcontroller: ATmega328P (with Arduino)
- LCD: 20x4 Character Sparkfun Display
- Wireless Transceiver: XBee Series 1 Module
- Current Sensors: Allegro ACS711
- Temperature Sensors: Maxim DS1624