Knowledge Solar Charge Controllers: MPPT vs. PWM

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A solar charge controller, also known as a solar regulator, manages the power going into and coming out of the battery of a photovoltaic system. It's fundamentally a voltage and current regulator that prevents a battery bank from overcharging. The charge controller regulates the current to the load, thereby protecting the electrochemical system from overdischarging. Furthermore, the controller is designed to prevent reverse current. It ensures no power gets back to solar panels that will drain the battery during the night when the voltage from the battery will be higher than that of the solar panel. The charge controller is therefore the energy manager in an off-grid PV system. It executes charging algorithms which allow the battery to be cycled under optimal conditions and deliver its rated capacity over its expected lifetime.

Solar charge controllers are available in different configurations. Pulse width modulation (PWM) and maximum power point tracking (MPPT) controllers are the two most commonly used types of charge controllers in today’s solar power systems. PWM controllers regulate the output voltage by adjusting the output switching transistor's duty cycle. A duty cycle is the pulse width multiplied by 100%. It defines the percentage of time when it is ON. The length of the duty cycle determines the power delivered to the battery. The charge controller monitors the battery voltage and gives as an output a binary signal that specifies the length of the duty cycle. An MPPT controller is designed to track the optimum current-voltage point on the current-voltage curve (maximum power point). It is a DC-to-DC converter that can step the higher solar panel voltage down to the charging voltage of the battery. The DC-to-DC converter serves to decouple the solar array and load voltages so that the MPPT controller can maximize the efficiency at which the solar panels deliver electricity to a load, whilst charging the battery at a voltage that is well matched to the required-state voltage level of the battery. An MPPT controller "looks" for the point at which the photovoltaic module produces maximum power, and then performs a voltage/current conversion to change it to exact values that the battery requires.

PWM Advantages
  • Ability to recover lost battery capacity and deter the formation of sulfate deposits.
  • Increased the charge acceptance of the battery.
  • High performance in low power (specifically low current) charging applications.
  • Allow lower amount of power applied to the batteries when the batteries are almost fully charged.
  • Relatively constant harvesting efficiency regardless of the size of the system.
  • Allows the battery to be fully charged with less stress on the battery.
  • Dramatically increases the charge acceptance of the battery.
  • Reduce battery heating and gassing.
  • Fewer electronic components and less thermal stress, resulting a longer lifespan of the controller.
  • Automatically adjust for battery aging.
  • Self-regulate for voltage drops and temperature effects.

PWM Disadvantages
  • Perform well in a subtropical to tropical climate but operate less efficiently in cold climates.
  • The PV module input nominal voltage must match battery’s nominal voltage.
  • Limited capacity for system growth.
  • Cannot be used effectively with 60A panels.

MPPT Advantages
  • PV array voltage can be much higher than battery voltage.
  • Allow the battery to be always charged at the maximum power point for maximum charging efficiency.
  • Significantly outperform PWM controllers in cold temperatures.
  • Ability to maximize energy harvest in any environmental conditions.

MPPT Disadvantages
  • Reduced harvesting efficiencies when used in low power applications.
  • More expensive than PWM controllers.
  • Generally larger in physical size.

PWM regulation is typically recommended for use in smaller systems where “boost” benefits are minimal. MPPT is recommended for 150W-200W or higher sized systems such that these system can take advantage of the “boost” benefits of MPPT. The greatest benefit of MPPT regulation is observed in colder climates.
 
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