PWM vs MPPT Solar Charging: Key Differences

This article compares PWM and MPPT solar charging and outlines the key differences between PWM and MPPT solar charge controllers.

Introduction

A solar charge controller functions much like a regular battery charger, regulating the flow of current from a solar panel to a battery. There are two primary types of charge controllers used between a solar panel’s output and a battery’s input: PWM charge controllers and MPPT charge controllers. Both methods are used to efficiently charge batteries using solar panels. The main functions of these charge controllers are:

• Preventing battery overcharging.
• Preventing battery deep discharge.

What is PWM Solar Charging?

PWM stands for Pulse Width Modulation, or Pulse Duration Modulation. It essentially functions as an electronic switch.

In this method, the average voltage (or current) supplied to a load is controlled by rapidly switching the connection between the power supply and the load. It turns ON and OFF between the power supply and load based on the battery’s charging status. During normal charging, a PWM controller delivers as much current as needed from the solar PV panel to the battery. When the battery reaches its target voltage, the PWM charge controller disconnects the battery from the PV panel to prevent overcharging.

The voltage output from a PWM generator can be calculated using the following equation:

V_out = V_max * Duty Cycle (%)

Where, Duty Cycle (%) = [ T_ON / (T_ON + T_OFF) ] * 100

Here, T_ON + T_OFF = the period of the pulse width modulated waveform, which is equal to 1/Frequency.

PWM charge controllers operate close to the maximum power point, though often slightly above it. PWM controllers are typically more affordable and come in smaller sizes. Low-pass filters (usually RC type) are often used to convert the PWM output to the desired DC voltage. Due to having fewer electronic components in their design, PWM controllers generally have a longer lifespan. Virtually all modern microcontrollers include PWM functionality, making it easy to develop a PWM solar charge controller.

Figure: Solar Electricity Block Diagram

What is MPPT Solar Charging?

The optimal ratio of current (I) to voltage (V) on a PV panel’s power curve to generate the most power is known as the MPPT, or Maximum Power Point. The MPPT curve varies depending on irradiation conditions.

MPPT provides an indirect connection between the PV panel and the battery. It functions as a DC-to-DC voltage converter. It’s especially well-suited when the battery voltage is less than the PV panel array’s output. MPPT utilizes the extra PV voltage from the solar array to generate extra current at a lower voltage without power loss. This increases the overall efficiency of the solar system.

DC-to-DC converters come in two main types: buck converters and boost converters. Buck converters are used when the output voltage is less than the input voltage, while boost converters are used when the output voltage needs to be greater than the input voltage.

Difference between PWM and MPPT Solar Charge Controllers

The following table highlights the key differences between PWM and MPPT solar charge controller types.