The solar charge controller is an essential component for battery-backed up solar PV systems – grid-tied and off-grid. They are the medium that accepts the solar array’s power and charges the batteries. There are two main types of solar charge controllers on the market: Pulse Width Modulation (PWM) and Maximum Power Point Tracking (MPPT).
What’s Pulse Width Modulation (PWM)?
Before residential 60-cell panels, the most common panels on the market were 36-cell panels. Such panels typically had a voltage around 12V, perfect for charging a 12V battery bank. If one were to charge a 24V battery bank, he or she could stack two 36-cell panels in series to do so. However, connecting the panels directly to the batteries could damage the batteries by overcharging and improper current flow.
The solution to this is a PWM charge controller. These controllers effectively modulate the current through a “switch” like mechanism where the current is turned ON and OFF to increase or decrease the current and thus the voltage. By modulating current and voltage to meet that of the battery, the battery can be charged appropriately without damage or overcharging. PWM charge controllers are an effective choice for smaller off-grid systems since the solar panels often have a relatively close voltage to that of the batteries.
Downside of PWM Charge Controllers with Higher Wattage Panels?
Nowadays, since 60-cell and 72-cell panels have an open-circuit voltage (Voc) up to ~40-50V, and a maximum power point voltage (Vmp) around ~30-40V, PWM controllers would be restricting much of the potential power output of the solar panels. As shown in the graph below, if the voltage decreases, the power from solar greatly decreases.
The major downside to using a PWM controller with a higher wattage array is that the power is cut significantly from what it could be. When using a PWM charge controller for higher wattage panels (i.e. 260W+), the short-circuit current (Isc) of the PV array must be less than the rated maximum charging current of the charge controller.
In conjunction, the Voc of the array must be less than the rated maximum input voltage of the charge controller. However, for smaller applications, PWM controllers are a good choice because there is a relatively lower power loss associated with lower voltage panels when using PWM.
Are MPPT Charge Controllers the Best Choice for Higher Wattage Panels?
MPPT Charge Controllers are the standard controllers that are used in almost all contemporary PV arrays. MPPT controllers actively optimize the voltage and current of a PV array for its maximum power output and are utilized in all newer microinverters and string inverters.
For larger systems with higher wattage panels, MPPT charge controllers are the best choice for charging your batteries. These controllers often have the flexibility to accommodate the current and voltage of most systems and is a necessity for high power output. These are the best choice for higher wattage panels.