How to Find the Right Charge Controller
Solar PV systems with batteries use both Maximum Power Point Tracking (MPPT) and Pulse Width Modulation (PWM) charge controllers. Both serve the basic purpose of regulating the amount of charge that flows into the system’s battery pack, and to ensure that the batteries do not get over- or undercharged.
However, there are important factors that decide which type of charge controller is best suited to a particular PV system.
Factor 1: Ambient Temperature
MPPT charge controllers are able to utilize a solar array’s excess voltage, i.e., the voltage produced over and above what is required to charge a PV system’s batteries (usually 12V). They are therefore preferred for colder ambient temperatures, because as the ambient temperature falls, the voltage produced by the solar array gradually increases, and after a certain point exceeds the 12V required to charge the battery pack.
On the other hand, rising ambient temperatures have the opposite effect on array generated voltage, and after a point there exists no net difference between the array’s voltage and what is required to charge the batteries. As PWM charge controllers are not capable of charging batteries with any excess input voltage, they are ideally suited to warmer ambient temperatures.
Factor 2: System Size and Cost
MPPT charge controllers tend to be more expensive than their PWM counterparts, and they are designed to be best utilized in systems rated above 150 kW. Therefore PWM charge controllers are better suited to smaller systems – typically residential rooftop systems and small commercial applications.
Factor 3: Array to Load Ratio
In a system where the array size is relatively large compared to the load being catered to, the system’s batteries will tend to remain at close to their full charge.
In such a case, PWM charge controllers are a better choice as they can efficiently regulate the charging of the batteries, without having to resort to the more expensive MPPT charge controllers.
Factor 4: Type of Solar Module
Off-grid solar modules – typically 36-cell modules – are compatible with both PWM and MPPT charge controllers. However, some newer grid-tied solar modules are no longer of the 36-cell variety. Instead they use 60 cells, whose voltage generation can be too high for charging 12 V batteries.
In this scenario, MPPT charge controllers are the best suited technology, as PWM charge controllers are unable to utilize the excess voltage generated by each module.