With many different solar charge controllers on the market, it is difficult to know which the best option is, but in truth, every model belongs to one of two types: MPPT or PWM. Here, we explain how each of these technologies works.
How do PWM solar charge controllers work?
A Pulse Width Modulation (PWM) works as a switch connecting solar panels to batteries, and not as a DC to DC converter, which is why this charge controller does not fully take advantage of the I-V curve of the panels. This charge controller modulates a pulse coming from the panels to the battery, limiting the voltage according to the charging stage of the battery, instead of converting it.
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Since the PWM charge controller only limits the maximum voltage, this translates from power losses. The reason for power losses is that the voltage set point for the battery may not be the most optimum point in the I-V or P-V curve of the solar panel. In other words, setting the voltage to 12V without adjusting the current to match the maximum power point of the curve, will result in power losses.
PWM solar charge controllers are quite cheap, and ideal for small-scale PV systems. Since these charge controllers operate at an efficiency of 75-80%, they can produce 25-20% power losses to the system.
How do MPPT solar charge controllers work?
The Maximum Power Point Tracking (MPPT) solar charge controller maximizes the power extraction from the solar panels by following an algorithm that allows it to track the maximum power point of the I-V curve (point generally marked as Pm in the I-V curve). To match this Pm value (which varies across the day) at the voltage of the battery, the electrical current is increased. In other words, this type of charge controller will always be delivering the maximum possible amount of power from the panels.
By using an MPPT charge controller, it is possible to obtain a 92-95% DC to DC conversion efficiency. With this charge controller, you do not have to match the voltage of the battery since it will maximize the solar power regardless of the battery voltage you are working with.
PWM and MPPT charge controllers are always being compared. While PWM controllers are cheaper, MPPT is much more advanced, making them better in the long run. Here we compare some of the pros and cons of each one:
Efficiency
The best advantage of MPPT solar charge controllers against PWM is the efficiency. While the PWM solar charge controller reduces the voltage of the I-V curve, causing power losses of up to 25%, MPPT uses advanced microcontrollers to track the maximum power point on the I-V curve. This can be done by making a DC to DC conversion that matches this power value to the corresponding voltage and current values for the batteries, barely causing 5-8% power losses to the system.
This difference in efficiency is highly associated with the capability of MPPT solar charge controllers of tracking the maximum power point of operation, functionality that PWM models do not have.
Price
PWM solar charge controllers are cheaper because they are a much simpler technology; however, they leave behind higher power losses. PWM solar charge controller is better used for small and portable PV systems, where losses are not as considerable as in large systems and cost reductions are important. Meanwhile, for large or home-size types of systems, an MPPT is definitely required despite the increased price.
Voltage of the Panels
In PWM solar charge controllers, the voltage of the array has to be close enough to the voltage of the batteries in order to avoid substantial power losses. This is not the case for MPPT controllers, where the voltage of the solar array can exceed that of the batteries by far.
Temperature compensation
Most MPPT solar charge controllers include built-in temperature compensation systems, regulating the output voltage of the device to the ideal voltage for battery charging. Only modern PWM charge controllers include this option, but they usually require an external battery temperature sensor.
