Solar Charge Controller Types – Shunt, PWM, MPPT
A solar charge controller is a device that manages the charging of a battery from a solar module or array of modules. Solar charge controllers come in three types, though the market is dominated more and more by just one of those.
A shunt controller is just an ON/OFF switch. When the battery voltage is low (ie needs a charge) the switch turns ON and energy flows from the solar array to the battery. When the battery voltage is high (ie charged) the switch turns OFF and charging stops.
Shunt controllers are the simplest type but also the crudest and offer the least efficient battery charging. (Imagine trying to fill a glass with water from a kitchen tap that only had 2 settings: OFF or full ON.) There are few shunt controllers available today and only in small sizes and generally for specific applications.
If you take a shunt controller and turn the switch ON and OFF rapidly you can control the average charging voltage that is applied to the battery. That’s PWM, or Pulse Width Modulation. This technology allows for multi-stage charging including Bulk, Absorb, and Float stages, which significantly improves charging efficiency and charging control, thus improving battery performance and lifespan.
PWM controllers were the industry standard for many years and include well known controllers like the Trace/Xantrex C-series and the Morningstar Sunsaver, ProStar and TriStar lines of controllers, among others. They are still used in industrial and remote applications where small, simple, and highly robust equipment is a priority, and many models have been ruggedized for industrial applications.
Batteries have unique charging requirements for optimum performance. Solar modules have unique electrical characteristics that change throughout the day. If we are to achieve maximum efficiency and effectiveness in charging we need some kind of conversion or translation device that allows the solar array to operate at maximum efficiency while, at the same time, providing optimal charging to the battery. That’s exactly what an MPPT, or Maximum Power Point Tracking controller does – it operates the solar array at its point of maximum power (the Maximum Power Point, or MPP) and independently applies optimal charging to the battery bank.
The vast majority of solar charge controllers in use today are MPPT controllers and include units by companies like Outback Power, Magnum Energy, Morningstar, Midnite Solar, Schneider, and others.
MPPT controllers often have superior programability and features like data logging, which can be very valuable for troubleshooting.