Charge Controllers 101

A charge controller is used to keep the voltage across the battery within acceptable limits. The charge controller automatically tapers, stops, or diverts power when batteries become fully charged. Charge controller capacities range from 4 A to 100 A and multiple charge controllers can be used in parallel for larger systems. Some charge controllers offer additional features including charge status display, data logging, automatic battery equalization charging, generator starting, and even lighting controls.

The simplest charge controllers disconnect the power source when the battery reaches a set voltage, and turn it on when a low voltage set point is reached. Pulse Width Modulated (PWM) charge controllers turn on and off very rapidly, maintaining the batteries at full charge voltage, which results in quicker and more complete battery charging. Maximum Power Point Tracking (MPPT) charge controllers optimize the voltage of the PV array to maximize total power output then convert that to the correct voltage to charge the battery. This process significantly increases the power from a solar array, particularly in low temperatures when battery voltage is significantly below the PV array voltage. Most MPPT charge controllers work with higher array voltages, which can greatly reduce the required wire size between the array and the charge controller. While more expensive than PWM controllers, MPPT charge controllers can boost system performance by up to 30% making them very cost effective.

MPPT Charge Controllers

The wattages for recommended maximum nameplate PV array sizes can be exceeded by up to 20% without damaging the controller, but some “clipping” of potential peak current may occur under cool, clear conditions at the peak of the day. While exceeding these wattages may reduce power harvest at peak times of the day, the total daily amp-hours delivered to the battery bank will be greater because the larger array will produce more power in less-than-peak conditions such as mornings, afternoons, and in hazy or cloudy weather.

PWM Charge Controllers
It is important to note that PWM charge controllers have limited voltage correction capabilities and should only be used with 36 or 72-cell modules in series or parallel to match the battery voltage.

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