Charge Controllers for Solar Electric Off Grid Systems

Charge Controllers

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A charge controller is an electronic voltage regulator, used in off-grid systems and grid-tie systems with battery backup, that controls the flow of power from the charging source to the battery. The charge controller automatically tapers, stops, or diverts the charge when batteries become fully charged. Some charge controllers have metering and datalogging capability to show system operation parameters and battery charge status. Load controllers have low battery load disconnect to prevent overdischarge.Lighting controllers have built-in light controls to turn on lights at night. Load diversion controllers are generally used for hydro and wind power systems in order to transfer energy to heat sink to avoid overcharging of batteries.

Charge controller capacities ranges from 4 amps to 80 amps and multiple charge controllers can be used in parallel for larger systems. The simplest charge controllers turn off the charge when the battery reaches a voltage near full charge, and turn it on when the voltage drops about one volt. Pulse width modulated (PWM) charge controllers turn on and off very rapidly, holding the batteries at full charge, making better use of available power.

Maximum power point tracking (MPPT) charge controllers take power from the charging source at a voltage where it can put out the most power (its maximum power point) and convert that to the correct voltage to charge the battery. This technique significantly increases the power from a solar array, especially when batteries are discharged, battery voltage is low, and the temperature is low causing the maximum power point voltage of the solar modules to be high. Most MPPT charge controllers can take an array voltage much higher that what is required by the batteries, allowing the use of modules with higher peak power voltage, designed for grid-tie use.

A higher voltage solar array also allows smaller wire to be used between the array and the charge controller, which can save wire and installation cost in large systems. Maximum power point tracking allows a PV array to deliver up to 30% more power to a battery than it would if it were connected directly to the battery.