Our batteries are expensive components of our off-grid solar installations. So it is a good idea to protect them from over-charging and deep discharge. To do this we use charge controllers. Charge controllers sense the voltage on the battery. If it is lower than a given threshold then they disconnect the battery from the load. If it is greater than a given threshold then they disconnect the battery from the source. In some designs, the current generated by the PV module is sent to a solar heater or some other circuit, when the battery is disconnected from the source. Pulse Width Modulation or PWM charge controllers work in an interesting way. When the battery is near full charge, they send short charging pulses  to the battery and when it is nearly discharged, they send long, frequent charging pulses to the battery. These can cause radio interference. MPPT charge controllers are much more efficient than PWM charge controllers. They check the output of the PV modules and compare it to the battery voltage. Then they electronically compute and adjust the voltage to present to the battery, so that power losses are minimum. The featured image is from: http://studer-inno.com/upload/pictures/VarioString_web.jpg
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