MPPT has in many ways revolutionized the PV industry. Higher-voltage PV arrays can now be used, reducing wire cost, improving efficiency, and increasing array performance under less-than-ideal conditions (clouds, low-horizon sunlight). But what exactly is MPPT, and how does it work?
Take a 12 V PV module, put it out in the sun, and measure the open-circuit voltage (Voc)—it will be much higher than 12 V, likely approaching 20 V. The reason that “12 V” nominal modules need to produce more than 12 V is that its voltage is the electrical “pressure” in a circuit, and has to be higher than the battery voltage to “push” energy into those batteries. PV modules perform best at cold temperatures, but their voltage will decrease as they get warmer—by about 0.5% per degree centigrade. A “12 V” nominal battery will rise to more than 14 V as the batteries approach a full state of charge. Higher PV voltage is essential, especially in hot weather; to push amps into the battery.
But what happens to excess voltage, especially in cold weather? Without MPPT, the battery bank “clamps” the array voltage to just above battery voltage, and those extra volts aren’t used to produce power.
To extract this extra power, a MPPT circuit will monitor the incoming power and alter the resistance (load) the array sees until it finds the array’s MPP voltage. While the MPPT circuit keeps the array operating at its maximum power point, it then uses a DC-to-DC converter circuit to lower the output voltage (and increase the output amps) to the battery bank.
The “MPPT vs. Non-MPPT” graph shows that the output of an MPPT controller operates at the “knee” of the I-V curve—at the maximum power point. As sunlight intensity, array voltage, and battery bank voltage change throughout the day, the MPPT controller automatically readjusts to put the maximum current into the batteries.