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So called heart of any off-grid connected system (autonomous system), charge controllers regulates electric current flowing through the system. Located at the node of the system, controller basically controls the charging and discharging of the battery in the system. Two major technologies are employed.
Pulse Width Modulation (PWM): These controllers use MOSFET or power transistors at very high frequencies to pulse the charge current on and off in order to maintain a constant battery voltage. Used in a series or shunt configuration the duration of the cycle will vary depending upon the battery voltage and the available charge current. These controllers offer excellent charging characteristics for solar arrays of up to 40 amps, however, due to their high switching frequency may cause noise on some telecommunications equipment.
The relays used within these controllers offer some limitations which affect performance and useful life. To extend life, relay's on/off range will be widened to reduce their cycling though this results in less efficient battery charging. Where currents are high, these relays may have a bypass circuit to temporarily handle the current during the cycle. Solid state switching devices will cycle almost endlessly without damage to them, however, the resultant voltage drop will lead to some heat generation.
Maximum Power Point Tracking (MPPT): MPPT’s are smart DC to DC converters that optimize the match between the solar array and the battery bank. While gains of 50% in solar module output are possible the typical wattage gain using an MPPT is 10 – 15% vs. a PWM controller. For example, Shell Solar’s SP75 is rated at 4.4 amps @ 17 volts that is 4.4 times 17 = 74.8 watts. But this 75 watts does NOT equal 75 watts of charging capacity since your battery is only charging near 13.5 V. The output of a solar module is characterized by a performance curve of voltage versus current known as its I-V curve. For crystalline modules, the current remains fairly constant as the voltage changes relative to the voltage of battery it is charging. A battery charging at 13 V is only using 57.2 watts of power not the full 75 watts – a loss of about 24%. In an extreme case, such as a fully discharged battery at 10.5 volts, you would get nearly 7 amps at 10.5 volts from the MPPT into the battery! MPPT's are most effective under these conditions: Cloudy or hazy days - when the extra power is needed the most. Cold weather - solar module output increases in cold temperatures during the winter when sun hours are low and you need the most power. Low battery charge - the lower the state of charge in your battery, the more current a MPPT puts into them - another time when the extra power is needed the most. With higher voltage solar arrays of 300 Wp or more. Below this size, it may be more cost effective to simply add another solar module. (courtesy: www.matrixenergy)
