In an off grid system, the charge controller is THE most important piece of equipment, because it acts as the intermediate between the PV array and the battery bank; which means it is the sole piece of hardware managing the energy extraction from the PV array – a role designated to the inverter in a grid tied system.
There are numerous charge controllers on the market, but there are some very key factors that separate a top of the line charge controller, from one that is more or less junk.
A premier charge controller will have a buck – boost functionality; manufacturers will vary in what they call this function, however the description remains the same. These charge controllers contain hardware that is capable of separating the DC input from the DC output and manipulating each independently. For example, consider the following scenario:
A PV module has a Vmp value of 30 VDC, an Imp value of 8.5 Amperes, and an ISC value of 9.2 Amperes and is connected via a charge controller, to a 12V battery bank. A charge controller with this function will be able to accept an input voltage of 30V and 8.5A for a total of 255 Watts, and then deliver upwards of 20A to the battery bank at 12V; as you can see, it acts as sort of a variable DC transformer.
Alternately, a simple PWM charge controller will not have that ability. Using the same module, under this new scenario, the module will be intermittently connected and disconnected directly to the battery bank. Unfortunately what this means is that when the module IS delivering power to the battery bank, it is doing so at the voltage of the battery bank. The same PV module used in the example above, would now deliver somewhere on the order of 9 amperes at a voltage of 12V (because it is connected directly to the battery bank), and thus would only be delivering 108 Watts!
So in this example, the cheap charge controller is only delivering 42% (108 W / 255 W) of the electricity that the array is capable of generating!
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MPPT Tracking refers to the ability of the hardware to determine the maximum operating point of the PV array, and thus extract as much energy as possible each day.