To MPPT or to not MPPT, that is the Question

Do I need a MPPT charge controller for my system?  This is a question that comes up for any off-grid solar array.  And it’s an important question not just for your wallet, but your system as well. 

Remember, the whole goal of the array is to charge your battery bank.  Therefore, we want to harvest as much amperage from the array to fully charge our battery bank.  It doesn’t matter if you do this with or without an MPPT charge controller.  What matters is that your batteries always come back to a full charge as well as never dip below 70% depth of discharge.

So how does an MPPT help in the above goal is the real question?  First of all, what is an MPPT charge controller?  A maximum power point tracking (MPPT) charge controller is a device that can change its internal resistance in order to output the maximum possible power from your solar array.  It does this through algorithms used by a DC to DC converter.  In simple terms, this means that the charge controller can take any excess voltage your array produces and convert that into a higher amperage output.  And again, our goal is to harvest as much amperage from the array to fill our battery banks amp-hours.

In any off-grid design, the array voltage is always higher than the battery bank nominal voltage.  This allows for the charge controller to provide enough voltage for the different charging cycles.  For example, a 24 volt battery bank will require around 28 volts for the absorption charging cycle.  Also, the voltage of the array has to be increased to account for voltage drop and temperature change as well.  With this in mind, there is always excess voltage to be used for the MPPT charge controller to carry out its function.

Why wouldn’t I choose an MPPT charge controller?  Right now they sound awesome.  And they are awesome.  In reality, any off-grid design can be done without them though.  Every design simply has to pair the modules with the battery bank.  In areas or at times with less sunlight hours, the array size and battery bank size just has to be increased.  However, there will be a point where the costs of increasing the array size and battery bank exceed the cost of an MPPT charge controller.  In general, if you are in an area or time of year that has limited direct sunlight hours (2-3 hours/day), an MPPT charge controller can really benefit you.

Take your time planning your off-grid array.  In the end you will save money and have a great performing array that last a long time.