# How does an MPPT charge controller control the input voltage?

I understand how buck/boost converters change the output voltage, but how is an MPPT converter able to control the input voltage/current while keeping the same output voltage? I understand the theory of MPPT, but not the implementation, and every search for "how MPPT works" leads to how the theory works.

• An MPPT controller does not keep the same output voltage. It allows the output voltage to vary in order to control the amount of power being transferred -- typically using it to charge a battery at a variable rate. – Dave Tweed Jun 23 '19 at 1:16
• @DaveTweed Really? My understanding of MPPT controllers on the implementation level is way off then. – Daffy Jun 23 '19 at 1:28
• They try to maximize power, hence the name. So the voltage doesn't have to stay the same. – hekete Jun 23 '19 at 6:08
• @DaveTweed "An MPPT controller does not keep the same output voltage" unless the output voltage is dictated by some other factor, such as a connected battery (in short time windows, the voltage is constant and same as the battery voltage) or the "other" MPPT connected parallel. – ceremcem Nov 29 '19 at 16:45
• @ceremcem: Even then, the battery terminal voltage varies in response to the charge current. – Dave Tweed Nov 29 '19 at 18:23

An MPPT basically works by attempting to present a solar panel with the ideal "resistance".

In the following image, the red line shows the current-vs-voltage curve of a typical solar panel. The basic idea is that for zero current (open-circuit), they provide maximum output voltage, and as you increase the current output (i.e. you go from infinite resistance to zero resistance) the voltage of the solar panel decreases. Somewhere in this range, there is an ideal amount of "resistance" that when connected to the solar panel, will extract the most power. Or, in other words, there is an ideal amount of current to "pull" from the solar panel that will provide the maximum amount of power

The way MPPTs do this is by having some kind of DC-DC converter, such as a buck-boost converter, in which they carefully control the output current of the solar panel (i.e. the input current of the MPPT) such that the maximum power is obtained. Essentially, they behave as if they were variable resistors, trying to find the best spot to consume the most amount of power. Of course, this inevitable makes the output voltage variable, as this input current adjustment will come from the DC-DC converter's duty cycle.

Additionally, some MPPT algorithms are smarter than others, since sometimes there are multiple power maximums: EDIT:

As requested, images were taken from here and here. Unfortunately these pages are not very informative.

• So the initial assumption of "constant output voltage" was wrong. That's what I wasn't understanding. – Daffy Jun 23 '19 at 2:06
• Yep. If you want to have an MPPT with a constant output voltage, you'd need to have an intermediate energy storage (such as a capacitor bank) after the first DC/DC converter, and then place another DC/DC converter after your energy storage. But it would kind of beat the point of the MPPT if the second converter doesn't output enough power. – Chi Jun 23 '19 at 2:10
• Can you please provide links or citations for the graphics that you copied into your answer? Thanks. – Elliot Alderson Jun 23 '19 at 12:55