Understand first how a PV cell operates and secondarily what the conditions that the PV will see in the environment, such as temp variation and illumination variation as clouds go by.
PV cell is an illumination based current source that is clamped in maximum voltage level by the cell's inherent diode. There is shunt loss resistance (which for a good quality cell is minor) and series resistance (primarily due to the metal feed and resistance to cell PN body junction).
If a PV cell is left unloaded in the sun, all the generated illumination current is shunted down the inherent diode within the cell PN junction. The voltage drop will be just like a forward biased diode voltage drop. For silicon it will be 0.6 to 0.8 vdc per cell depending on how much current shunts through the cell.
If a PV cell is short circuited (or shunted with an amp meter) it will show a current based on the illumination level. The illumination current generated has a minor dependence on temperature so this amp meter shunt make a good illumination level indicator.
MPPT point is a load on the cell (or series of cells) that allows the highest possible voltage across the PV cell while allowing only a small amount of illumination generated current (2-3%) to be shunted down the cell's inherent diode. This yields the best V x I product output or maximum power for a given level of illumination generated cell current.
Since the inherent cell diode voltage drop is a function of temperature, just like a regular diode, MPPT optimum loaded voltage point will vary depending on what is the temp of the cell. As cell get hotter in the sun it produces less power (for same illumination level) since the cell inherent diode voltage will be lower as temperature goes up. Neglecting cell shunt resistance, the series resistance in the PV circuit path will have more voltage drop, the more the output illumination current is.
Now to switching power supplies. Most switching power supplies are output voltage regulated switching power supplies (perhaps with a maximum output current limit for protection) that also require a relatively low impedance input source, i.e. a voltage source for their input. Hook this type of a buck or boost switcher to a high impedance current source, like a PV panel, and they will likely result in a switcher that oscillates with unpredictable results.
A switching power converter for a PV panel must be regulated by input voltage and current to load the PV array for maximum power draw, as well as secondary feedback from output voltage and current to protect the battery from overcharging or excessive charging current. When the battery side reaches maximum charge voltage or current the switcher will no longer be operated in MPPT mode.
Assuming the output power from the MPPT switcher is not limited, then the MPPT regulation is solely a function of allowing the switcher to draw more current from the PV array (increase the load on PV array) to point where the voltage starts to drop. A simple MPPT switcher can just regulate based on input voltage but this does not take into account PV temp variation or series resistance voltage drop to PV cells. This is sort of equivalent to saying first order approximation for a silicon diode voltage drop is always 0.6 vdc. Normal MPPT voltage for a moderately sun warmed PV cell is around 0.52 vdc per cell. If you are in a hot desert it may be as low as 0.42 vdc for silicon. If you are in a cold environment it might be as high as 0.68 vdc per cell. A more sophisticated MPPT controller will adjust for this by searching out the best V x I product.
This MPPT algorithm is not as simple as it might sound given the changing environment of clouds passing by on a partly cloudy day and the fact the cell cools down when shaded by clouds and heats up when in the clear sun. Just about all MPPT controller do a load 'hunting' process to search for the best loading to yield best V x I product. Again, not as easy as it might seem since this hunting process, by default, means the cell is not being operated at it maximum V x I product during some of the time spent in the hunting process. The amount of time spent hunting can degrade the overall average MPPT power delivered.
Here is the frustration for the MPPT hunting algorithm: Just when you hunted your way to find the best V * I product, the sun again dips behind a cloud and the illumination generated current drops significantly. If the switcher does not react quickly to lighten the load on the PV array the input voltage will drop like a rock. Input voltage is detected to quickly react. Also realize the cooling down of the cells in the cloud shaded condition and the heating when sun re-appears occurs at a much slower rate then rather quick illumination level change. So you must decide when is 'good enough'. Hunt too much and you lose the MPPT overall benefit.
A single parameter sense feedback control system is difficult for most folks to achieve proper loop stability of the switcher. The PV MPPT controller is a multi-sensed, multiple loop feedback control system. A very complicated design task.
MPPT controller usually do not justify themselves for PV arrays less then 1000 watts. Just a simple PWM controller is more cost effect for smaller power arrays. PWM controller is nothing but an ON-OFF chopping switch allowing the PV illumination current to pass directly to batteries or not. This regulates the maximum average charge current to battery and turns off (or low duty cycles) when battery full charge voltage is reached. PV array must always be greater then maximum charge state battery voltage, but it is desireable to not be excessively higher, as PV load voltage when switch is ON is always going to drop to battery voltage.