Help on MPPT for solar panels

I recommend that you use either buck or boost with a battery that is always below or always above panel voltage respectively. eg buck with a 12V battery system, boost with a 24V battery system. This is because buck-boost converters that swap from one to other mode usually have a low efficiency range in the changeover area and converters that buck-boost by inverting, tend to be lower efficiency as all delivered energy is stored in magnetics and converted.

If you have a choice of battery voltage I'd lean towards a buck converter as these tend to be more efficient than boost converters.

If you have choice of battery voltage, using a battery whose V_max_in is only slightly less the panels Vout_min operating makes it easier to get good efficiency.

Note that MPPT is getting increasingly hard tyo justify financially due to the falling cost of PV panels. MPPT will give you maybe 25% gains in low and variable light conditions but in a well designed system will give much less gain at full sun. Low panel [prices mean that adding say 10% panel capacity may be cheaper than using an MPPT controller. MPPT still makes sense in situations where absolute panel size or fitted weight is constrained by non financial aspects. (eg available room on a yacht or vehicle).

The point of MPPT (maximum power point tracking) is to continuously adjust the load presented to the solar array for maximum possible power transfer. The output voltage from the solar array is affected by the current - if the current is too high, the voltage will drop and if the current is too low, then the voltage will saturate. This is represented by the I-V characteristic of the solar array, and it varies with temperature and illumination.

Basically, the way this works is the input resistance of the controller is adjusted such that the product of the voltage and current (power) drawn from the panels is the largest. The controller is generally configured in a similar manner to a buck DC to DC converter, and the duty cycle of the switching transistor is adjusted to change the current draw from the array. The voltage and current from the array are monitored by the controller, and the duty cycle (current draw) is adjusted to keep the efficiency high. According to https://en.wikipedia.org/wiki/Maximum_power_point_tracking , there seem to be several different methods of doing this. The simplest method is called 'perturb and observe', and the idea is to adjust the operating point slightly and then check to see if the new operating point is better or worse than the old one.

One possible problem with MPPT is that the output power will vary continuously, therefore it is necessary to have some sort of load-smoothing capability in order to fully utilize the power from the array. Generally this is done with a large battery bank. The batteries will be charged from the array with varying charge current as the power from the array fluctuates, ensuring that the maximum amount of power from the array can be captured.

Partial shading is where some parts of the solar array are in shade and as a result perform differently. MPPT controllers connected to the entire array cannot compensate for this, and the efficiency will suffer. MPPT controllers connected to individual panels can track the best power point on each panel individually and can compensate for partial shading to some degree.