Bucking and MPPT?

KJINTF wrote:
OK guess I'm a bit dense
I now understand what it is you are going on about
It's the magic tracking, the "T" in MPPT we are discussing

The magic in a quality MPPT controller is hidden in the tracking function.
Finding a moving target as in Vmp can be difficult, array sweeps, output interruptions, etc all controlled by the tracking algorithm. Yes some older Blue Sky units were simply set it and forget it, IMO a joke. That might work somewhat OK for a fixed install but can not possibly do good for a RV's install where sun shade temp changes etc.. are part of the mix.

Tracking functions include finding and holding on to the Vmp of the array. It has several other tasks for example limiting output current by reducing the input power (the reason why many MPPT controllers can be connected to larger than "allowed" PV array), It moves away from the Vmp thus not harvesting all the available energy. Typically the PWM charging function Bulk ABS Float etc is in constant communication with the tracking function. A quality controller will always have tracking enabled 100% of the time (when actually passing current the loads), which is sometimes not true for the buck conversion stage that can be bypassed when the array voltage drops to low. The Buck conversion output is directly controlled by the tracking function. Tracking simply reduces the input power limiting the output current that can charge the battery with the PWM function. Most good PWM operations will go from 0% to a full 100% another place to limit/increase current to the loads.

Now all we need are measurements of Array Voltages/Currents as well as Battery voltages/currents.

That data will show you what is happening in each of the charging modes

I have a morningstar MPPT controller that I use with three "12V" panels in series. It's interesting to watch it track when first connected to the panels. Watching my Trimetric, in bright sun, it will output about 5A into a well discharged battery bank. After about 30 seconds, the current starts to rise quickly to near 12A as it begins to search for optimal Vmp. In full sun it will soon hit the 15A limit of the controller. In partial sun it will start converging with the the current going up and down as it searches, finally settling down. Then, if it is suddenly shaded for a few minutes the process will repeat Not sure what the algorithm is to determine when to search, but interesting to watch. If I pull off a panel and drop to two panels the process will repeat as well to find Vmp.

I suspect it starts assuming a Vmp in the range of 18V or so for a single panel then does a search to find the real Vmp.