Sorry, but your understanding lacks the fact that a SCC can not increase loading without somewhere to send loading. Current is drawn not sent. Yes it attempts to find the maximum power point. However that is only up to the max charge current and not beyond (This might be a few amps either side of rating). It really is not a matter of cheap versus expensive charge controllers in that way. It is more matter of duty cycle rated components.
Loads drive supply not the other way around.
I do not want to get into a big argument here, but I have read up quite a bit on MPPT charge controllers and even looked at code to build one with an Arduino or Raspberry Pi. As a DC to DC converter, there is a definite risk of blowing up the switching components. In a perfect world, the input power from the PV solar panel would change very slowly and the MPPT would have no issue tracking it and limiting when it comes up to maximum power. And 99% of the time, it will live.
We do not live in a perfect world and MPPT code gets very complex to do it well. There is a huge trade off for tracking speed and efficiency. Let's try a worst case here. The panels are getting 50% irradiance which lowers the current from 21 amps to just 10.5 amps, but the maximum power voltage is still around 22 per panel, or 44 volts total. That is 462 wats into the 600 watt rated MPPT input. The MPPT is working great and tracking it without a problem. The switching FET into the DC to DC converter is running at maybe 70% duty to convert that to the 12 volt (actually 13 volts at about 35 amps. But now the cloud moves and the panel not only gets full direct sun, the nice white clouds around the area also reflect some light onto the panels. The available current can jump up to near the full 21 amps very quickly. The voltage also rises with the increased light. The next pulse of the DC to DC converter is still 70% duty cycle. The current into the inductor climbs much faster and can even saturate. If it saturates the coil, the current will rise even faster. This one pulse could damage the switching FET before the charge controller even sees the change in output current. Better DC to DC converter circuits get around this problem by adding another shut to measure the input inductor current and use a feed forward loop to immediately turn off the switching FET early at a safe maximum inductor current. This mode is called current controller PWM. It needs more components and costs more to build and you still need the output current measuring and feedback control.
In most circuits, yes, the load determines how much current it will draw, but an MPPT charge controller is a different beat. It is a current source into the battery. If the battery was directly connected to the solar panel, the current would be lower but the voltage would climb too high and fry the battery. The MPPT software has to work in (at least) 2 different modes. Most of the time it is hunting to pull the most power it can from the solar panels. And that is what causes the problem in this case. The cheap controllers depend on the fact that solar panels are a current source. Even if the DC to DC converter goes to 100% duty cycle, they know the circuit can handle the 15 amps rating they listed in the manual. That makes it so they don't have to bother adding circuits or writing code to limit the input current any further.
On my cheap controller, it will not even let me set the output current. It just produces all it can until the battery voltage reaches the set point. Then it will finally switch mode and adjust the duty cycle down to stop the output voltage from climbing.
The short answer here is this...
If the MPPT input lists a maximum ISC current input, DO NOT EXCEED THAT RATING.
If you wire up more panels and exceed that current rating, it may work, for months, maybe years. But all it takes is one time the sun hits the panels just right and the controller goes pop and you have no warranty because you fed it more current than it was rated for. In this case, the true current source is the solar panels. And they are a CURRENT SOURCE. If they say they can put out 21 amps and the input is only rated for 16 amps maximum, you are likely to have a problem. Connect the panels to a resistor, and you will see the current climb directly with the amount of light hitting the panels, up until you exceed the VMP voltage. On the flip side, if you change the resistor value, (the load) the voltage will change a lot, but the current will change very little. That is a current source, not a voltage source, and the whole reason we need MPPT control to find the peak power.
Yes, a Victron charge controller will happily current limit all day long while over paneled. So will Enphase inverters. But even these list a maximum safe ISC current. But it is typically much higher. In the case of this particular PECRON power station, it is rated up to 16 amps. That hits the maximum 600 watts at just 37.5 volts, even though it can handle 95 volts. So yes, you can safely over panel it by more than double. But keep the total ISC current below 16 amps.
I will not reply about this again. If you want to do it and it goes up in smoke in a few weeks, it's not my fault.
