Correct my logic please (still a n00b) on my thought experiment:
If I had enough light to get a panel to about 50% of its output (low light rough numbers)
This is more than you might think, but we can use it. Actually, given the current you've chosen, we're getting 42.5% rated power.
100w panel would be 12v @ 2.5a
No. It would be ~17V @ 2.5A = 42.5W (using Vmp)
4x 25w panels in series would be 48v @ 0.5a
If they are 12V panels, then they'd be closer to 78V @ 0.54A = 42.5W (using Vmp)
Now, because the PWM controller doesn't do anything until that magical 13.5v mark, 12v @ 2.5a = 0 watts to the batteries.
No. That is not the case with any SCC I've ever seen. The PWM will charge at below 13.5V. The PWM will "see" battery voltage on one side, and it will "see" panel Voc (~20V+) on the PV side. It will then short the panel to the battery allowing the 2.5A to flow at battery voltage, 2.5A * 12V+ = 30W+ (depends on battery voltage). Once the battery approaches full at say 14.4V, the power is 14.4* 2.5 = 36W.
With the MPPT controller the 25w panels would be 48v @ 0.5a = 25w to the batteries.
No. You'd get the 42.5W calculated above minus inefficiencies.
However, you'd also get the 42.5W from the single 100W panel as well as it can work at its Vmp of 17V on the MPPT instead of being forced down to battery voltage by the PWM.
It's the PWM's forcing of the attached solar panels to battery voltage that reduces the power collected where the MPPT allows the panels to operate at or near their Vmp value.
So if someone needed to deal with lots of cloud cover the multiple smaller panels and MPPT would be more effective over time than a larger panel with a PWM by a significant margin over time, right? The single larger panel would produce more amperage when it got the light to work with, but only under near perfect conditions?
No. Your two configurations would produce almost exactly the same amount of power on an MPPT. In fact, the 4X 25W in series would likely be slightly less due to inefficiencies associated with converting the 78V down to ~12V for charging vs. converting 17V down to ~12V. This kind of jump from high voltage is typically 1-2% less efficient than a jump from about 1.5X battery voltage (18V).
Please refer to my prior post... if a PWM would get you 100Wh for a given day, an MPPT would get you 110-120Wh on that same day - 10-20% more. You've tried to create a scenario that changes that relationship dramatically.
100W w/PWM will produce 30-36W depending on battery voltage
100W w/MPPT will produce 42.5W
4S 25W w/MPPT will produce 42.5W
4S 25W w/PWM will probably damage it due to over-volt, but if you could, it would only produce 12-14.4V * 0.5A = 6-7.2W
MISSING CONCEPT: The presence of light induces Voc. It doesn't take much to get full panel Voc.
This is 14 minutes BEFORE SUNRISE @ 6:37am this morning:
My panels hit 104V before the MPPT started pulling current - no sun on them. That first bump in current you see is 0.1A, and that pulled the voltage down to the 80s.
The point is to demonstrate and ANY panel in low light conditions is going to rapidly yield > 12V. It's the INTENSITY of the light that determines how much current will flow.
The BIG question here is...
Did the other SCC fix the problem?