MPPT vs PWM charge controller in an always cloudy scenario

[Sunking]

Well I was at 20.7 VOC today.. then with both panels ON I was at 19.2 at the terminals inside the charge controller..

You may be correct.

As your batteries reach full capacity, assuming there is no load device taking power, you will see the input voltage go up as the duty cycle decreases toward ZERO. It will never actually go all th eway to 0% because of the battery self discharge rate which if the battery is in good shape should be about .001C so for a 100 AH battery a trickel charge is about .001 x 100 = .1 amp.

What you have to keep in mind which eludes most people is a solar panel is a current source, not a voltage source like a battery. A current source has a source impedance which will dictate how high of a voltage it can reach before the current collapses.

So let's say I have a 20 volt 5 amp current source. That means it can supply 5 amps up to a 20/5 4 ohm load. So if I short out the current source I will have 0 volts @ 5 amps. If I connect a 1 ohm load I will have 5 volt @ 5 amps and so on until I reach 4 ohms.

So put that into a solar panel spec of say your typical 100 watt 12 volt battery panel and you will get a Vmp 0f 18 volts and Imp of 5.55 amp. That means at maximum solar input the panel can deliver its rated power into a 18 / 5.55 = 3.24 ohms. If I were to connect say a 2 ohm load the panel output would drop to 11.1 volts @ 5.55 amps or 61 watts.

[epsgunner]

So today with a good battery I get BULK and 12.9V.. nothing else is showing on the meter..



It does say 13.4 Volts coming off the panels..

I guess thats the difference.. the battery needs nothing.. I even have a 12v car bulb attached and nothing..