Oh so by using a straight up SMPS if you keep trying to get the voltage to be 13.5, once the solar panel voltage goes below a certain point, then it shuts off, while with MPPT you can still sorta charge the battery with a lower voltage? Some might also do a voltage boost I'd imagine.
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Though I guess you could probably use a much higher voltage solar panel or two in series and probably get away with more using a basic SMPS topology.
No, it has nothing to do with which voltage you select or what your panel voltage is, the problem is the way DC/DC converters operate. They are all designed with the assumption that the source is CV (constant voltage). A solar panel is only ~CV to the right of the maximum power point (Vpanel > Vmp). To the left of the peak a solar panel is CC, and DC/DC converters do not work correctly with a CC source.
Oh and what do you mean by dropping input impedance? isin't impedance basically resistance, but for AC? How would that be controlled and how does that come into play for solar panels?
resistance is for DC
reactance is for AC
impedance is the combination of resistance and reactance. It can be DC or AC or both. It's a more generic term than either resistance or reactance.
In this case, he's referring to the relationship between voltage and current (remember ohm's law) at the solar panel output (SMPS input).
Assume a SMPS is at steady state. Steady input voltage, steady input current, steady output voltage, steady output current, Pin = Pout (assuming 100% efficiency), everything is happy. Now what happens if the load increases slightly, even a milliwatt. Well Pin != Pout anymore, the SMPS needs to pull more power from the source in order to deliver this extra power to the load. How does it do that? All SMPS do this by reducing their input impedance, which means for a CV source, you get the same input voltage, more input current, which gets you that extra power.
This
does not work with a CC source. With a CC source, lowering the input impedance gets you the same input current and LESS input voltage, which means LESS input power. This causes the SMPS to lower its input impedance even more, getting it even LESS power, and this feedback loop continues until the panel voltage drops below the SMPS minimum input voltage and the SMPS browns out.
All of this means that a regular SMPS running off of a solar panel
cannot run the load when the requested power is more than the panel can provide, and when the requested power is more than the panel can provide, you only get the requested power. It's the worst of both worlds. With an MPPT you always get whatever the panel can provide. With other less sophisticated panel controllers, such as PWM, or the one that Seekonk has been describing, you might not get the full power that the panel can provide, but you'll be relatively close. Anything is better than a SMPS with a fixed output voltage/current connected directly to the panel.