ANOTHER D*** Solar Question!!!!

jrnymn7 wrote:
" "Imp" is just a value of I on the I curve which I is the same as the Isc you would get at the time"

I'm having difficulty wrapping my head around that. It seems to me Imp and Isc could be quite different at any given moment. If two identical panels were hooked to two identical banks, at identical voltages, it seems to me a pwm cc and an mppt cc could/would utilize different I values... the pwm using Isc, and the mppt being somewhere on the curve that coincides with the mpp.

The IV curve for the panel is there all the time no matter what kind of controller you use.

The panel's Isc, taken when it is disconnected, will be any 'value' of I at the moment depending on isolation. That is the I you get to the battery at the same moment while connected when using PWM controllers.

The I you get to the battery using MPPT with its buck converter, when in MPPT mode, is a sort of accidental amount that results from a different set of circumstances. It is neither Imp nor Isc.

The panel will have a voltage and an Isc sitting out there in the sun. Its voltage will be reduced by the panel temperature being warmer from the sun, but its Isc will not be reduced.

Depending on its voltage and insolation for what Isc is at the moment, the panel will have some Watts sitting out there.

The controller will find the MPP, which is at a certain voltage on the knee of the IV curve (yes, that same curve for the panel) It will have its INPUT WATTS from the panel. Imp will be an accidental sort of amount from dividing that Wattage by the voltage the MPPT chose to get MPP (Vmp). It could be any sort of amps amount, but it is unlikely to be the "rated Imp" because the panel being hot, will not be up to its "rated watts"

The MPPT buck converter has an "efficiency" which depends on a bunch of things but two big ones are:
1. how much bucking. It is less efficient to go from 48 to 12 than from 24 to 12.
2. internal controller temperature. It is better to have the insides of the controller cooler. The fins on the outside are only so helpful. It would be better if the controller had openings and a fan like converters and inverters do. At least keep it out of the sun in the shade and try to keep it in open air.

So now you have the OUTPUT watts from the controller which is a bit less than its input watts. At this point it is the battery voltage that sets the amps you will now get to the battery on that path. The OUTPUT watts are the same, but with a low battery voltage you get more amps by dividing the output watts by battery voltage.

So with MPPT the amps you get to the battery are sort of an accidental by-product after all that other stuff has happened.

With PWM you don't have to cross your fingers and hope to get at least some amps like you do with MPPT. :) You get the I that is the same as the panel's Isc of the moment every time, no fooling around.

Sometimes when conditions are right--low panel temperature and low battery voltage, you will get more amps with the MPPT than with the PWM. Other times you could get fewer amps.

I prefer the certainty of knowing what amps I am supposed to be getting, so I would rather have a pair of 12s for my 230w and PWM instead of a 24v 230w and MPPT, but I suffer along as best I can with the big 230 and the MPPT because of the price being so much less to buy the big 230 instead of the two 12s.

YMMV :)