Dave Tweed wrote:

> I've only been skimming this thread up til now, so I'm not sure where
> this concept of measuring the open-circuit voltage of the panel crept
> into the discussion.
>
> The V-I curve of a panel is far from linear, or even constant, so the
> open-circuit voltage has very little to do with the maximum-power
> point. You need to measure the actual output voltage and current of the
> panel simultaneously in order to calculate and maximize the power.


This only needs to be done once. Unlike the original
discussion of a "general PIC MPPT system" which would
definitely benefit from being closed loop and constantly
self tuning, this thread has evolved into a particular
situation for a commercial product. In his case the panel
is a known constant and can be characterised before
production.

> Although you will want to vary the operating point occasionally to see
> whether the optimum value has changed, the zero-current point is not
> the most useful place to look.

I don't think this is true. A solar panel produces an
OC voltage depending on the level of light available.
They can be used as light sensors. By doing the simple
(and QUICK) OC voltage test the PIC knows the level of
illumination, and since the panel has been characterised
beforehand the PIC can simply select a PWM duty cycle
based on that one quick test. The battery voltage will
change very little compared to the panel voltage, but
since the battery voltage is measured too it can be
compensated simply in software.

The benefits of a very simple and fast 1mS OC voltage test,
coupled with a "brainless" PWM output selected by a lookup
table are significant since his PIC is needed for other
tasks and this is NOT just meant to be an MPPT device.
-Roman

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