Now why didn't I say that. ;o)
-Roman


Olin Lathrop wrote:
>
> > With a system like battery + solar panel the part that most affects
> > efficiency is the one with the higher Ri, and that is the panel, so the
> > mppt system should concentrate on that point, if you cannot or do not
> want
> > to cover both.  In practice you could measure the ac ripple on the input
> > capacitors of the smpsu and scale it (knowing their esr or something) to
> > obtain the required information.
>
> Instead of measuring parameters that effect efficiency, why not just
> measure the end result and tweak the solar panel voltage (or current
> depending on how you want to look at it) to get the best result.  This way
> you don't have to assume anything about the intervening mechanisms, and
> you get optimal operation by definition.
>
> For example, if the solar panel is to charge a battery, then the best end
> result is the highest battery charge current.  Let's assume that you've
> wired up the solar panel such that under useful illumination its voltage
> always exceeds the battery voltage.  First you put a big fat capacitor
> accross the solar panel, then put a buck converter between the capacitor
> and the battery.  The capacitor is big enough so that the individual
> pulses of charge going into the buck converter don't significantly alter
> the capacitor voltage, and therefore the solar panel operating point.
> However, in the aggregate, the duration and frequency of buck pulses
> control the current draw, and therefore adjust the voltage on the
> capacitor, which is also the solar panel operating voltage.
>
> You really don't give a rat's butt what voltage, power, and current the
> solar panel is running at as long as the battery charge current is maximum
> for the given illumination.  There is also no need to know the solar panel
> impedence, open circuit voltage at high noon in Arizon, it's voltage
> versus current curves, or what day of the week it was made on.  All you
> need to do is turn the buck converter up or down to get the maximum
> desired output, which in this case is the battery charge current.  All the
> solar panel characteristics and the buck converter ineficiencies will be
> automatically adjusted to by Feedback Magic(tm).
>
> The only tricky issue is that the buck converter to charge current
> transfer is not monotonic.  It has a maximum point somewhere within the
> range, and falls off on either side of that point.  This kind of
> maximum-finding is poorly suited to analog feedback, but can easily be
> handled by digital logic.  A small PIC with an A/D and PWM module running
> at 32KHz should be able to handle the task just fine.

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