OK - how about this for simplicity.
Forget about measuring the current(s) altogether and just measure the input
voltage.
The maximum power transfer will be when the conversion circuit input
resistance equals the source resistance of the PV array.
So, depending on the input time constant, but probaly only for a few mS,
turn the boost (or buck or whatever) converter off, measure the open
circuit voltage developed, and then adjust the switching pulse mark/space
to get 1/2 this value when charging the battery.
This need only be done every 10 seconds or so and most of the unconverted
energy will be recovered from the filter cap anyway.

The advantage is that the optomisation is always known, if the voltage is
high - then just increase the on-time slightly. - You do not have to dither
the mark/space ratio around to find a peak.

This does require that the PV characteristic is such that maximum power is
developed at best resistive match but I would expect this to be very close
to the case anyway.

Can anyone see a problem with this - I think it might be worth a try as it
is very easy to physically implement.

Richard P





                    Roman Black
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                    Sent by: pic          Subject:     Re: [EE]: Solar charging of batteries
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                    22/01/03 14:14
                    Please respond
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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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