Peter Mcalpine wrote:
>
> ok, with an MPPT system, I am not 100% confident I understand
> what I need to measure with my uP..
> I can disconnect load from the cell - and measure open solar
> voltage level.. I also know at that time what my battery is
> with no charge.

Yes, use 2 ADC channels; to measure open panel volts
and standing battery volts.

> So, I then connect my SMPS and ramp it through and see where
> a maximum current output is?
> (given that I have enough solar voltage at the time etc)

No, you then check your data table in the PIC, which
gives the MPPT PWM width for each panel voltage, even
a small table like 32 entries will work well. This avoids
the need to measure current which is a dead loss in a
micropower circuit as it would be a resistive shunt,
and avoids the time lost in a ramping test.

Testing the open circuit panel volts only takes a few
mS, you can then do the PWM for a few seconds so power
lost in the test is almost nothing. Remember it charges
the large input cap during the open circuit voltage
test so much of that energy is not lost anyway if you
keep the test time short enough, ie once that cap voltage
rises and stabilises the test is done.

> also, if it is a SMPS circuit and I went for an IC for that
> solution, I suppose I could use an DA output from my uP to
> the feedback port on the SMPS IC instead of a PWM control..

Does your micro have a PWM output? That solves lots of
issues. Or if you have a DAC output, you can use a cmos
7555 timer as a SMPS chip, all you need is to adjust the
volts on its threshold pin to set PWM duty cycle. Plenty
of circuits on the net and much cheaper and lower power
use than a SMPS chip.

> P.S. what is the recommended 'fast' charge for NiMh generally?

You should really avoid NiMH for this app, NiCd is much
better suited for unknown charge currents, slow constant
charge etc.
-Roman

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