At 04:49 AM 05/12/2009, you wrote:
> > Hi Folks!
> > I have some solar cells that almost appear to be made to charge Lion
> > batteries from discarded cell phones! I use these battery solar cell
> > combos
> > to rig little LED flashlights!
> > If I connect the PV directly charging the battery I get almost 40 mA
> > charging current, however of course in shade or darkness it discharges the
> > battery, so i always use a diode!
> > However that bit of voltage drop is critical enough to then only get 12mA
> > charge current!
> > I have been thinking about using a transistor voltage drop (<.3V) or maybe
> > a
> > FET, but I'm not sure I am imagining correctly that the circits I'm
> > thinking
> > off would indeed prevent reverse currents!
>
>You would be lucky to get less than about 0.3V drop with a Shottky diode.
>
>You can use a FET driven by a comparator so that the FET is switched when
>the PV voltage is > battery voltage. The opamp circuit will take a small
>amount if current - under 1 mA without much effort.

It's a little trickier problem than that.. the chip I suggested that the OP
et. al look at attempts to servo the MOSFET forward voltage to a small fixed
value (25mV, IIRC) so that the direction of current flow can be determined.
That value is chosen to be reliably greater than the offset voltage of a
real (imperfect) comparator, probably plus some hysteresis. If you 
have a hefty
MOSFET, when it is on, the voltage across it (and thus the difference between
PV and battery voltages) will be both very small and poorly controlled
(ie. it will be difficult to reliably determine the polarity).

Of course this is a specific problem and perhaps a less general answer would
work out to be simpler. Eg. briefly turn the MOSFET off every second or
whatever, and compare the two voltages. Maybe use a.. PIC or something. ;-)

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



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