Reposted with correct tag!

> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
Behalf
> Of Tobias Gogolin
> Sent: 03 December 2009 03:26
> To: Microcontroller discussion list - Public.
> Subject: Lower voltage drop than diode?
> 
> 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!
> 
> So if anybody could point me to some well established circuits for
this
> purpose I'd be very grateful!

This is a fairly simple (and quite cunning IMO) circuit that does what
you want:

http://www.solarbotics.net/library/circuits/misc_switching.html

Note that there will be a small quiescent current drawn from the battery
(mainly through R3), the value of which will depend on your battery
voltage which you don't seem to have stated.  At 4.2v (fully charge
lithium polymer voltage) LTSpice says it will be of the order of 45uA
with the component values shown.  You can probably reduce this somewhat
by increasing the value R3 since the circuit is designed for lower
voltages.

Regards

Mike

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