> 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.

Yoy may also be able to get FETs with gate turn on voltages of ~= 0 volts. 
This is done by trapping charge in a floating gate so the FET is biased to 
just before turn on.

The opamp method is liable to be more accessible and cheaper.


           Russell McMahon

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist