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More  Solar Cells; Lithium Ion Battery Chargers

Eight Solar Cell Lithium Ion Battery Charger
May 21, 2013

Eight Solar Cell Lithium Ion Battery Charger
A single lithium ion battery cell needs 4.2v from a current source to insure it is fully charged.  A solar panel with just enough individual cells wired in series to produce the needed voltage would be more efficient than one with more cells for a given surface area.  If you divide 4.2 by eight you come up with a figure of 0.525 volts. 

 

That means if an 8 cell solar panel were used, each cell would need to produce 0.525 volts to insure it could charge a lithium ion battery.  As the curve below indicates, a typical solar cell does indeed produce such a voltage and could be used to charge the battery but only if the charge circuit had a very low voltage drop to maintain the 4.2v. 

Note that at one full standard sun intensity, only about 70% of the peak current is available at the needed 0.525 volt point.  However, as a lithium ion battery reaches the target 4.2 volts, the charge current naturally tapers off, so this reduction in current may not translate into much of a reduction in overall efficiency.

An ideal charger circuit would limit the charging battery voltage to 4.2v but also have a very low voltage drop, when delivering current to the battery.  Also, during night time, when the solar panel is not producing any current, the circuit would need to block current from flowing back into the solar panel from the battery.  These two needs often conflict with each other.  The circuit below supplies both needs.  A photodiode is used to detect daylight conditions and switches off the charge circuit at night.  Two p-channel FETs form a series pass element to regulate the voltage and blocks leakage current when turned off.  With the components shown, the voltage drop is just 0.02v at 250ma of current.  Larger solar panels are also possible.  Two 2v 400ma panels as shown below can be wired in series. Such panels should be able to harvest enough to fully charge a 2.5 Amp hour cell in one sunlit day.  Larger 5 Amp hour cells are also available.
8 Cell 250ma Solar Panel  2.2 Amp-hour Lithium Ion Battery
   
2v 400ma-hour Solar Panel  4.9 Amp-hour Lithium Ion Battery

Click on Drawing Below to view PDF version of Schematic

8 Solar Cell Lithium Ion Battery Charger

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