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More Solar Cell Circuits, Battery Chargers

Eight Solar Cell Lithium Ion Battery Charger Version 2
June 15, 2013

Jim Wilber looked at my design for a battery charger and sent me some suggested changes.  He suggested using the LTC4412 ideal diode IC to switch off the current path from the solar cell to the battery in darkness.  He also noted that the 2.5v regulator from Seiko I had selected was no longer in production and suggested using the shunt reference.  I think his suggestions made a lot of sense. 
In my design, in dark conditions, the first MOSFET Q1 would not turn off until well after the point where the solar panel no longer produces current, allowing some leakage current to flow from the battery into the solar panel.  I have included these changes in the circuit shown below.  I also included a day/night sensor which can be used to turn on a light at night.  The phototransistor should be the type which is colored violet which contains a daylight filter.

The LTC4412 ideal diode from Linear Technology monitors the voltage from the solar cell and the battery.  When the voltage from the solar cells is 20mv higher than the battery, the output turns on the p-channel device Q1.  In darkness, when the solar cells are no longer producing current, the voltage from the solar panel will be less than the battery.  The ideal diode IC A1 will then turn off the transistor Q1, blocking the current path from the battery to the solar panel.  In a similar fashion, in darkness, the 2.5 volt reference will drop to a lower voltage, being starved of current from the solar panel, turning off the voltage regulator circuit.  The result is that Q2 is turned off hard.  With both devices turned off, no current flows into or out from the solar panel.  As in the last circuit, the op Amp A2 limits the charging voltage across the lithium ion battery to 4.2v when the solar panel supplies current.

 

Click on Drawing Below to view PDF version of Schematic

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