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More  Solar Cells Circuits

9v Solar Panels Converted to 4.5v Panels - July 26, 2017
designed by David A. Johnson, P.E.

Three 1.2v NiMH
cells wired in series are perfect for powering white LEDs.  The 3.6 volts from such a battery bank is a good match for a 3v white LED. Only a single resistor is needed between the battery and the LED to limit the current.  The ideal solar panel voltage when charging three 1.2v NiMH
cells, is 4.5v, using 9 solar cells.  A 1N5817 Schottky diode is usually wired between the solar panel and the battery pack, to prevent battery current from leaking back into the panel at night.  A voltage of about 4.08v is needed to charge three NiMH
cells in series.  When you add about 0.4v for the voltage drop across the Schottky diode, you get 4.4v.  Thus, a 4.5v solar panel is perfect.

  The solar panel itself limits the battery charging current, based on the panelís maximum current rating.  NiMH
cells donít require any fancy charge control circuits.  But, I do suggest keeping the charging current to about C/10, where C is the milliamp-hour rating of the battery.  However, it is difficult to find panels with 9 cells.  Most panels from China have 8 or 10 cells. One solution is to purchase an 18 cell 9v solar panel, such as the one shown below, and modify the panel slightly.  As the photo below depicts, most 9v panels are made of two sections of 9 cells each, wired in series.  By cutting the underside copper trace of the panel and soldering two jumper wires, the panel can be converted into two 9 cell sections wired in parallel to form a 4.5v panel, with twice the current as a 9v panel.  The 2 watt panel below can then produce about 450ma at 4.5v.  Larger 4 watt panels are also available and can be modified the same way to generate about 890ma at 4.5v.  I have also seen some 24 cell 12v panels, which could be converted into a 6 volt panel using the same cut and jumper method.

9v Solar Panels 9v Solar Panel
  12v 4 Watt Solar Panel

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