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Improved Solar Powered Path Light
By: Dave Johnson
     February 28, 2011

Over the last decade or so solar powered garden and path lights have become quite popular.  However, over the last few years I have seen a gradual decline in their use.  I think one of the reasons for this is a huge drop in quality for these devices.  Chinese manufactures have whittled down their designs until they just barely work.  Devices fail after a few months of operation, batteries freeze, wires break, solar panels become cloudy and crack and night time operation becomes shorter and shorter.  After several attempts, I think many home owners have stopped buying these things.  However, I think the basic concept is still viable, if only a device were designed and built to really work properly.

I have outlined below some ways to use the guts of an existing path light and turn it into something that actually works.

Design concepts and considerations

One path light area which has seen huge a reduction in quality over the years is the solar panel.  The latest trend by the Chinese manufacturers is to install the minimum size and number of solar cells possible in those solar energy gathering squares as shown by the examples below.  Notice how small the actual solar cell area is relative to the cover plate.  If you really want to improve a path light design, this is one area to focus on.

Solar Path Lights with Poor Quality Solar Panels

There are many solar path light designs. I would recommend one which has enough room on the top of the assembly for a square type solar panel at least two inches by two inches.  Some designs have room for even larger panels.  Those larger units might be nice when more light needs to be produced.
I would recommend a solar panel with 10 cells.  This gives an open voltage of about 5 volts.  This voltage is about right for charging a single lithium ion battery cell or three NiMH cells.  Panels are available which range in size from 4 square inches of area to nearly 16 square inches.  Be sure to buy a panel which nearly fills the square with solar cells.  The highest efficiency cells can produce about 140mw per square inch but most will be closer to 100mw per square inch.  Also, buy panels which use mono-crystalline or poly-crystalline cells.  Don’t buy thin film or amorphous solar cells.  Those have much lower efficiencies.  A 10 cell, four square inch panel should be able to pump 80ma of current into the battery.  A 9 square inch panel should crank out 200ma of current and a monster 16 square inch panel should be able to charge the battery with 350ma of current.
10 Cell 2” x 2” Solar Panel   10 Cell 3” x 3” Solar Panel 10 Cell 2.5” x 4” Solar Panel     
     

10 Cell 4” x 4’ Solar Panel

Path Light with Large Panel

The next consideration is the battery type.  Many of the current path light designs use a single (nickel metal hydride) NiMH rechargeable cell.  They then install a voltage boost circuit to convert the 1.2v from the battery into a higher voltage for the white LED light.  I single battery will not store enough energy during daylight hours to provide a reasonable amount of light through the night.  I think multiple NiMH cells should be used.  NiMH cells are quite common and are available in the AAA and AA sizes.  AAA cells should have a rating of at least 800ma-hours, while AA cells should be around 2500ma-hours.  I would recommend using a battery holder for the three cells.
AA 2500maH NiMH Cells AAA 850maH NiMH Cells  3 Cell Battery Holder
     
The other battery choice is a single lithium ion rechargeable cell.  These are more expensive but they should work fine for this application.  They are the battery of choice in cell phones, MP3 players and laptop computers.  The most common package is a flat plastic assembly with a capacity ranging from 1000ma-hours to 2000ma-hours.  You can easily solder wires onto the end electrodes.  A less common package looks much like a NiMH AA cylindrical cell.   These are offered in a capacity ranging from 1000ma-hours to 2500ma-hours.
1000maH Lithium Ion Cell  2500mah Lithium Ion Cell   1000maH Lithium Ion Cell
     
Although there are some intelligent battery charger circuits available, I think for this application you can use a constant voltage type interface.  The solar panel itself will limit the charging current, equal to the short circuit current rating of the panel.  I would suggest charging the 3 cell NiMH battery pack at about 4 volts and the lithium battery at 4.2v.  I would also suggest keeping the charge current limited to about C/5, where C is the battery milliamp-hour capacity. An 800ma-hour AAA battery pack should charge well with 160ma of current.  A 2500ma-hour AA battery pack could be charged with 500ma of current, if you can find a large enough solar panel to generate that much current.  With a C/5 charge current or less, the battery should be fully charged in the 6 hours of sunlight for a typical day.
To prevent current from back feeding into the solar panel from the battery at night, most designs call for the installation of a diode between the panel and the battery.  However, the voltage from a 10 cell panel is so close to the desired battery charging voltage that the voltage drop across a conventional diode would be too high.  Even a low voltage drop schottky diode would be too high. I think the design calls for an ideal diode circuit.   A good ideal diode design would keep the voltage drop to less than 0.05 volts.  To limit the voltage to the battery, a low voltage drop voltage regulator will also be needed between the ideal diode and the battery.  A constant current LED driver circuit, which drops very little voltage, will also keep the circuit efficient and keep the light emitted by the LED constant. A good quality white LED should be used in the path light.  Some designs call for a LED with a broad emission pattern others work well with a narrow pattern device.  In my Book, Cree makes the best white LEDs.  Don’t be fooled by eBay sale devices, claiming to be bright devices.  More often than not, those device fade rapidly.  Cree LEDs tend to be rock solid.  The LED current should be about 20ma for standard LEDs and if a larger solar panel is used, a 1 watt LED assembly can be used with the current set at over 100ma. Be sure to use a heat sink on the higher power LED.  Heat will shorten the LED’s life.  Finally, a simple voltage comparator could be used to detect when the solar panel is receiving enough sunlight to begin charging the battery and to turn off the LED light.  A circuit containing all these elements is shown below. 
Standard LED Package    1 Watt LED

 

Please send comments to me


March 2011     Issue 15

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