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  Experimenter's Corner- [Issue 2, October 2009]

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Light Powered Cell Phone Battery Charger
By: Dave Johnson

I’ve been thinking about another way to charge a cell phone battery without wires.  The usual method, as outlined in my “wireless battery charger” post, is with a magnetic field coupled circuit. This works but is somewhat complex and expensive.

I often wondered if light could be used.  I imagine some type of paste-on solar panel.  Perhaps it could be attached to the back of the cell phone to charge the battery from sunlight or from some other light source.
When exposed to light, the small panel would route current to the battery.  To make this method work for a wireless battery charger for an indoor night stand, we would need a powerful infrared light source with a low profile.  But, would it work?  How much power could we route to the battery?
I looked at my own phone and measured about 4 square inches of possible light collecting area on its back.  A solar panel consisting of 9 cells, should produce enough voltage to charge a typical lithium ion battery.  Let’s suppose we used some good quality solar cells.  These should be able to convert about 15% of sunlight into electricity.  This works out to about 0.1 watts per square inch.  With 4 square inches, that means it could collect about 0.4 watts of power.  The charge current might be about 100ma, with an end of charge voltage of about 4.2 volts. On a typical day, with 6 hours of sunlight, that would yield about 600ma-hours of juice to the battery.  This is not enough to fully charge a dead battery but it is certainly enough to restore about 50% of the total energy needed.  It could certainly top off a battery, allowing it to operate for a much longer period.  But, how would this solar panel work if illuminated with infrared light from some LEDs?

The curve below is the typical light sensitivity for a silicon solar cell.  Note that when illuminated with near infrared light of about 900nm, the light to electricity conversion efficiency is at maximum.  It turns out that if 900nm light were used the conversion efficiency is about 50%.  This is not bad.  The efficiency of LEDs has improved a great deal over the years.  Many modern infrared LEDs have an electricity to light conversion of about 25%.  If several of those were used as an energy source, the overall efficiency could reach 12% to the battery.  High power infrared LEDs do exist.  I purchased some 5W units not long ago from eBay.  In theory, two of these units might result in about 1 watt of battery charging power, if the cell phone’s solar panel was placed directly over the LEDs.  This would place the charging current at about 250ma, which is even more than possible with direct sunlight.  However, some additional losses would be expected, lowering the overall efficiency.

As a quick experiment, I fired up one of my 5W infrared LEDs that I bought on eBay and aimed the light at a nearby one square inch solar cell.  I measured the short circuit current at 250ma.  With a four square inch solar panel consisting of 9 cells, the charge current should reach about 100ma if the LED light were evenly dispersed onto the solar panel.  If two LEDs were used, perhaps the panel current could approach 200ma.  So, this indeed looks promising.  A commercial battery charging pad might also use an array of infrared LEDs as shown below.  This might be easier to build and would more evenly illuminate the panel.  I have also seen some arrays of LED chips mounted onto ceramic wafers.  The ceramic conducts heat much better than standard circuit board material.  Still, I think about 10 watts would be the maximum practical power put onto the array.

Based on my crude test, it does look like a properly designed LED array should be able to induce up to 1 watt of power from the solar panel.  Assuming a 1 Amp-hour battery, this means the battery could be fully charged in about 4 hours.  A typical 8 hour night, would be plenty of time to fully charge a dead battery.


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October 2009      Issue 2

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