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Buddhist Temple Statue Light

Wily was just about to crawl into his warm bed and take a short nap when his phone rang.  The call was coming from half way around the world.  A lighting engineer needed Wilyís help.  The engineer wanted to illuminate a stature in a Buddhist temple. 

The caller had already selected a one watt white LEDs, which was mounted in a reflective ceiling assembly.  The light would shine down onto the top of a statue.  The light was to run every hour 7 days a week. 

OK, so why did they need Wilyís help?  The kicker was that there was no utility electricity for the light.  It was going to have to be powered by solar energy. They had plenty of room for the hardware needed. They would going to mount the solar panel on the roof and put the rest of the hardware at the back of the building.  They would run wires to the LED light.  But, they just didnít know where to begin in putting together the solar power bits and pieces.  Wily agreed to take on the project.
    Buddhist Statue    1w LED Ceiling Light 

Wily did some quick calculations.  Wilyís plan was to use a solar panel to charge a battery during the day, so the stored energy could keep the light going at night. A 1 watt LED running 24 hours would chew up 24 watt-hours of energy per day.  Wily figured that the storage battery should store at least twice that figure, to take care of cloudy days

Wily had many options for what kind of battery to use.  He could use a 3.6v lithium ion cell.  As the typical discharge curve below shows, the battery voltage remains above the 3.2v needed by the LED for most of the battery life.  This voltage would mean the constant current drive circuit for the LED would be fairly simple.  But, to last the required 48 hours, the battery capacity would have to be greater than 15 Amp-hours.  This would mean he would have to wire several batteries in parallel.

Wily could also use two 2v lead acid cells.  Again, the 4v battery voltage would be close to that needed by the LED. This would allow him to use a simple linear LED current control circuit.  A company called Cyclon makes a nice 25 Amp-hour cell, which would insure enough energy storage for 48 hours plus a good 50% margin.

Lithium Ion Discharge
Voltage Vs Time

Wily also knew that there were some nice 1.2v 10 Amp-hour NiMH
cells available. Wiring three of them in series would give him 3.6v. But, he would have to wire two cells in parallel to insure a 20 Amp-hour rating.  That would mean a 6 cell bank.  Wily also didnít like charging NiMH
cells, which usually require a more complex charging circuit to maintain a long battery life.
25 Amp-hour
Seal Lead Acid Battery
  Finally, Wily could go with a very conventional sealed 12v lead acid battery.  These are quite common and could make it easy to replace a worn battery when the time came.  But, the higher voltage would mean a bit more complex switch mode LED driver circuit would be needed.

In the end, after giving all these battery options some consideration, Wily decided to go with the more conventional 12v battery.  Wily knew that the battery should not be discharged much below the 50% point.  That put the total required battery stored energy rating at about 100 watt-hours.  A battery with a 10 Amp-hour rating should be good enough for this application.
  10 Amp-hour 12v Seal Lead Acid Battery
Next, Wily shifted his attention to the solar panel.  Wily liked to use 6 hours of sunlight per day to estimate the size of the solar panel.  If the panel needs to fully charge the battery and run the LED during those 6 hours, the panel should put out about 10 watts of power.  Panels containing 36 solar cells are common for 12v systems. They are usually wired in a 4 x 9 series array.

Wily looked at the LED specs again. The light they wanted to use had a 3.2v forward voltage at a forward current of about 300ma of current. Wily had many options for a driving the LED.  Since he was planning on using the 12v battery, he decided to use an off-the-shelf switch mode type converter, designed to drive the LED with a constant 300ma of current. 

Between the solar panel and the 12v battery, Wily decided to include an off-the-shelf battery charge controller.  The controller will insure that the battery would not be overcharged.  This would mean a longer battery life.

12v 10 watt Solar Panel
Wilyís complete system is illustrated below. He suggested that the battery, charge controller and LED driver module all be mounted inside a vented box.  Other than that, he left the final wiring up to the engineer in charge of the lighting system.

Issue 2 -2011     February

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Wily Widget

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