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Solar Powered Flood Light Using NiMH
December 20, 2013

Solar Powered Flood Light Using NiMH
There are many solar powered garden lights available.  Most of the devices offered at the home improvement stores have very poor performance. Their batteries are too small, the light launched is weak and the solar panels are undersized. None of the devices I have tested would operate through a typical night. 

Over the years, I have modified several garden lights to make them more practical.  In this design I show how to build a light which uses 3 AA size rechargeable NiMH
cells and a 4v eight cell solar panel to produce a useful light, which launches a nice broad flood pattern and will operate through the longest night.

A typical white LED needs about 3.2v at a current of about 20ma. If the light used five LEDs wired in parallel, they would draw a total of 100ma of current from the battery.   A long winter night might last 16 hours so the battery would need to store at least 1600ma-hours.

If we added another 50% for cloudy days, that would put the needed battery Amp-hour size at about 2400ma-hours.  Several battery configurations are possible to power this kind of light.  A 3.6v rechargeable lithium ion cell with a 2400ma-hour rating would be nearly ideal but they are hard to and are expensive.  Another option is to use three very common AA size 1.2v NiMH
cells, wired in series, to produce a 3.6v supply. Most AA cells have a capacity of about 2500ma-hours, so they would be a good choice for this application.  NiMH
cells also have a fairly low internal resistance, keeping the available voltage high during the discharge process.  NiMH
cells would not need any charge control if the charging current is about C/10, where C is the Amp-hour capacity of the battery.

Three series wired NiMH
cells need about 4 volts to insure they are full charged. Assuming about 0.5 volts per cell, the minimum number of solar cells to charge the battery is eight.  However, the charge control circuit will need to be very efficient with a very low voltage drop, since there would not be much of a margin between what the solar panel can deliver and what the battery needs.  Also, during the night, the current pathway from the battery back to the solar panel needs to be blocked, so the battery is not discharged when there is no sun.  These two requirements can be met with a quality p-channel MOSET wired backwards.  During the day, the device is turned on, providing a low resistance path between the solar panel and the battery.  At night, the device is turned off blocking any discharge current.

A typical day should provide about 8 hours of sunlight.  Working backwards, if the 2500ma-hour battery is nearly fully discharged, and only 8 hours of sunlight is available, then the solar panel should be rated at about 300ma.

An inexpensive phototransistor is used to detect sunlight.  A small Schmitt trigger inverter is used to turn on and off the p-channel FET linking current from the solar panel to the battery.  The same signal also turns on and off the LED current.

An op-amp is used to control the LED current.  With components shown, the circuit only requires a 0.05 volts across a shunt resistor to maintain current control.

8 Solar Cell 250ma Solar Panel    AA 1.2v NiMH
Modified Solar Garden Light

Click on Drawing Below to view PDF version of Schematic

5 LED Solar Garden Light by David A. Johnson


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