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.