Project: Solar Battery Charger Using the LM317
A small solar panel makes an excellent battery charger for AA and AAA rechargeable batteries. Only a few components are required and construction is very simple making this a perfect first renewable energy project.
Rechargeable AA and AAA batteries have a voltage of around 1.2 Volts when fully charged. Therefore 2 in series gives a total of 2.4 Volts, 4 in series 4.8 Volts. Common solar panel output voltages are 3 Volts
In this example we will make a solar charger using a 6 Volt 250ma Solar Panel to charge four 800mah AAA batteries. The batteries can be put into a couple of 2 x AAA battery holders and wired in series (link the positive output from one battery holder to the negative of the other).
This gives us 4 x 1.2 = 4.8 Volts with a capacity of 800mah - therefore we want a charging current of around 80ma. According to the table above, a 15 Ohm resistor gives a fixed current of 83.3 milliamps which will be perfect.
The power loss in the resistor is again given by Ohm's Law as I * I * R = 0.0833 * 0.0833 * 15 = 0.10 Watts, therefore we can safely use a standard 0.25 Watt 15 Ohm resistor.
Provided by: http://www.reuk.co.uk/Solar-Battery-Charger-With-LM317T.htm
