|After much experimentation, I have zeroed in on a few basic designs which work great with three NiMH cells. I have found that a battery pack using quality NiMH cells performed much better than other designs using questionable lithium ion cells. I have often been very disappointed with the results using lithium cells. Cells marked with a 5000ma-hour capacity tested as only having 500ma-hours. The lithium battery cell also requires a more complex charging circuit, while the NiMH cells just need a minimum voltage. In some units, I use three AAA cells with an 800ma-hour capacity. With others I use 3 AA cells with a larger 2500ma-hour capacity. When I really want a bright light, I use 6 AA cells, wired in a series parallel network, producing a 3.6v 5000ma-hour pack. In all those cases, the working voltage is 3.6v from the NiMH battery pack. This voltage is just about right with newer white LEDs which have a forward voltage of about 3 volts. With the battery voltage close to the operating voltage of the LED I can use just a simple series resistor to control the maximum current through the LED.
A NiMH battery cells needs about 1.36 volts to achieve a full charge. When you multiply 1.36v by three, you get 4.08v. A typical 8 cell solar panel will generated about 4 volts, so the panel voltage and battery voltage is a good match. However, to prevent current from leaking back into the solar panel from the battery at night, a diode is usually placed between the solar panel and the battery. A schottky diode, such as a 1N5817, is typically used for this application. But, the 0.3v drop across the diode would mean a less than a desirable voltage to the 3 cell NiMH battery pack. One way to solve this problem is to use an ideal diode circuit, instead of single diode. The circuit below is an example of a path light using a ¼ watt white
LED, three 2500ma-hour AA NiMH cells and an ideal diode circuit. The dual comparator IC is used for both the ideal diode function and to turn on the LED at darkness.
Q1 is turned on when the A1A comparator circuit detects a change in current direction of the solar panel. Q2 is turned on by A1B when the voltage across R1 reaches 2 volts, indicating that current is trying to flow from the battery to the solar panel. The battery leakage current through R1 is small enough to be ignored, since a much higher current would flow into the LED from the battery during darkness.