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.