The first item I am redesigning is the charge controller PCB...
I am currently in process of designing this simple circuit on a single-sided PCB with the wire terminals in the same location as revision 1. I'm using the open source tool, KiCad for the first time. This board will be so simple I'll etch a prototype PCB myself.
The controller receives the solar panel power and regulates the charge going to the battery, cutting off charging when the battery is fully charged. The circuit uses an input from solar panel, and can charge a Nickel Metal Hydrate (NiMH) battery.
o Cost reduction strategy: The solar panel is specified to have 0.5W maximum output power, so the power source is limited. My sample measurement was actually only 0.25W. For revision 1 PCB design, I overlooked the power source capability; and specified the use of a charge regulator IC, which would be needed in the case there is more power potential available than is needed (for example, AC wall socket source). Because the available panel power source to charge the batteries is low, it is not necessary to regulate the charging current.
o The Solar panel power is 0.5W, at 5V the available current is 100mA maximum. The batteries can accept up to 125mA charge current.
o Since the current is limited by the panel power, we only need to regulate the voltage. Using a general purpose diode, we prevent the battery voltage from exceeding the panel voltage. A very small value resistor in series can be used as a small power divider to insure the battery is not over powered.
o Optional: using a zener diode, we clamp the battery voltage to a maximum of 4.7V. I doubt this zener diode will be needed in the end, but want to test it.
o Optional: using a re-settable fuse (such as a Raychem polyfuse) in place of the series resistor can be used if we find a reason for more robust protection of overpowering the battery. I also doubt we will need a fuse, as the solar panel and series resistor limit the power.
