Charging Circuit

This one is a pretty straightforward hookup of the MCP73831 charging circuit, with one addition: D302 is there so that the charging circuit does not have to treat the whole device as a battery to be charged. If USB is connected, VIN will get the full 5V (minus the voltage drop of a Schottky diode). If not, the battery is used (again minus the voltage drop). Finally, both diodes need to be there so that the battery doesn't feed back to the charger input and try to charge itself. The programming resistor tells the resistor to charge the battery at a maximum of 100mA. This is 1C for a 100mAh battery, the smallest one I have and the one that flies with the Rocketometer.

The status light uses a resistor from the 1.5k pack in the USB connection.

Other versions of this circuit have had a voltage divider between VLIPO and GND so as to allow the MCU to measure the battery voltage. I haven't used it in a while, and it does draw some current, so it is gone from this circuit.