SparkFun 5V/1A LiPo Charger/Booster Hookup Guide
Contributors:
MTaylor
MTaylor 
Charging a Battery
When a voltage is supplied to the charger, the MCP73831 charge IC comes alive and starts making decisions on how to regulate. First, it regulates current to 500mA until a certain voltage is reached, then it regulates voltage until current goes (close) to zero. When this happens, the charger IC shuts off.
As shown in the MCP73831 datasheet, charge current drops as battery voltage rises
The charger IC is making decisions based on the battery voltage and output load, and can be tricked into an invalid mode. Follow these rules to get a reliable charge:
- Turn on the battery switch before connecting the charger.
- Consume no more than 20mA from the output terminals during charging.
Using small batteries: It's not recommended, but if you use sub amp-hour batteries you may find they don't charge correctly. By default, the board is set to charge at 500mA. 500mA is too much for them, so the protection circuits get in the way and creates a situation where the battery is dumping the input current rather than using it to charge. If you decide to go this route, swap RPROG with a 10K resistor to limit the charge circuit to 100mA. To calculate the programming resistor needed with the small batteries, you can use the following formula from the datasheet: