If you need to power a 5V circuit with a 3.7V battery you can use this simple Sparkfun charger. It can provide up to 1A of current to its output
The circuit is built by feeding a charge controller IC MCP73831 to the LiPo port and the input of a boost controller PAM2401. Various connection types are provided for battery, charging source, and switch to allow application flexibility, and two LEDs provide information on system status. If you need more than 5V from your project, SparkFun LiPo amplifiers / chargers can also be daisy-chained for a higher return on your investment.
Charger, microUSB, 500mA
Booster, 5V, 1A output
Form factor for our 1,000mAh batteries
Battery isolation switch with optional external connections
Enable pin broken out
Disabled current less than 10uA
LED indicators for power and charge
Super-clean output signal (high-frequency switching)