The PowerBoost 1000C is based around a TPS61090 boost converter from TI. This boost converter offers some excellent features to give freedom to the user
Adafruit, to make the charger even more useful, stuck a smart load-sharing LiPoly charger. This charger is powered from the microUSB jack and will recharge any compatible (3.7 V / 4.2 V) LiIon or LiPoly at a max rate of 1000 mA. There are two LEDs for monitoring the charge rate.
Thanks to the build-in load-sharing, it will switch over to the USB power supply when available.