From the wiki on the NASA Docking System:
Once mated the NDS interface can transfer power, data, commands, air, communication and in future implementations will be able to transfer water, fuel, oxidiser and pressurant as well.
So they could certainly top off their charge if they need to (assuming they have a NDS installed by the time CST-100 gets there).
According to Boeing's Design Considerations, page 3:
The CST-100 can operate autonomously for up to 60 hours of free-flight, and is designed for Day One rendezvous with a Day Two backup opportunity. The vehicle can stay docked to a host complex for up to 210 days while provided with one kW of keep-alive power.