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.[1]
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.