The Shimano Di2 Internal battery charger makes charging your Di2 electronic shifting system a snap. With its indicator lights that tell you when it's charging and when it's fully charged, you don't have to guess. Plus, a full charge takes just 1. 5 hours. But don't worry too much about missing a charge - your system will only need to be charged every couple months.
For the charging on internal SHIMANO DI2 batteries (BT-DN110), the SM-BCR2 also serves as a PC linkage device for E-TUBE PROJECT firmware updates and settings.