If you have an internal battery for your Di2 setup, then you'll definitely need the Shimano E-Tube Di2 Internal Battery Charger. Why? Well, so you can recharge your battery and continue to shift your bike 9 months from now. It plugs into a 3 or 5 port junction box to charge the battery without having to remove it. Man, Shimano is always making our lives easier. NOT compatible with 7970 Di2 systems.