You don't have to use the button cells. You can still wire up the shifters inthe traditional way and run them off the main battery. The 8170 and 9270 series levers can be used either wired or in wireless mode.
Ah, clever cookies. So you could more easily retrofit wireless to a non-Di2 bike but use the same shifters on a proper wired setup. Neat. I've been thinking of making the Sequoia the missus uses Di2. Actually I've just had a look and hers looks like it might in fact have Di2 ports.
You don't have to use the button cells. You can still wire up the shifters inthe traditional way and run them off the main battery. The 8170 and 9270 series levers can be used either wired or in wireless mode.