Most riders going with Di2 these days are opting for the seatpost battery especially on frames factory designed for internal Di2 routing. The seatpost battery cleverly hides the battery and is slightly lighter than the standard battery.
A few seat posts are specifically made to accept the Di2 seat post battery, but the Ritchey Di2 Seatpost Battery Mount Securely hides your Di2 Battery within your standard seatpost. It comes in two sizes 27.2-28.6mm and 30.9-31.6mm to fit virtually all modern round seatposts.
Does not include Di2 battery - sold separately