For 2014 and newer Cervelo S5 framesets, the user has the option of the internal, cylindrical battery (usually thought of a seatpost version). Under the downtube of the frame, towards the bottom bracket, there is a mounting boss option for external batteries. These bosses allow you to employ the internal battery via this specially made Cervelo Internal Battery Mount for S5. The composite material is shaped for cylindrical battery, which then gets affixed to the mount via two zip ties. The battery is connected via wire (and tested) to the shifting system, then inserted into the downtube from the removable bottom bracket guide. Following all instructions, you then bolt the mount from the exterior via the aforementioned mounting holes.

It sounds complex, but it is actually pretty straightforward once you see the mount bracket itself and expose the entry via the removable bottom bracket. The system has several advantages over a seatpost option. First, you don't have to constantly ensure proper seatpost height or be concerned about pulling the wire out too far. Second, the weight is kept low in the frame. Third, the system is quite secure yet easy to access for charging. Fourth, the mount may not be as inexpensive as some of the aftermarket seatpost mounts, but it far more reliable, and much less expensive than some of the other proprietary systems offered.