Originally Posted by rm -rf

Perhaps the older downtube external batteries had the button?

But the newer seatpost battery setups use the front junction box that's normally mounted under the stem. It has a charging USB port on the side, a red/green status light, and a setup button on the bottom surface. (There appear to be aftermarket mounts to avoid the strap around the stem.)

And you can check the charge level by holding down a shift paddle for a couple of seconds, even while riding.

An example found on the web:
[IM G]http://i.imgur.com/69aiNxj.jpg[/IMG]
[IM G]http://imgur.com/69aiNxj[/IMG]