In 4.5 years, I haven't had to take it out, but it is secured in some sort of (commercially available) mounting device in the seatpost. There is enough slack in the wire so that I can pull out the seatpost with the battery still mounted inside, and I guess I should be able to release the battery from its mount in the seatpost when the time comes to replace it. (My frame-builder set it all up, so I don't know exactly what would have to be done, but I assume it is a straightforward procedure. I charge the battery from an external junction box strapped to the bottom of my handlebar stem.
Some examples on Amazon