Kinesis do (or used to) a seatpost with a circlip to fix the battery inside the post rather than the stupid bung thing that inevitably slips eventually.
That's it. When I have tried - or the mechanic that fitted me a new battery in the middle of TCR in 2019 tried - to keep my battery inside the seat post, it has always fallen out.
Kinesis do (or used to) a seatpost with a circlip to fix the battery inside the post rather than the stupid bung thing that inevitably slips eventually.