How to limit charging to a user defined limit? My UleFone Armor 2 has a 4700mAh battery and I would like to limit the charging to 60% to reduce battery aging. The batteries in most of the phones are not user-replaceable and I am worried that a factory/service battery replacement might affect water resistance negatively.

How can I limit the charging to 60%? (But not limiting charging speed. I love fast charging.)

Lenovo ThinkPads have a built-in feature that does this, called “battery preservation mode”.
There now is a hardware+app solution for that, it's called Chargie. https://chargie.org

Basically, Chargie is a Bluetooth switch that is linked to an app on your phone. You set the desired level of charging, the app runs in the background and stops the charge at the percentage you set. 

*disclosure: I am the CEO of Lighty Electronics, we have developed this solution and this isn't meant to be a spammy post with a link, but rather a very relevant solution to this issue. As far as I know, there are no other manufacturers of such devices.