Samsung has a feature called "Protect Battery" since Android 12 (other manufacturers may also have it). When it is switched on, the charging process stops at 85% (i.e. not only the threshold described by bookreader , but also the 85% display value).
If you are out of range of power sockets for a longer time, you can simply deactivate "Protect Battery" beforehand and the device will charge to 100%.
If this really helps to extend the life of the battery, I would also think it would be cool to have such a toggle on GrapheneOS.