Overview

Built a free, open-source menu bar app in Swift that stops charging at the level you choose, from 20 to 100 percent.
Added sailing, which lets the battery drift a few points below the limit before charging again, and heat protection that pauses charging while the battery is hot.
Built top-up to 100 percent once, forced discharge down to a target, and scheduled calibration cycles.
Kept 30 days of battery history with a 24-hour chart in the menu, and stopped charging during sleep where the Mac allows it.
Worked around macOS 27 firmware, where Apple locked the old charging switches, by using the system limit at 80 percent and above and switching the charger off at lower limits.
Shipped a privileged helper service, a command-line tool and Shortcuts actions, with a Homebrew cask and releases under the MIT license.

chargnr — Battery Charge Limiter for Apple Silicon