Technical report on the development of Amendment 1 to UN GTR No. 25 on in-vehicle battery durability for electrified heavy-duty vehicles. The GTR provides a harmonised methodology to monitor battery health over time and introduces minimum performance requirements for battery durability. Development involved approximately 29 IWG meetings over three years with 50 to 60 attendees. The framework centres on minimum performance requirements for in-vehicle batteries, readable on-board battery health monitors, in-use verification procedures, and data collection processes. Three test methods were adopted for measuring usable battery energy: discharge by driving on test track, discharge by driving on road, and discharge using bidirectional power supply system, plus an alternative chassis dynamometer method. The GTR includes an optional annex with minimum performance requirements based on state of certified energy.