Proposal to establish harmonised methods for determining in-vehicle battery durability performance of heavy duty pure electric vehicles and heavy duty off-vehicle charging hybrid electric vehicles. The GTR introduces a State of Certified Energy monitor to track battery health over time, defines minimum performance requirements as optional provisions by contracting parties, and specifies in-use verification procedures in two parts: Part A verifies monitor accuracy through measurement of usable battery energy using four alternative test methods, and Part B verifies battery durability against minimum performance requirements through remote data collection from vehicle samples. Virtual distance calculations account for energy used in vehicle-to-grid, power take-off, and non-traction applications.