Retired EV batteries, which lose too much capacity for automotive use but retain 70–80% of their original charge, are being repurposed as megawatt-scale grid energy storage. Companies like B2U Storage Solutions, Redwood Materials, and Moment Energy are leading this effort, with recent announcements including partnerships with Waymo and General Motors. The process involves rigorous health testing using battery management system data, electrochemical impedance spectroscopy, and ML-based diagnostics to assess remaining capacity, internal resistance, and cell consistency. Batteries passing inspection are grouped by chemistry and wired into parallel matrix architectures inside containerized systems with real-time cell-level monitoring. Currently representing just 2–3% of deployed storage capacity, second-life batteries could reach 20–25% within a decade if diagnostics improve and recycling costs remain high. The economics ultimately depend on battery chemistry — LFP batteries favor repurposing, while nickel- and cobalt-rich chemistries may yield more value through recycling.
