Old Rivian battery packs are getting a second act, this time helping run Rivian's own factory in Normal, Illinois. In a notable full-circle move, the electric vehicle maker said retired vehicle batteries will be turned into a stationary energy storage system designed to cut power costs and support a more flexible grid. According to a report from InsideEVs, Rivian said it reached an agreement with Redwood Materials to install a 10-megawatt-hour e…