(Reuters) - Proterra Inc will provide batteries for electric transit vans that Lightning eMotors is building to meet growing demand from companies looking to electrify their fleets, the commercial electric vehicle startups said on Thursday. Proterra and Lightning eMotors, both slated to go public via reverse mergers with special purpose acquisition companies (SPACs), did not disclose financial details of the agreement. SPACs are shell companies that raise money through an IPO of its shares to take another company public within two years. Lightning eMotors, which has merged with Gigacapital3 Inc in a deal valued at $823 million, said the first transit vans with Proterra batteries are expected to be delivered to customers later this year. The Colorado-based company said it aims to expand its annual manufacturing capacity to 3,000 vehicles this year, up from currently 1,000. It buys the chassis of delivery and passenger vans, buses and trucks from companies including Ford Motors Co and General Motors Co, which it then turns into electric models and equips with its own technology. Proterra in January announced a merger with ArcLight Clean Transition Corp in a deal valued at $1.6 billion. The California-based company provides electric buses and school buses to public transit agencies, cities and school districts. It also sells batteries and charging infrastructure to fleet managers. Proterra and Lightning eMotors are part of a range of commercial EV makers, including Rivian, Fisker Inc and Nikola Corp, seeking to build on the success of Tesla Inc. The industry has been expanding quickly at a time of growing interest from delivery fleet operators including United Parcel Service Inc and Amazon.com Inc. Transit agencies have also been looking to electrify fleets in response to regulatory pressure and in an effort to reduce greenhouse gas emissions.