SPARKS, Nev.—Tesla Motors Inc. is scrambling to finish building its massive $5 billion battery factory here years ahead of schedule to meet demand for its coming cheaper sedan and provide power for new types of vehicles Chief Executive Elon Musk says are under development.

Tesla has doubled the number of people constructing the “gigafactory,” which sits on more than 3,000 acres near Reno. Now, 1,000 workers build seven days a week on two shifts in an effort to start churning out lithium-ion cells by late 2016.

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