Last month, Elon Musk tweeted that a "major new Tesla product line -- not a car -- will be unveiled" on April 30. Photographer: Simon Dawson/Bloomberg Tesla Motors Inc. is signing up big customers like Wal-Mart and Cargill, accelerating efforts to become a leader in energy storage -- a new market that's poised to boost sales and profit at the electric vehicle pioneer. Next week, Telsa will make a deeper push beyond the car business when it unveils batteries for homes and utilities.