DETROIT — The CEO of Stellantis NV says employees of its joint-venture electric-vehicle battery manufacturing plants will be unionized if they wish. “That is very fine with us, very, very fine,” Carlos Tavares said during a virtual video conference from the Detroit auto show. “Why? Because we need to discuss with somebody, so we are very fine with unions. We have zero problem with unions. We may disagree … that’s fine.” Organizing the battery plants is critical for members of unions like the United Auto Workers and Unifor in Canada as production of engines has an expiration date because of t…