SEOUL, Sept 28 (Reuters) - South Korea"s SK Innovation Co Ltd (096770.KS) plans to invest 5.1 trillion won ($4.32 billion) to build battery production facilities in the United States through its battery joint venture with Ford Motor Co (F.N) through 2027, the company said in a regulatory filing on Tuesday. The South Korean battery maker, which supplies electric car batteries to Ford Motor and Hyundai Motor Co (005380.KS), among others, has battery production sites in the United States, Hungary, China and South Korea. ($1 = 1,179.6200 won)