OSAKA, Japan (Reuters) -The battery arm of Japan's Panasonic will need to build four more factories to reach its target for a sharp boost in annual capacity of batteries for electric vehicles by 2031, its technology chief told Reuters. The comments by Shoichiro Watanabe of Panasonic Energy are the Tesla supplier's first clear indication of the number of additional factories it will need. In May, Panasonic Energy said it aimed to boost annual EV capacity to 200 gigawatt hours (GWh) by early 2031, or about four times its capacity in March this year.