Latest rumors claim that Samsung's first foldable phone - the Galaxy X - will be getting flexible batteries with capacities between 3,000 and 6,000 mAh. The foldable 7.3-inch OLED displays that will be featured with the Galaxy X are scheduled to enter production later this summer, and the devices should be ready for launch in Q1 2019, along with the Galaxy S10 models.
https://www.notebookcheck.net/Foldable-Samsung-Galaxy-X-smartphone-to-get-7-3-inch-screens-and-flexible-batteries.314028.0.html