The 3C listing for Galaxy S21 suggests that it will come with a 3,880mAh rated capacity with a typical capacity of around 4,000mAh. The Ultra variant was recently spotted on TUV Rheinland certification revealing a battery capacity of 5,000mAh.
We can expect the series to be more powerful than Galaxy S20 series as it is said to feature Samsung’s un-announced Exynos 1000 chipset and Snapdragon 875 in some markets. The Galaxy S21 lineup is most likely to run Android 11 OS with OneUI 3.0 on the top. Most of the specs of Galaxy S21 series devices is a mystery now but we can expect more details to surface online. Some reports suggest that the series may launch in February 2021.

Samsung Galaxy S21  live battery image with 4 800mAh capacity appears in Safety Korea certificate - 30Samsung Galaxy S21  live battery image with 4 800mAh capacity appears in Safety Korea certificate - 28