An internal source cited by Galaxyclub revealed the model number and battery capacity that will be used for the upcoming Galaxy S21 Ultra handset. In short, it’s the same as the S20 Ultra.
The battery model number is EB-BG998ABY and it’s rated at 4885mAh, so the typical capacity will surely be 5000mAh, which is the juice pack of the Galaxy S20 Ultra.
In contrast, the Galaxy S21 and S21 + will have an increase in battery capacity compared to their predecessors.
Of course, the upgraded Exynos and Snapdragon 5nm chipsets will bring efficiency improvements so we can expect better battery life, although there won’t be a capacity upgrade.
Source (in Dutch)