http://www.techradar.com/news/samsung-galaxy-s9-battery-size-revealed-and-you-probably-wont-like-it

The Samsung Galaxy S9 is shaping up to be a fairly small upgrade on the Samsung Galaxy S8 and there’s at least one major aspect that may not be upgraded at all – the battery.

UnionRepair (an online store) has listed replacements for the batteries destined for the Samsung Galaxy S9 and Samsung Galaxy S9 Plus and you can see that their capacities are 3,000mAh for the former and 3,500mAh for the latter, which is exactly the same as the batteries in the Galaxy S8 and Galaxy S8 Plus.