https://www.techradar.com/news/samsung-galaxy-note-9-could-have-a-massive-battery

The Samsung Galaxy Note 8 is a big phone, but its battery is rather small at 3,300mAh. However, the Samsung Galaxy Note 9 could massively improve things in that area, as it’s rumored to have a 4,000mAh battery.

That claim comes from leaker Ice universe, who has a reasonable track record and claims to be “100% sure” of the size.