That’s very odd o.O
I just did a search and it appears it is a common issue at least as recent as Mint 18 only offering about half the battery life as WIN 10.
https://forums.linuxmint.com/viewtopic.php?t=261641#:~:text=On%20manual%20and%20papers%20it%20boasts%20a%20battery,battery%20lasted%204%20hours%20max%20(all%20settings%20default).