I use High Sierra on a MacBook Pro with a Magic Mouse. I rotate among three pairs of rechargeable batteries.
Today I got the warning about low mouse battery charge when the charge was at 9%. But the previous battery warned at 12% or 14%, I don't remember for sure (but I remember being surprised that it was more than 10%).
What criteria does MacOS use to decide when to warn about the mouse battery charge, since it's obviously not a fixed percentage?