Why does my magic mouse lose the connection when I lift and place down again on surface?
It must be about the batteries but for what I've experienced so far it's not about energy, but about their length, at a very slight scale.

Since two days I had this problem again - it occured like almost 10 times this morning. So I removed the battery cover, and put a flat piece of folded paper towel upon the batteries, in a way that when I put the cover back again, it is slightly bent, just what it needs to ensure a firm pressure on the batteries, so they won't move at all.

My conclusion is : problem solved.

tl;dr : you can fix Apple's 80$ mouse with 2 sheets of toilet paper