I always leave the battery in - as pointed out by henry_crun it provides a UPS function. As for potential lifetime - I read an interesting post by a guy who repairs small powertools and batteries. He's uncovered a sneaky practice by Makita (but who knows what other firms might be doing the same). Apparently the power management software built into the battery has a fixed number of charge/discharge cycles programmed, and once this is reached it will no longer accept a charge...