I've read lots of opinions on this topic, mostly contradictory.
I just use the machine as I want, on mains most of the time, on battery if out and about. Based on the last 5 years or so, modern Li-Ion and NiMH batteries seem to have a "shelf life" of 2 - 4 years. Longer if you're lucky.
Unless you need the spare capacity, don't buy a second one when the laptop is new. They'll then both die of old age around the same time. I was that buyer...