So the leaf is known, at least the 2012 and older models, for not being very good at looking after and charging the 12v battery. I figured that out after having all sorts of issues with my 2012 leaf and they were resolved with a new battery and I've changed how I behave.. I now don't just plug in my car anytime I'm home as this pulls juice from the 12v all the time to keep the car charging system ready even if it's not charging. I also plug it in to a battery tender every few weeks. My 12v battery was replaced after 3 years.. but I hope that my new behavior will allow this replacement battery to last a little longer.. more like 7 yrs and getting closer to your average you mentioned. Most ICE cars are good at looking after 12v batteries and it's so strange that nissan didn't make the leaf be just as good.
i think that is what the other posters are saying and why they are asking about your location.. I'm Utah and the winters are cold and long enough to stress out a poorly maintained 12v battery.
Statler: Wake up you old fool. You slept through the show.
Waldorf: Who's a fool? You watched it.
2015 Leaf S (leased until May 2018, Bought out in Jan 2017)
2012 Leaf SL (purchased May 2015)