This data would be greatly enhanced if the number of charge cycles was also logged. From the graph it looks like that battery is deeply cycled frequently. That'

This data would be greatly enhanced if the number of charge cycles was also logged. From the graph it looks like that battery is deeply cycled frequently. That's hell on batteries. At my org (a school) we used to disallow charging our students' laptops in class. We did that for about three years. During that time, it was not uncommon for a battery to fail in the first year, with most failing after two. Since we started allowing charging, most batteries stay above failure threshold the students' whole four years here. My hypothesis has been that students charging opportunistically avoid the deep discharge cycles that are very bad for batteries.