Research suggesting that a LiIon cell isn't observably damaged by discharge to zero, but by a modest degree of reverse charge. But series packs discharged to zero are still a problem, as some cells in the pack will be reverse charged. https://www.nature.com/articles/srep30248 , or a more human approachable youtube video discussing the research, https://www.youtube.com/watch?v=sRwoYJyjZNo