The lithium ion technology is not good-natured, meaning that if charging or discharging continues once the first cell is fully charged or discharged respectively, this cell will be damaged! Lithium ion cells must not be overcharged or over-discharged, because non-reversible processes take place. Heavily overcharging or over-discharging them may also be dangerous under certain conditions. Other chemistries like NiCd, NiMH or Pb-acid  are good-natured. The VRLA (valve regulated lead acid) battery for example dissipates excessive energy during overcharging in an internal oxygen cycle and the good old open top lead-acid starts gassing (which is also essential to stir the electrolyte). This good-natured process is used to equalise the cells in a series connection during the last charging phase: when the first cells become fully charged, they just dissipate the excess energy. Equalisation is necessary, because the cells are manufactured with tolerances or mounted in different conditions, so that they do not behave in the same way - especially if they are old. The same is true for lithium-ion cells, but unfortunately, they cannot be equalised by carefully overcharging them. This means they would drift slowly apart so that after some time the useable capacity of the pack becomes very small. Active cell equalisation is essential - it should be part of a good battery management system.