A problem maybe,

when he adds the cell he gets, to the pack he has used for a while then maybe the used pack may have a different charge/disharge rate (from being used), and the new cell you add could be under charged/over charged/over discharged/under discharged during use and charging,

personnally I noticed this when i added some gp3300 to my 12 cell packs, after a charge the new ones I added where still cool, while the rest where hot.

So I would just wait for all 16.