What limits the max # series cells in a custom battery pack?
Balancing requirements become more stringent when many Li-ion cells are in series. If one cell is out of balance with respect to the others, it can present a safety issue as the pack is cycled, so usually the number of cells in series is limited to 7. New technology is emerging to balance high series cell count strings, so this rule of thumb may soon change which will enable the manufacture of many medical batteries.
