Cell Balancing.  During a charge or discharge, some cells will reach the bottom or top state of charge before the others.  At this point the charge or discharge must stop to avoid damaging these full or empty cells. That means there are some cells that will never become completerly full or empty and if the condition is left untreated, will cause the effective capacity of the battery to decrease.  To fix this problem, the charge on each cell in a series can be balanced with every other cell.  There are two primary methods of cell balancing.

  • Passive Cell Balancing.  During or after a charge or discharge, the voltage of all cells is checked and the voltage of the weakest cell becomes the reference.  At the end of a charge, the cell with highest voltage is the weak cell and so a switch turns on and current is drained from the cell until it is no longer the highest voltage cell. The process is repeated until all the cells are of approximately equal voltage.   A similar process can be done after the first cell reaches 100% discharge.  The process is generally wasteful of energy as the bled off current is usually dissipated as heat.
  • Active Cell Balancing.  The primary difference from passive balancing is that with active balancing, energy is transferred between cells until they are mostly equal.  The drawback is that this method is more complex and harder to implement.