This unbalanced pack means that every cycle delivers 10% less than the nameplate capacity, locking away the capacity you paid for and increasing degradation on every cell. The solution is battery balancing, or moving energy between cells to level them at the same SoC.
A battery pack is out of balance when any property or state of those cells differs. Imbalanced cells lock away otherwise usable energy and increase battery degradation. Batteries that are out of balance cannot be fully charged or fully discharged, and the imbalance causes cells to wear and degrade at accelerated rates.
Contributed Commentary by Anton Beck, Battery Product Manager, Epec When a lithium battery pack is designed using multiple cells in series, it is very important to design the electronic features to continually balance the cell voltages. This is not only for the performance of the battery pack, but also for optimal life cycles.
The meaning of battery balance is to keep the voltage of the lithium-ion battery cell or the voltage deviation of the battery pack within the expected range. So as to ensure that each battery cell remains in the same state during normal use, in order to avoid overcharging and over-discharging.
The battery pack is composed of 100 series cells, with each series cell storing 10 kWh of energy. All cells are fully charged at 100% SoC except for one cell that is out of balance and is only at 90% SoC. As a result of this one cell, the entire pack is storing 999 kWh of energy, or 1000 kWh less the 1kWh from the cell that is not fully charged.
needs two key things to balance a battery pack correctly: balancing circuitry and balancing algorithms. While a few methods exist to implement balancing circuitry, they all rely on balancing algorithms to know which cells to balance and when. So far, we have been assuming that the BMS knows the SoC and the amount of energy in each series cell.