Meaning
This operational condition occurs when the states of charge of individual cells connected in a battery pack drift apart over time. Typically resulting from slight variations in cell capacity, self discharge rates, or operational temperatures, state of charge divergence causes the pack’s usable capacity to decrease. It happens because individual cells reach their upper or lower voltage limits at different times, stopping the charge or discharge process prematurely.
Sourcing and engineering teams must evaluate this divergence to design effective battery management systems and to choose cells with tightly matched characteristics. If the divergence is left uncorrected, it leads to a significant loss of driving range or operating time for the end user.
Divergence Mechanics
The drift begins as cells with slightly different self discharge rates are allowed to sit or are subjected to varying temperatures in the pack. Cells that are hotter or have higher self discharge rates will lose charge faster than the others, leading to a difference in their states of charge. During the charging cycle, the cell with the highest state of charge will reach the voltage limit first, forcing the charger to stop before the other cells are fully charged.
Conversely, during discharge, the cell with the lowest state of charge will reach the low voltage cutoff first, ending the cycle prematurely. This limits the total energy that can be extracted from the battery pack.
Sourcing Sizing
Sourcing teams negotiating the purchase of battery management systems focus on the capacity of the balancing circuits to correct this drift. The contracts specify the balancing current and the algorithm used to equalize the cells during the charging process. If the balancing current is too low, the system will not be able to correct the divergence, and the pack’s performance will degrade quickly.
Buyers also look for cell suppliers who can provide cells with very low and consistent self discharge rates to minimize the drift in the first place. This careful sizing and selection of components helps ensure the long term reliability of the pack.
Balance Procurement
Sourcing contracts often define the maximum permissible drift in the state of charge after a specific number of storage or operating cycles. Quality audits are conducted to verify that the supplied cells and the battery management system are working together effectively to maintain the balance of the pack. If a supplier fails to meet these divergence limits, they can be held liable for the warranty costs associated with pack degradation.
This rigorous quality control ensures that the purchased battery packs will deliver consistent performance over their entire service life.
