Hybrid-electric and pure electric vehicles include a traction battery comprised of many cells. During repeated charge and discharge cycles, the state of charge of the cells may vary from cell to cell. To optimize traction battery usage, it is desirable that the state of charge of the cells be equalized. Cell balancing can equalize the states of charge and improve battery performance. To achieve proper cell balancing, an accurate state of charge value should be known for each cell. An accurate state of charge value may be achieved when a persistent excitation condition and an estimation convergence condition are met. If the conditions are not met, active excitation of the battery may be performed to improve the chances of meeting the conditions. Cell balancing may be initiated and terminated according to state of charge values estimated when the conditions are satisfied.