A battery management system includes at least one controller programmed to, in response to a battery current becoming approximately zero, output an open-circuit voltage based on a sequence of battery terminal voltages measured during a time interval in which the battery current remains approximately zero and while a charge polarization voltage is decreasing. The open-circuit voltage may be further based on a non-linear regression of the sequence of battery terminal voltages. The non-linear regression may minimize a mean-squared error between the battery terminal voltages and corresponding battery terminal voltage estimates. The at least one controller may command the battery current to zero for the time interval. The battery management system may be included in a vehicle with a traction battery.