Current sensorless state of charge estimation approach for onboard battery systems with an unknown current estimator

Current measurement is essential in a wide variety of lithium-ion battery applications, including electric vehicles and energy storage systems. In onboard battery management systems, regardless of the type of current sensor, it is difficult to ensure the accuracy of the current measurement due to various types of external noise and electrical interference between systems. In extreme cases, a malfunction of the current measurement can occur due to connection loss or sensor fault. Thus, the current measurement is error-prone, which easily decreases the accuracy of the state of charge estimator. This study presents an alternative method to estimate the state of charge of a battery system by estimating the engaged current by using an unknown current estimator instead of relying on erroneous current measurements. There are two main contributions: (1) A nonlinear state space representation of a lithium-ion battery cell is proposed by implicitly transforming the engaged current into internal state variables and equivalent parameters. (2) Based on the disturbance observer for a class of nonlinear systems, the unknown current estimator is established. The effectiveness of the proposed method is verified with a cylindrical battery cell and an experimental investigation with the onboard battery management system.