Method of determining the state of charge of a battery used in an electric vehicle

A method of determining the state of charge (SOC) of a battery used to provide power for propulsion of an electric vehicle comprising the steps of:

• establishing an initial value for the SOC and a noise covariance error;
• generating updated sigma points for a best estimate of the SOC and a noise covariance error matrix;
• determining the SOC based on a nonlinear battery model that is updated over a predetermined time interval, wherein the method includes linearizing the nonlinear model into a linear model using an unscented Kalman filter and wherein the determination of the SOC is based on a measurement error of battery voltage and a gain of the unscented Kalman filter;
• computing an update of the covariance error matrix based on an innovation sequence related to an error associated with predicting battery voltage, wherein the innovation sequence is a sequence known as normalized innovation squared (NIS) sequence;
• performing a consistency check of the NIS sequence by checking whether the NIS sequence lies within a predetermined confidence bounds;
• updating the covariance error if the consistency check of the NIS sequence determines that the NIS sequence lies outside of the confidence bounds;
• and
• repeating the aforementioned steps after the predetermined time interval has elapsed.