As over-charging and over-discharging can cause damage to batteries, to guarantee the stability and duration of batteries, it is necessary to avoid being over-charged and over-discharged. Therefore, knowing the batteries' state of charge (SOC) is essential. In this paper, the author describes in detail the procedure of building a SOC estimation algorithm. The SOC estimation algorithm is based on unscented Kalman filter. Moreover, changes of battery capacity and internal resistance are

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... . The impact of these changes on the estimation accuracy is explored. Experiments on an Iron Phosphate Li-ion battery cell with real load were conducted to justify this method. This method was verified to be not only precise but also stable.