State of Charge Observer for Hybrid Vehicles

Abstract: As a part of a research project, AB Volvo Technological Development is investigating the properties of traction batteries of different types. The aim is to find a suitable energy storage for electric/hybrid vehicles.

This master thesis examines the possibility of using a non-linear model of a traction battery to construct a device that accurately estimates the battery State of Charge (SoC) under typical hybrid operating conditions, i.e. the battery is rarely fully charged or discharged.

The work has consisted of three major parts. First, the battery model was fitted to measurement data from a specific NiCd battery cell. This model was then used to design a state derivative feedback observer which is implemented in a micro controller.

Certain precautions were taken when generating the source code, to that large parts of the code can be reused with other battery management applications.

The laboratory tests give good results compared to simpler methods, especially in the case of complicated discharge patterns or if the battery has aged and lost capacity. The observer method can, however, be further refined with the use of more complex models.