A Theoretical and Experimental Analysis of the Coulomb Counting Method and of the Estimation of the Electrified-Vehicles Electricity Balance in the WLTP

The battery of a vehicle with an electrified powertrain (Hybrid Electric Vehicle or Battery Electric Vehicle), is required to operate with highly dynamic power outputs, both for charging and discharging operation. Consequently, the battery current varies within an extensive range during operation and the battery temperature also changes. In some cases, the relationship between the current flow and the change in the electrical energy stored seems to be affected by inefficiencies, in literature described as current losses, and nonlinearities, typically associated with the complex chemical and physical processes taking place in the battery. When calculating the vehicle electrical energy consumption over a trip, the change in the electrical energy stored at vehicle-level has to be taken into account. This quantity, what we could call the vehicle electricity balance, is typically obtained through a time-based integration of the battery current of all the vehicle batteries during operation. In such cases, the charge storage efficiency is often assumed unitary, meaning that the conduction of electrical charges on the battery cables corresponds to an equal amount of charge being stored/released in/from the battery; this is also the case for the official energy consumption/CO₂ emissions certification procedure for light-duty vehicles. The Joint Research Centre has collected experimental data from different vehicles with electrified powertrains concerning the electrical energy use and the State Of Charge (SOC) of the traction battery communicated from the vehicle; the latter was used as a reference to quantify the actual vehicle electricity balance from a trip or driving cycle. This work investigates the accuracy of the simple Coulomb Counting method with unitary charge transfer efficiency for the quantification of the vehicle electricity balance. The simple-Coulomb-Counting-based and the vehicle-SOC-based estimations of the vehicle electricity balance are compared and presented; the significant deviations encountered for some vehicles, which might affect the certified CO₂ emissions and electrical energy consumption values, are discussed. Finally, an improved version of the Coulomb Counting method, which uses charge correction factors derived from experimental data to improve the estimation, is presented.