Advanced Battery Management Technologies for Electric Vehicles

4Battery State of Health Estimation

4.1 Background

The capability of a battery to store energy and provide a certain power decreases over battery lifetime because of aging. The state of health (SOH) is an indicator to quantify aging level of a battery in terms of capacity fade and/or power fade. It can be used to recognize an ongoing degradation of a battery, prevent its possible failure, and decide the replacement and removal of a battery system (or a battery pack) from electric vehicles (EVs).

The battery SOH can be defined as battery capacity decrease and internal resistance (or impedance) increase. It reflects the health of a battery cell in percentage; 100% stands for a fresh cell. When the battery capacity drops to 80% of the initial rated capacity, namely the SOH is 80%, the battery should be considered for replacement in an EV. Similarly, when the battery internal resistance doubles, which results in a significant power drop, the battery should also be considered for removal from an EV [1].

The SOH can be generally estimated by experimental and model‐based methods. The experimental methods analyze the stored data which affect battery lifetime during its cyclic operation to directly determine the SOH. The model‐based methods determine the SOH by estimating battery parameters that are senstive to degradation of a battery cell throughout the real‐time operation of the battery. Each group of methods can be further divided into different methods, as summarized in Figure 4.1 ...