SOC (State of Capacity) has been gradually replaced by SOH (State of Health) and become the mainstream of the focus on battery’s life cycle and health evaluation. Although there is still no standard for measurement, many studies have confirmed that SOH is able to provide a comprehensive and accurate estimation of a battery’s health status.
Recently, the most popular method of SOH evaluation is to compare the current status of a used battery with that of a new battery. During the process, several different parameters will be considered and then a “relative” health status will be calculated. The evaluation for a battery cell focuses on the differences between charge/discharge performances, while the battery pack focuses on the consistency among each single cell.
- Comparative index between new and used battery cells: capacity, internal resistance, voltage concentration ratio, Self-discharge rate, energy conversion efficiency, etc.
- Comparative index between new and used battery packs: capacity, energy conversion efficiency, consistency (internal resistance, voltage difference, temperature difference), etc.
- Complete Test (Available in Chen Tech Equipment) : Based on the testing standard IEEE 1188-2005, record the voltage, current, and temperature values from a full charge and discharge cycle, find the rates for different indexes, and calculate the SOH with pre-defined weights.
Diagram 1 : Example of IEEE 1188-2005 Test
- Rapid Test (Available in Chen Tech Equipment) : Measure the parameters while charging and discharging in certain status, or performing a specific charging/discharging pattern. Some automobile manufacturers collect BMS data regularly, and use the data to calculate SOH.
Diagram 2 : Example of IEEE PESC 2006,a two-pulse method proposed by Dr. Marin Coleman
- Precision Test : Besides the variation of voltage, current, and temperature during charge/ discharge process, extra information such as Entropy, Enthalpy, and X-ray is also collected and used to calculate the SOH. This method provides the most precise results as it comes from assessing chemical reaction of raw materials.
Diagram 3 : A thermodynamics method from Dr. Richid Yazami, a professor of Nanyang Technological University, Singapore
Comparison of Various Evaluation Methods
|Complete Test||Rapid Test||Precision Test|
|Requirements of Equipment Specs||Moderate||High (Pulse and mS data recording capability needed)||Extreme high (Instruments for electrochemistry study needed)|