Week 7 State of charge estimation

coulomb counting

The coulomb counting method is expedient for state-of-charge (SOC) estimation of lithium-ion batteries with high charging and discharging efficiencies. The charging and discharging characteristics are investigated and reveal that the coulomb counting method is convenient and accurate for estimating the SOC of lithium-ion batteries.

The design and development of a coulomb counting evaluation platform to be used for SOC and SOH measurement for a typical energy storage module, which in this case is a 24 V module,typically comprising seven or eight Li-ion cells. The evaluation platform is composed of a hardware system including an MCU and required interfaces and peripherals, embedded software for the SOC and SOH algorithm implementation, and a PC-based application software as a user interface for system configuration, and data display and analysis. The evaluation platform periodically measures the voltage value of each cell and the battery pack’s current and voltage, by means of appropriate ADCs and sensors, and will run the SOC estimation algorithm in real time. This algorithm will use measured voltage and current values and some other data collected by temperature sensors, and/or given by PC-based software application (such as constructor specifications from a database). The SOC estimation algorithm output will be sent to the PC graphical user interface for dynamic display and database updating

Coulomb Counting Method
The coulomb counting method, also known as ampere hour counting and current integration, is the most common technique for calculating the SOC. This method employs battery current readings mathematically
integrated over the usage period to calculate SOC values given by

SOC= SOC(t0) + 1/Crated ∫ (Ib – Iloss) dt

where SOC(t0) is the initial SOC, Crated is the rated capacity, Ib is the battery
current, and Iloss is the current consumed by the loss reactions.

The coulomb counting method then calculates the remaining capacitysimply by accumulating the charge transferred in or out of the battery. The accuracy of this method resorts primarily to a precise measurement of the battery current and accurate estimation of the initial SOC. With a preknown capacity, which might be memorized or initially estimated by the operating conditions, the SOC of a battery can be calculated by integrating the charging and discharging currents over the operating periods. However, the releasable charge is always less than the stored charge in the charging and discharging cycle. In other words, there are losses during charging and discharging. These losses, in addition with the self discharging, cause accumulating errors. For more precise SOC
estimation, these factors should be taken into account. In addition, the SOC should be recalibrated on a regular basis and the declination of the releasable capacity should be considered for more precise estimation