Week 7 State of charge estimation
AIM : To define coulomb counting and find how BMS implements coulomb counting for SOC estimation
State Of Charge of Battery
*SOC of a battery is defined as the ratio of its current capacity to the nominal capacity The nominal capacity is given by the manufacturer and represents the maximum amount of charge that can be stored in the battery. The SOC can be defined as follows:
*SOC estimation is a fundamental challenge for battery use. The SOC of a battery, which is used to describe its remaining capacity, is a very important parameter for a control strategy.
*As the SOC is an important parameter, which reflects the battery performance, so accurate estimation of the SOC can not only protect battery, prevent overdischarge, and improve the battery life but also allow the application to make rational control strategies to save energy.
* A battery is a chemical energy storage source, and this chemical energy cannot be directly accessed. This issue makes the estimation of the SOC of a battery difficult. However we calculate SOC using advanced methods.
* Different methods to calculate SOC
(i) Direct measurement: This method uses physical battery properties, such as the voltage and impedance of the battery.
(ii) Book-keeping estimation: This method uses discharging current as the input and integrates the discharging current over time to calculate the SOC.
(iii) Adaptive systems: The adaptive systems are self-designing and can automatically adjust the SOC for different discharging conditions. Various new adaptive systems for SOC estimation have been developed.
(iv) Hybrid methods: The hybrid models benefit from the advantages of each SOC estimation method and allow a globally optimal estimation performance.
Book keeping estimation :
Book-keeping estimation method uses battery discharging current data as input. This method permits to include some internal battery effects as self-discharge, capacity-loss, and discharging efficiency. Two kinds of book-keeping estimation methods have been employed: Coulomb counting method and modified Coulomb counting method.
(i) Coulomb counting method : It is a technique used to track the state of charge of a battery. The Coulomb counting method measures the discharging current of a battery and integrates the discharging current over time in order to estimate SOC.
SOC is calculated by following equation,
(ii) Modified Coulomb counting method:
*To improve the Coulomb counting method, a new technique called modified Coulomb counting method is proposed. The modified Coulomb counting method uses the corrected current to improve the accuracy of estimation.
*The corrected current is the function of discharging current. There is a quadratic relationship between the corrected current and discharging current of battery. By practice of experimental data, corrected current is calculated by
where K2,K1&K0 are constant values obtained from the practice experimental data.
*In modified Coulomb counting method, SOC is calculated by the following equation,
*The experimental results show that the accuracy of the modified Coulomb counting method is superior to the conventional Coulomb counting method.
BMS implements coulomb counting for SOC estimation
* The above system works according to the equation
* Here temperature and current are given as inputs. Gain block is attached with current signal. Since temperature affects SOC ,capacity is varied with respect to it.
* An integrator block is used which integrates the discharging current over time in order to estimate SOC. Current is divided by capacity Qn.
* This helps us to find out state of charge of a battery.