Week 7 State of charge estimation
Study – What is coulomb counting? How BMS implements coulomb counting for SOC estimation?
Coulomb counting is method to estimate state of charge of a battery. State is one of the most important parameter of battery and it is defined as the ratio of current capacity and nominal capacity. There are many methods to estimate SOC like – terminal voltage method, open circuit method, direct measurement, coulomb method, modified coulomb method etc.
Coulomb counting is an elegant solution for specially li-ion batteries, which have high coulombic efficiency. Losses reduces the total energy delivered and available energy will be less than delivered, but coulomb counting works well. Coulomb counting method takes the values of current and integrates them over the time to find SOC. Current value will be provided by ammeter. SOC equation-
`SOC(t) = SOC(t-1) - int(n*I(t).dt)/Q_n`
Where-
`I(t)` = discharging current
`SOC(t)`= previously estimated SOC
`Q_n` = battery capacity
`n` = discharge efficiency
Coulomb counting method has simple structure and ease of implementation, however it does not consider battery temperature, cycle life and battery history during SOC calculation. Battery efficiency is influenced by battery temperature which causes inaccurate SOC calculation. It is very necessary to calibrate initial SOC value for this method.
Implementation in BMS - Coulomb counting method can be implanted in BMS with a suitable algorithm. The following chart is giving general idea, how SOC estimation is going to work. This algorithm with can be implemented with state machine in Simulink, which can be further uploaded in flesh memory of BMS hardware using code generation option.
Now look at inside of a Simulink model of battery management system. In this model SOC is estimated with different methods – Kalman filter and coulomb counting. Coulomb counting method is taking pack current values and divided by capacity values, which is given by a look up table to show variation of capacity with temperature. Further values are integrated to estimate charge.
