Week 7 State of charge estimation
A) Simulation of BMS Model
The BMS model consists of a BMS ECU is with its various monitoring and control algorithms is connected to a block with the representation of the battery pack .
BMS ECU is the main operating state of the BMS. The state machine has four parallel states :
- The first one defines the variable BMS_State which indicates standby, driving, charging, and fault.
- The second state switches the fault state ON in case a current, voltage, or temperature value reaches an unsafe level.
- The third and fourth states define the contactor ON and OFF switching sequence for the charger and inverter.
- The third and fourth states define the contactor ON and OFF switching sequence for the charger and inverter.
The dashboard is used to control and identify the test case states in the state machine. It consists of the test sequence variant subsystem which holds three different test cases. This dashboard also helps in determining the fault states and other parameters as shown in the diagram below. The green color indicates that the system is working without any errors while the red signal will indicate that there is a fault in the system.
CASE-1: drive-balance-charge-balance, there are no errors and faults :
When the simulation begins, initially the vehicle is in a driving state and the SOC will decrease for this time period. After a time period of 3000seconds, the next step is balancing mode and the vehicle is in standby condition. After another 1000 seconds, the vehicle is in the charging phase and the battery's SOC increases almost to 100%. The next phase is the post charging phase in which the vehicle again goes for the standby mode.
The input cycle can be seen in the BMS_State plot in which the vehicle initially undergoes driving, then charging till SOC= 90% and then later in the standby state.
-During Charging the Battery pack voltage is increased up to 4.2V. The cell temperature increases during the driving and charging cycle and starts falling off in standby mode.
-The SOC drops to less than 50% after the driving cycle and then later put into charging until SOC Reaches 100%. The current supplied to the EV during the driving phase can be seen in the current plot.
CASE-2:Drive-charge, High-temperature fault :
After 10,000 sec when the driving cycle is over the fault condition occurs and so the SOC does not increase even for the charging phase.
cell temperature increases during driving, and since it is higher than recommended, BMS stops the battery from charging, so we can see a gradual decrease in temperature
CASE-3:charging, there is no fault :
The SOC increases from 80 to 100% and then remains in standby mode.
During charging SOC increases from 80 to 100%, temperature increases during charging and is reduced in the standby state of the BMS.
B)Coulomb counting
Coulomb counting is a technique used to track the State of Charge of a battery pack. It works by integrating the active flowing current over time to derive the total sum of energy entering or leaving the battery pack. This produces a capacity that is typically measured in Amp-hours.
The Coulomb counting technique measures the discharging current of a battery and integrates the discharging current over time in order to estimate SOC. Coulomb counting method is done to estimate the, discharging current, and previously estimated SCC values. Integrating the current into or out of a battery gives the relative value of its charge.
Coulomb counting needs a starting point. If the initial charge in the battery is known, from then on coulomb counting can be used to calculate the charge. The cell voltage during discharge is lower than during charge, so the energy discharged is lower than the energy during charging, even though the charge in and out is the same.
Implementation of coulomb counting in BMS:
The estimation of the SOC of a Li-Ion battery in this method is based on monitoring both the voltage and the current. The operation mode of the battery is recognized by the direction of current through the battery system.. Coulomb counter /10 is used to track the SOC when the battery is charged or discharged. The amount of charges /iQ in an operating period T is obtained by temporal integration of a measurement of charging and discharging of current.
The estimation of the SOC of a Li-ion battery by the developed method is based on monitoring the voltage, current, and temperature. The operation mode of the battery is recognized by the direction of the current through the battery system.