Week 7 State of charge estimation

1. Here it is asked to compare the 3 test results of BMS closed loop harness dashboard model.

first test

In this tesy its driving mode for 3000 seconds, then balancing mode for 1000 seconds, then charging mode for 5000 seconds, then the remaing 11000 seconds it is for standby mode i.e balacing mode. As the total simulation time is 20000 seconds.

Model to be simulated is:

After simulation the LED used here is all green which means there is no any falut occured in this test. The Scope results are as follows:

As there are 6 cell module used ao total 96 cells in series.

The observation from the results are:

From 0 to 3000 seconds it is in driving mode as shown in BMS state. During that mode, cell voltage and cell current keeps on fluctuating based on the drive cycle data. Cell temperature also keeps on increasing as the usage of  discharge current in battery is more . As the initial SOC is 80% it start to greater extend upto 45% in case of extended Kalman filter. Then the SoC is shown as 48% for unscented Kalman filter (UKF). Soc is showing 53% in case of coulomb counter.. During driving state balance command is 0.

From 3000 to 4000 seconds there is no usage of battery current and voltage as the battery is in stand by state. Also the temperature decreses to some extend as the battery is been not used either for chargeing or discharging. Similarly SOC is constant for this period.

From 4000 to 9000 seconds, the battery is in charging mode as shown in BMS state. As we know the battery li-ion battery charges in CC-CV mode i.e constant current constan voltage mode. Constant current is for shorter duration of 4000 to 4400 seconds. But for the remaining time it is is constant voltage mode which is for around 4500 to 7500 seconds. So time duration for CV is longer then CC mode. During this charging mode, the temperature of the battery increases lienearly. Once the temperature reached its maximum limit coolant is switched ON, so that the temperature start decreasing.

From 9000 to 20000 seconds, the battery is is stantdy mode, so voltage is maintained also the used pack current is zero during this state. As load or charger is not conectd, the current used is zero. During this time, the temperature further decreases as the battery is not in use. Soc is almost 100% in case of coloumb counter measurement. But in case of EKF and UKF the SOC is bit lower near to 95%. So the conclusion, Kalman filter is more accurate for estimation of SOC rather than coloumb counter.

Second test:

In this test, it is been in driving state for 10000 seconds, and for 10000 seconds it is in charging state.

From 0 to 10000 seconds it is in driving mode as shown in BMS state. During that mode, cell voltage and cell current keeps on fluctuating based on the drive cycle data. Cell temperature also keeps on increasing as the usage of  discharge current in battery is more . As the initial SOC is 80% it start to greater extend upto 45% in case of extended Kalman filter. Then the SoC is shown as 48% for unscented Kalman filter (UKF). Soc is showing 53% in case of coulomb counter.. During driving state balance command is 0.

From 10000 to 20000 seconds, the battery is in charging mode as shown in BMS state. As we know the battery li-ion battery charges in CC-CV mode i.e constant current constan voltage mode. Constant current is for shorter duration of 10000 to 11000 seconds. But for the remaining time it is is constant voltage mode which is for around 11000 to 13000 seconds. So time duration for CV is longer then CC mode. During this charging mode, the temperature of the battery increases lienearly. Once the temperature reached its maximum limit coolant is switched ON, so that the temperature start decreasing.

Third Test:

In this test , it is being in charging mode for 20000 seconds.

Scope output is :

As it is in charging mode initially it is in CC mode for short duration, then is moves to CV mode where the current keep on decreasing exponentially. As before 2000 seconds the SOC of the battery reached to 100% from 80%  in coulomb conter model. But in case of EKF and UKF the SOC has reached to it maximum limit of 94% which can't be reached more than this because of balancing issue between the cells. Similarly the temperature of the battery pack increases till it reaches the full SOC . Once the OSC is full the temperature start to decrease linerarly.

2.

A feature or device that measures the accumulated energy added to and removed from a battery, allowing accurate estimates of battery charge level. Also it is a technique used to track the state of charge of a battery pack. It work by integrating the current over time to derive.

Coloumb counter method:

In general, the 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(t)=Q(t)/Qn                  

The Coulomb counting method 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 SOC(t) , which is estimated from the discharging current, I(t) , and previously estimated SOC values, SOC(t-1) . SOC is calculated by the following equation:

                    SOC(t)=SOC(t-1)+(I(t)/Qn)dt

But there are several factors that affect the accuracy of Coulomb counting method including temperature, battery history, discharge current, and cycle life.

How BMS implements coulomb counting for SOC estimation?

Li-ion batteries are charged by  a specific charge method called constant-current-constant-voltage method(CC-CV).  Based on different C-rate of discharge current, the voltage dip will vary.

Functions of BMS:

1. Monitoring of battery current, voltage (individual cell voltages as well as pack voltage)
2. Temperature monitoring (pack temp)
3. State Of Charge(SOC) and Sate Of Health(SOH estimation)
4. Balancing of cells
5. Generating Safety Critical alarm and derating the output if required.
6. Emergency Shutdown in Critical state.
7. 

Coulomb counting will note down or monitor the voltage of battery and current of battery i.e  Vbat and Ibat. Also temperature is also taken by using thermal sensors.

The counter works on the principle of recogonized direction of current from current sensing element.

When the current Ibat = 0, then compensation of self discharge losses will be taken into consideration.

The amount of charge dQ in an period of t is obtained by integrating battery current with respect to time.

If dQ is negative then battery is discharging, its positive in case of vice versa.

Modes of operation:

1. Charging mode:

                                           Qgained(T+t)=Qgained(T)+dQ

 This represents the charge accumulated over a time t with initial condition at time T.

                                           SOC(T+t)=SOC(T)+dSOC

 This is the instantaneous charge accumulated in the battery. The value of dSOC(T+t) is:

                                       dSOC(T+t)=Qgained(T+t)/Qrated

As we know DOD + SOC = 1. Depth of discharge is just the toggle of SOC.

2. Discharge mode:

Here the coulomb counter has to take the lost charges into consideration.

                                        dSOC(T+t)=Qgained(T+t)/Qrated

                                              DOD(T+t)=DOD(T)+dDOD

                                           DOD(T+t)=Qlost(T+t)/Qrated

Here DOD is measured with the charges lost formula.

In case of above model, the closed loop BMS the SOC calculation is doen by using 2 methods for comaparison, 

Coulomb method

Kalmans filter  method (EKF and UKF) 

It is shown as follows:

  In case of Coulombs counter method:

Both current and temperature of battery pack is taken,. The current signal is fed into a gain block of 1/3600 to convert it into ampere hour. The temperature signal is fed into a 1D look up table. Both these values are processed and fed into a integrator block with z-transform. The output of this block is the SOC of the system.

Second case is UKF and EKF, Here the cell voltahe is taken and is fed Kalman filter. This output is fed into selector switch. The output of this switch is the SOC of the battery.