Week 7 State of charge estimation

Q1. Battery management system: -

State request 1: -

 

 

Observation: - 

1. The first graph indicates the cell voltage of all the cells involved in the battery pack. we first observed that cell voltages change as the current flows in and out. at the beginning of the simulation, the voltages are slightly different because we initialize the model with a slight associate of imbalance the voltages are deflected from the range of 3.1 v to 3.9 volts for the period of 0.3*10^4 sec, and then there is a balances state of all voltage of their respective cells and then there is a sudden hike in cell voltage till they reach 4.2 volts and at the end of the simulation, the values converge towards one another and trying to balance the voltages till the end still there is a small difference in their voltages and the voltage line flows between 4 to 4.2 volt. because the system goes on standby mode.
2. In charging state during the constant current phase current (current is deflecting in the range of 19 A TO -70 for 0.310^4 sec) is debated because the maximum cell voltage is high, enough compared to prescribe 4.4-volt limit that an excessively high current control the voltage beyond the threshold significantly limiting the life of the battery
3. The temperature (it's starting from the range of 289-degree Celsius and at the end, all the cells have a different temperature) traces show the significant discrepancy between the hottest and coldest cell the reason is mainly because of symmetry in the merger layer in terms of thermal behaviour. cell no 6 is significantly hotter the cell no 1 because it is thermally insulated on one side. even if the maximum temperature reached during the simulation is not concerned in terms of safety. the temperature difference at cell no 6 at its higher temperature will eventually cost much faster degradation of cell 6 compared to cell 1 leading to undesirable and unevenness in cell condition. Therefore, we need an active thermal management system to keep thermal differences within a few degrees Celsius.
4. graph no 3 show the soc of three different battery cell each performed with different matter yellow corresponds to column counting, blue corresponds to ukf and orange to ekf, the initial soc is 75%, however, the simulation so estimates need to initialize it to 80% to set their capability to recover.it is apparent that column counting never does because it has no way to realize it is wrong due to the absence of voltage information. both common filter algorithms on the other hand recover from the initial error within the first hour of simulated time. with the ekf slightly outperforming then ukf.
5. The other scopes indicate the BMS state at each of the balance command signals respectively.

State request 2: -

 

 

1. At the initial state the voltage of all the cells is deflecting in the same manner and dropping slightly in the range of 3 to 3.9 volt it has 4 fluctuation zones and in between each of these fluctuating zones, they are stable for a while and this phase lasts for 8692 sec. (Throughout the simulation BMS has put on driving mode).
2. On the other hand, the current graph has flowed in exactly the same manner but in the opposite direction when voltage is dropping, the current required is high and current flows in the range of -70 A to 20 A.
3. The temperature of all the cells initiating from 288-degree Celsius and they get scattered all the cells have a different temperature) traces show the significant discrepancy between the hottest and coldest cells the reason is mainly because of symmetry in the merger layer in terms of thermal behaviour. cell no 6 is significantly hotter the cell no 1 because it is thermally insulated on one side. even if the maximum temperature reached during the simulation is not concerned in terms of safety.
4. The cells are initially at 80% of soc and then they're falling at different periods. to set their capability to recover.it is apparent that column counting never does because it has no way to realize it is wrong due to the absence of voltage information. both common filter algorithms on the other hand recover from the initial error within the first hour of simulated time. with the ekf slightly outperforming then ukf then it is apparent that column counting never does because it has no way to realize it is wrong due to the absence of voltage information. both common filter algorithms on the other hand recover from initial error within the first hour of simulated time. with the elf slightly outperforming the ukf.
5. The other scopes indicate the BMS state at each of the balance command signals respectively. where it's on driving mode and command signals on zero lines.

State request 3: -

 

1. All the cells are initiated at different voltages the where the 6 the one having the higher voltage of 4.2 volts and the 1st one having near about 4.1 v and remaining falling under this range BMS on charging state throughout the simulation. so, the voltage graph moves along in a straight line because the voltage supply continues. There has some deflection of voltage lines of different cells but that is a very less amount of change in voltage .at the end of the simulation BMS tries to balance the voltage very much
2. The battery pack current is initiated with 20 A and then it falls to zero current till its complete simulation time of 0.2*10^4 sec and then it maintains the constant current because it is put on charging state and it remains constant throughout the simulation
3. All the cells initiate with 287-degree Celsius and then all the cells maintain the straight line but with different temperatures range. the end all the cells have a different temperature) traces show the significant discrepancy between the hottest and coldest cells the reason is mainly because of symmetry in the merger layer in terms of thermal behaviour. cell no 6 is significantly hotter the cell no 1 because it is thermally insulated on one side. even if the maximum temperature reached during the simulation is not concerned in terms of safety. the temperature difference at cell no 6 at its higher temperature will eventually cost much faster degradation of cell 6 compared to cell 1 leading to undesirable and unevenness in cell condition. therefore, need an active thermal management system to keep thermal differences within a few degrees Celsius.
4. The initial soc of three different cells is at 80% as the. as the system is put on charging state the soc hikes up to 100% for the yellow cell, ukf and ekf initially having different charging rates but after 0.410^4 sec they having equal state of charge of 91% and it maintains throughout the simulation .t is apparent that column counting never does because it has no way to realize it is wrong due to the absence of voltage information. both common filter algorithms on the other hand recover from the initial error. ekf and urf slightly outperforming then coulomb counting.
5. the other scopes indicate the BMS state at each of the balance command signals respectively. BMS on charging state and battery command passing signal between 0 to 1.

Q2.

Ans: -

Coulomb Counting Method: -

The Coulomb counting method measures the discharging current of a battery and integrates the discharging current over time to estimate SOC. Coulomb counting method is done to estimate the SOC(t), which is estimated from the discharging current, 1(t), and previously estimated SOC values, SOC(t-1). SOC is calculated by the following equation:

`SOC (T)=SOC (t-1)+(I(t))/Qn*∆t`

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

Modified Coulomb Counting Method: -

To improve the Coulomb counting method, a new technique called the 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 the battery. By the practice of experimental data, the corrected current is calculated by the following form:

`Ic(t)= k2I(t)^2 + k1T (t) + k0,`

where K1, K2, and KO are constant values obtained from the practice experimental data. In the modified Coulomb counting method, SOC is calculated by the following equation:

  `SOC (T)=SOC (t-1)+(Ic(t))/(Qn) ∆t`

The experimental results show that the accuracy of the modified Coulomb counting method is superior to the conventional Coulomb counting method.

SOC Estimation in BMS system: -

In the above model there are a few subsystems like BMS ECU and PLANT (battery pack)

before we charge our vehicle, we need an accurate Estimation of battery SOC, in the battery system we don't measure the state of charge we measure different things like battery pack current cell voltage and cell voltage which we taking signals as input from the plant. -after entering the plant block, we can see the state machine, SOC estimation, and balancing logic block in which we are going to study SOC estimation.

In this SOC estimation block we can see three different techniques for the state of charge estimation. all three techniques have their own merits and limitations. The first method is columb counting and the other two are unscented(ukf) and extended Kalman filter(ekf). here we are concern with the columb counting method.

In coulomb counting method block -it takes two input signals such as current and temp and the current signal goes to gain block and the temp signal goes to lookup tables of battery time and current. and both the blocks are connected to the divided block and these signals are connected to the discrete-time integrator to track the charge and which finally helps to calculate the state of charge. which consist of integrating a current and doesn't leave the cell for that here we using discrete-time integrator to give the track of the cell charge of the time the benefits of this are its simplicity and its low capitation cost and the drawbacks are an accumulation of current sensor error and its inability to recover from the wrong initial condition. from the lack of feedback from voltage measurements. Here we used the same formula which is mentioned above.