Week 7 State of charge estimation

1. Battery Harness Dashboard

the battery closed loop harness model is as shown. This dashboard consists of a BMS_state request subsystem which selects the variant based on the signal given from the test sequence variant. This request is ent to the BMS_ closed loop model to get BMS_input, BMS_output and BMS_info. The bms input goes back to the bms plant model to work in closed loop.

The BMS output and Info is used to analyse and get the information about the BMS. The dashboard also has lights to indicate fault state, high temperature, over voltage, charger connected, voltage sensor, over current, low temperature, under voltage and inverter information.

The test sequence is selected based on the test sequence variant value given.

Test Case 1:

The first case consists of driving, balancing, charging and balancing and post charger balancing for 3000,1000,5000 and remaining respectively. It can be clearly seen that until 9000 seconds there are changes and after which it goes to post charger balancing mode.

Test Case 2:

Test Sequence 2 has Driving and charging for 10000 seconds for both cases.

Test case 2 works for 10000 seconds until driving is achieved and the charging is continued.

Test Case 3:

The cell voltages , cell temperatures, SOC, Pack Current, BMS State, Bal cmd can be seen from the scope. Test case 3 is for charging, The SOC becomes maximum and remains constant, the pack current reduces to zero. The cell voltages are stabilized to a nominal voltage value.

Columb Counting to estimate SOC:

Coulomb counting is a technique used to track the SOC of a battery pack. It works by integrating the active flowing current (measured in amps) over time to derive the total sum of energy entering or leaving the battery pack. This produces a capacity that is typically measured in Ah.

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:

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

In the above model, the current is divided by the capacity based o the loop table which gives the capacity of the battery based on the temperature value.This current divided by capacity is integrated overtime to get SOC of the battery. This is the Columb Counting method followed.