Week 7 State of charge estimation

Aim :- Simulate the three test cases from harness dashboard and write a report on the results.

Objective :- Explain coulomb counting and also explain how BMS implements coulomb counting for SOC estimation.

Simulate the three test cases from the harness dashboard and write report on it.

(1)Theory :-

What is Coulomb counting,

The estimation of the state of charge (SOC) of a lithium battery is technically difficult, particularly in applications that don’t fully charge the battery or fully discharge it. Such an application is a hybrid electric vehicle (HEV). The challenge stems from the fact that lithium batteries have a very flat voltage discharge characteristic. There is little change of voltage from a 70% SOC to a 20% SOC. In fact, voltage changes due to temperature change are similar to the change of voltage due to discharge, making it essential to compensate for battery temperature if SOC is to be derived from voltage.

Coulomb counting is a technique used to track the state of charge 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 Amp-hours.

Rather than coulomb counting there are other methods are also available to measure SOC,

Voltage method :-

Measuring state-of-charge by voltage is simple, but it can be inaccurate because cell materials and temperature affect the voltage. The most blatant error of the voltage-based SoC occurs when disturbing a battery with a charge or discharge. The resulting agitation distorts the voltage and it no longer represents a correct SoC reference. To get accurate readings, the battery needs to rest in the open circuit state for at least four hours; battery manufacturers recommend 24 hours for lead acid. This makes the voltage-based SoC method impractical for a battery in active duty.

Hydrometer :-

The hydrometer offers an alternative to measuring SoC of flooded lead acid batteries. Here is how it works: When the lead acid battery accepts charge, the sulfuric acid gets heavier, causing the specific gravity (SG) to increase. As the SoC decreases through discharge, the sulfuric acid removes itself from the electrolyte and binds to the plate, forming lead sulfate. The density of the electrolyte becomes lighter and more water-like, and the specific gravity gets lower.

Impedance Spectroscopy :-

Battery state-of-charge can also be estimated with impedance spectroscopy using the Spectro complex modeling method. This allows taking SoC readings with a steady parasitic load of 30A. Voltage polarization and surface charge do not affect the reading as SoC is measured independently of voltage. This opens applications in automotive manufacturing where some batteries are discharged longer than others during testing and debugging and need charging before transit. Measuring SoC by impedance spectroscopy can also be used for load leveling systems where a battery is continuously under charge and discharge.

Implementation of Coulomb counting using BMS,

To determine available capacity, coulomb counting relies on the integration of the current drawn from and supplied to a battery over time, `tau``epsilon[t0 t1]`, as given by

SOC(`t`) = SOC`(t0)`-`int_0^1 (eta(tau)I(tau))/C_b^o d(tau)`

where,

I is the current.

η is the efficiency during charge and discharge operations.

Simulation of Li ion Battery Management System :-

(2) Theory

Battery management system (BMS) is technology dedicated to the oversight of a battery pack, which is an assembly of battery cells, electrically organized in a row x column matrix configuration to enable delivery of targeted range of voltage and current for a duration of time against expected load scenarios. The oversight that a BMS provides usually includes:

• Monitoring the battery
• Providing battery protection
• Estimating the battery’s operational state
• Continually optimizing battery performance
• Reporting operational status to external devices

Here, the term “battery” implies the entire pack; however, the monitoring and control functions are specifically applied to individual cells, or groups of cells called modules in the overall battery pack assembly. Lithium-ion rechargeable cells have the highest energy density and are the standard choice for battery packs for many consumer products, from laptops to electric vehicles. While they perform superbly, they can be rather unforgiving if operated outside a generally tight safe operating area (SOA), with outcomes ranging from compromising the battery performance to outright dangerous consequences.

BMS Model by Mathworks

The BMS model is combination of the various sub-systems which are BMS ECU,BMS plant,and the Dashboard.

BMS ECU :

It is the main component of the BMS or also called it as a brain of the BMS which calculates all the required thing that is needed to the Smooth working of the Battery or the BMS.Charging and discharging current,the state of charge,cell balancing and the state machines which provides the control logic these things are controlled or monitor by the ECU.The calculated values are pass through a bus which then relayed to the bus plant.

BMS State Machine :

This subsystem is the control logic behind the BMS.

Inside the State Machine ,

BMS Plant :

This subsystem is the actuator of the BMS.The battery pack subsystem is a combination of battery modules.There are two options,1 module or 16 module.1module is selected by default and it has 6 cells.The module subsystem provides details of parameters like cell voltage,cell temperature,pack voltage,and pack current.The second subsystem is the current charging circuit which is activated during the charging operation.The output system is called charger_load where the battery charger and motor drive are the outputs and depending on driving or charging condition the required subsystem is selected.

BMS dashboard :

Test Harness Dashboard : Testing different scenarios and conditions

Test No 1

No Fault condition in the Vehicle,

1). Battery is in the driving mode for first 3000 sec.

2). For next 1000 secs it is cell balancing as the BMS has come to a standby.This will generally only happen when the vehicle has comes to a stop for a long time.

3). The next 5000 seconds are for charging.

4). BMS continues to be in the standby for the remaining 11000 sec.

Output,

1. Fluctuations in the cell voltage and pack current until 3000sec indicates that the vehicle was in the driving mode.

2. Between 4000sec and 9000sec there is constant current and constant voltage charging happening from 4000 to 4600 sec it is constant current and then the remaining is constant voltage.

3. During CC the SOC increases much more quickly and then it slows down during CV as a precaution mechanism to avoid charging past the voltage llimit.

4. There is also moderate difference in the temp of cell 1 and cell 6 when the vehicle comes to a stop at 3000sec

5. The SOC graph shows the estimation of SOC through different methods with yellow corresponding to coulomb counting,blue for unscented kalman filter,and orange for extended kalman filter.

Test No 2

Here the simulation run for the 20000 sec,in which the first 5000 sec is for the discharging means vehicle is in the driving mode.And rest of the time is for charging of the battery.

Output,

1

1. In the voltage plot as we can see the voltage get drops due to discharging of the battery or vehicle is in the driving mode but after 5000sec it got settled to a constant value because vehicle is in the steady condition or in charging mode.

2. The second plot with pack current represents the current drawn and given to the battery while charging and discharging.

3. In the third plot which shows the temp of the cell.While charging and discharging the temp of the cells got increased.

4. The SOC increases curve decreases as we discharge the battery and increases while charging.

5. The 6 no plot shows the cell balancing of all cells.The cell balancing starts when the battery start to charge and at the onset of the fault the battery charging is stopped but the balancing happens.

Test No 3

No fault condition in the vehicle,

Here the battery is only in the charging mode.

Output,

1. Since the battery is only in the charging mode, the sudden influx of current from 0 to 30A is seen within the first 5sec,due to the surge of electrons in the cells.This also explains the drastic rise in temp in all cells.

2. Cell balancing is in process since the begining.

3. EKF has outperformed UKF by a little bit again,whilw coulomb counting method for SOC approximation seems the most inaccurate one.We can say this because a soc of 100% is highly unlikely and with coulomb counting it might be displaying 100% because it is only pre programmed to do so at the end of CC CV charging.EKF shows a SOC of 93% while UKF displays 94.5% which seems more realistic and accurate.