Due to the Emission of CO2, CO, Suspended particulates, volatile organic compounds, into the atmosphere are released from traditional IC Engine Vehicle. As an increasing number of people use public and personal transportation, the amount of air pollution increases every single day. Consequently, electric vehicles are becoming more and more popular.
An electric vehicle generally contains the following major components: an electric motor, a motor controller, a traction battery, a battery management system, a plug-in charger that can be operated separately from the vehicle, a wiring system, a regenerative braking system, a vehicle body, and a frame. The battery management system is one of the most important components, especially when using lithium-ion batteries. Starting, lighting & other accessories use
Currently, three types of traction batteries are available: the lead-acid, nickel-metal hydride, and lithium-ion batteries. Lithium-ion batteries have several advantages over the other two types of batteries, and they perform well if they are operated using an effective battery management system.
Lithium is the lightest metal with the greatest electrochemical potential and the largest energy density per weight of all metals found in nature. Using lithium as the anode, rechargeable batteries could provide high voltage, excellent capacity and a remarkably high-energy-density. However, lithium is inherently unstable, especially during charging. Therefore, lithium ions have replaced lithium metals in many applications because they are safer than lithium metals with only slightly lower energy density. Nevertheless, certain precautions should be made during charging and discharging. During Charging ions move from Cathode (+) to Anode (-) and discharge is vice versa.
The lithium-ion battery requires almost no maintenance during its lifecycle, which is an advantage that other batteries do not have. No scheduled cycling is required, and there is no memory effect in the battery. Furthermore, the lithium-ion battery is well suited for electric vehicles because its self-discharge rate is less than half of the discharge rate of lead-acid and NiMH batteries.
Lithium ion batteries comprise a family of battery chemistries that employ various combinations of anode and cathode materials. Each combination has different advantages and disadvantages in terms of safety, performance, cost, and other parameters. The most prominent technology for Automotive applications is Lithium – Nickel – cobalt – Aluminium (NCA), Lithium – Nickel-Manganese-Cobalt (NMC), Lithium – Manganese- spinel (LMO), Lithium titanate (LTO) & Lithium-iron phosphate (LFP). Despite the advantages of lithium-ion batteries, they also have certain drawbacks. Lithium ions are brittle. All Automotive battery chemistries require elaborate monitoring, balancing, & cooling systems to control the chemical release of energy, prevent thermal runaway, & ensure a reasonably long-life span for the cells.
To maintain the safe operation of these batteries, they require a protective device to be built into each pack. This device also referred to as the battery management system (BMS), limits the peak voltage of each cell during charging and prevents the cell voltage from dropping below a threshold during discharging. The BMS also controls the maximum charging and discharging currents and monitors the cell temperature.
LITHIUM BATTERY CHALLENGES
The operating temperature and voltage are the most important parameters that determine the performance of lithium-ion cells. Figure 1 and 2 shows that the cell operating voltage, current and temperature must be maintained within the area indicated by the green box labelled “Safe Operation Area” (SOA) at all times. The cell could be permanently damaged if it is operated outside the safety zone.
The batteries could be charged above its rated voltage or be discharged under the recommended voltage. If the recommended upper limit of 4.2 V were exceeded during charging, the excessive current would flow and result in lithium plating and overheating. On the other hand, overly discharging the cells or storing the cells for extended periods of time would cause the cell voltage to fall below its lower limit, typically 2.5 V. This could progressively break down the electrode.
The operating temperature of lithium-ion cells should be carefully controlled because excessively high or low temperatures could damage the cell. Temperature-related damages could be grouped into three types: low-temperature operational impact, high-temperature operational impact and thermal runaway. While the effects of voltage and temperature on cell failures are immediately apparent, their effects on the lifecycle of the cells are not as obvious. However, the cumulative effects of these digressions may affect the lifetime of the cells.
Figure 3 shows that the lifecycles of the cell would be reduced if its operating temperature falls below approximately 10 °C. Similarly, their lifecycles would be reduced if the cells were operated above 40 °C. Furthermore, thermal runaway would occur when the temperature reached 60 °C. The thermal management system, which is part of the BMS, must always be designed to keep the cells operating within its limitation.
It is clear from the discussion above that the goal of the BMS is to keep the cells operating within their safety zone; this could be achieved using safety devices such as protection circuits and thermal management systems.
BATTERY MANAGEMENT SYSTEMS (BMS)
There are different types of BMSs that are used to avoid battery failures. The most common type is a battery monitoring system that records the key operational parameters such as voltage, current and the internal temperature of the battery along with the ambient temperature during charging and discharging. The system provides inputs to the protection devices so that the monitoring circuits could generate alarms and even disconnect the battery from the load or charger if any of the parameters exceed the values set by the safety zone.
The battery is the only power source in pure electric vehicles. Therefore, the BMS in this type of application should include battery monitoring and protection systems, a system that keeps the battery ready to deliver full power when necessary and a system that can extend the life of the battery. The BMS should include systems that control the charging regime and those that manage thermal issues. In addition, it must interface with other onboard systems such as the motor controller, the climate controller, the communications bus, the safety system, and the vehicle controller.
The definition of a BMS may differ depending on the application, the basic task of BMS could be defined in a below-mentioned manner:
- It should ensure that the energy of the battery is optimized to power the product.
- It should ensure that the risk of damaging the battery is minimal.
- It should monitor & control the charging and discharging process of the battery.
According to the definition, the basic tasks of the BMS are identical to its objectives. Although different types of BMS have different objectives, the typical BMS follows three objectives:
- It protects the battery cell from abuse and damage.
- It extends the battery life as long as possible
- It makes sure the battery is always ready to be used.
FUNCTIONS OF BATTERY MANAGEMENT SYSTEMS (BMS)
- BMS is to keep the battery from operating out of its safety zone.
- The BMS must protect the cell from any eventuality during discharging.
- Batteries are more frequently damaged by inappropriate charging
- For Lithium-ion Batteries a 2-stage charging method called the Constant Current – Constant Voltage (CC-CV) is used.
- During the 1st stage (CC) the charger produces a Constant Current that increases the battery voltage
- In the 2nd stage, When the battery voltage reaches a constant valve, & the battery becomes nearly full, it enters the Constant Voltage (CV) stage. At this stage, the charger maintains the Constant voltage as the battery current decays exponentially until the battery finished charging.
STATE-OF-CHARGE (SOC) / DEPTH OF DISCHARGE
- The BMS has to keep track of the SOC of the battery
- The SOC sends a signal to the user and control the charging and discharging process
- There are 3 methods of determining SOC:
- Through Direct measurement
- Through Coulomb Counting
- Through the combination of the above 2 techniques
- The SOH is a measurement that reflects the general condition of a battery & its ability to deliver the specified performance compared with a fresh battery
- Any parameter such as cell impedance or conductance that changes with age could be used to indicate the SOH of the CELL.
- Cell balancing is a method of compensating weaker cells by equalizing the charge on all cells in the chain to extend the overall battery life.
- Two methods of cell balancing are used Active Cell Balancing & Passive Cell balancing
- In Active cell balancing the charge from stronger cells is removed & delivered to the weaker cells
- In Passive cell balancing, different techniques are used to find the cells with the highest charge in the pack, as indicated by higher cell voltage. Then the excess energy is removed through a bypass resistor until the voltage or charge matches the voltage on the weaker cells.
- The Logbook Function of the BMS would hold important data such as initial conditions or a set of standard conditions for comparison to maintain SOH of battery
- This function of BMS is provided through a Datalink used to monitor performance, log data, provide diagnostics or set system parameters.
- The choice of the communication protocol is not determined by the battery instead it is determined by the application of the battery.
- The BMS used IN BEV must communicate with vehicle ECU’s & Traction Motor controller to ensure the proper operation of the vehicle
- Usually CAN bus Protocol is used in BEV
There are 3 BMS Topologies:
- Distributed Topology – The Voltage Monitors & Discharge balancers with digital communications that can cut off the charger & reports its status are placed on each cell.
- Modular Topology – Several Slave Controllers are used to consolidating the data to a master controller.
- Centralized Topology – A Centralized master control unit is directly connected to each cell of the battery pack.
BATTERY MANAGEMENT SYSTEM CIRCUIT
When it comes to Electric Vehicles, the battery industry is developing innovative solutions faster than ever. This is due to the adoption of advanced battery technologies for electric and hybrid transport applications. Batteries are a key enabler for electric transportation. According to researchers, advanced battery technologies could give electric vehicles more than 200 miles of charge in as little as 10 minutes. Electric vehicle manufacturers and battery manufacturers are making progress in developing new lithium-ion designs for a ground-breaking revolution in EV charging.
The development of battery tech and battery management systems relies on innovative R&D, advanced manufacturing processes and technical infrastructure to build sturdy and reliable battery components to support the growth of Electric Vehicles. Technical experts are developing cutting-edge solutions for rapid-charging batteries. The development and commercialization of hybrid and electric vehicle battery systems within the speciality EV battery market need to be addressed while assessing consumer demand, competing technologies and overcoming the challenges to commercialization. The tremendous growth in the demand for batteries is imposing a number of economic, environmental, and societal threats to the value chain. A sustainable solution of the battery value chain is required to ensure the positive impact of batteries.
– Article contributed by Prafulla Vejendla