Rechargeable batteries are used to power auxiliary systems and motors in electric vehicle applications. Among all rechargeable batteries, lithium-ion batteries will provide high efficiency for electric mobility because lithium-ion batteries have low self-discharge rate, wide operating range, maximum energy density and cycle high life.
To improve battery quality and safe operation, a battery management system (BMS) is used and it plays a vital role in the application of electric mobility.
this is why do we need a battery management system (BMS) in electric vehicles?
Why a Battery Management System (BMS) in an electric vehicle?
To prevent battery failures and mitigate potential hazardous situations, it is necessary to have a supervision system that ensures the batteries are working properly in the end application. This supervision system is called a battery management system (BMS).
Read more: Thermal Management Systems (TMS) for Electric Vehicles
Battery Management System Functions in EV
The main function of a BMS is to meet safety requirements. But there is more than that. Goals related to more efficient use of battery cells and extending their lifespan are also increasingly integrated into BMS design.
Although there is no single definition of a BMS, it should be designed with a minimum set of requirements such as-
- It should measure individual cell voltages
- The BMS should measure temperatures at various points as close to the battery as possible
- It must measure the currents passing through it
- The BMS must communicate information to the control units and take action to ensure that the battery will operate within safe limits
- The BMS must balance the battery cells either passively or actively
- And, the BMS should provide thermal management
Battery status settings
For the management of the batteries during the operation of the electric vehicle, to obtain the best performance and extend the life of the battery, it is necessary to monitor in real time different states inside the battery according to the management system. battery (BMS).
These states include state of health (SoH), state of charge (SoC), state of operation (SoF), charge acceptance (CA), etc.
State of Charge (SoC)
All vehicles have a fuel gauge, similarly EVs also have a battery state of charge (SoC) gauge. The BMS helps to indicate and show the driver the actual state of charge of the battery.
The SoC of a cell is a percentage value that expresses the remaining charge Q of a battery.
Definition of State of Health (SoH)
With the advance of battery degradation, the internal resistance of the battery increases while the capacity of the cell decreases. This leads to drastic changes in cell behavior and can render a cell unsuitable for its primary application, such as in an electric vehicle.
Therefore, it is necessary to follow cell degradation, using the state of health (SoH) parameter. Battery SoH characterized by slowly changing parameters, such as capacitance fade and resistance rise, varies with cycles and therefore must be monitored over a long period.
Setting Function State (SoF)
Simply put, SoF can be defined as a parameter that describes how a battery’s performance meets application requirements while in use.
The SoF can be either a percentage value, a concrete value in kW, for example, or even a binary value representing whether or not the battery is able to meet the demand of the application.
A more general definition of SoF can be the fraction of the ΔP (difference between the available power and the requested power) compared to the ΔPmax (difference between the maximum power the battery can supply and the requested power), i.e. say it is a percentage value that describes how much the current state of the battery (SoC, SoH, temperature, etc.) differs from the optimal state of the battery.
Acceptance of Charges (CA)
It indicates the maximum charging current that the battery can accept under the current conditions (SoC, SoH, temperature) and for a given charging voltage and is therefore very relevant for regenerative braking.
However, SoC and SoH cannot be measured directly by sensors, they are only monitored and reflected based on measured parameters such as voltage, current, temperature, and internal resistance.
Various methods have been developed for the estimation of SoC and SoH. Considering the practical applications, the methods can be roughly classified into online and offline methods.
Online methods can be used for real-time battery condition estimation. However, offline methods are not suitable during battery operations due to strict experimental designs or high computational costs.
Classification of Soc, SoH estimation methods
Beyond the basic functionality of a Hybrid Electric Vehicle (HEV)/Battery Electric Vehicle (BEV) BMS of measuring cell voltages, cell temperatures, and current flowing through the battery pack, the Automotive BMS shall provide methods of equalizing charge imbalances between individual cells in a multi-cell battery system to increase both cell life and usable energy in each discharge cycle.
Battery Cell Balancing
Imbalances between cells are exacerbated by continuous charge/discharge cycles if left uncorrected, resulting in cell gapping. Cells with less total capacity than the best performing cells in the system can become overloaded, leading to premature cell degradation.
This degradation leads to capacitance fading and therefore accelerates the initial problem. Additionally, this overcharging can become a safety hazard as it can cause active battery components to react with each other and cause thermal runaway.
Causes of Imbalance in Battery Cells
Classification systems for balancing methods
There are different classification systems for balancing methods.
Static methods: Methods executed before the pack is running or not controllable by the BMS once the pack is running.
Dynamic methods: balancing methods controllable by the BMS, themselves divided into active and passive methods. The active cell balancing technique uses inductive charge shuttle or capacitive charge shuttle to transfer charge between cells.
This technique has proven to be an effective approach as it transfers energy to where the energy is needed instead of wasting it.
However, this requires adding additional components to the system, which in turn results in increased costs. The passive cell balancing technique uses the idea of discharging cells through a bypass path that is primarily dissipative in nature.
It is simpler and easier to implement than active balancing techniques because the bypass can be external or built-in, making the system more cost effective in either case. However, since all excess energy is dissipated as heat, battery run time is negatively affected and is less likely to be used during discharge.
The adoption of precise cell balancing results in a higher capacity for the intended application because the state of charge (SoC) that can be achieved is higher.
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