Battery Management System in an Electric Vehicle – The Ultimate Guide – Get an Electric Vehicle

A battery management system is an electronic system that manages a rechargeable battery.

A battery management system is the brain of a battery

A battery with an integrated battery management system is a smart battery. A smart battery charger that can be connected to the smart battery BMS via CAN bus or any other communication protocol charges the battery. It is necessary to use a smart battery charger to charge a smart battery.

Why BMS in electric vehicles?

Li-ion battery, one of common batteries who use in a electric vehicle are expensive and prone to damage. Battery life is important!!

The chances of Li-Ion battery thermal runaway are much higher and each cell must be monitored individually to prevent damage.

Mobile apps would only be effective (battery lasts long) if the battery is working at maximum efficiency. With a good thermal and Powerful management, a battery must be used optimally to get the most out of it.

Battery management system functions

The main function of a BMS is to protect the battery from damage under a wide range of operating conditions.

Operation of a battery outside the safe operating area is prevented

  1. Internal switches to isolate the battery when it tends to operate outside the safe operating zone.
  2. Ask devices to reduce battery power demand
  3. Active environmental control through radiators, coolers and fans

Other functions of BMS are

  • Energy management
  • Cell Balancing
  • Control your environment
  • Etc.

A brief explanation of the functions of a battery management system

A battery management system has the following functions.

It monitors voltage, current, temperature, state of charge (SOC), state of health (SOH), depth of discharge (DOD), state of power (SOP) and liquid flow cooling.

Using the above parameters, a BMS calculates some useful values ​​and helps protect the battery.

Here are some of them.

Maximum charge current, Maximum discharge current, Energy delivered since last charge, Internal impedance of a cell, Charge stored in the battery, Total energy delivered since first use, Total operating time since first use, Total number of rounds.

What is a BMS [Video]

A battery management system communicates with low-level hardware via sensors and with human-machine interfaces (HMIs) via the CAN bus. The instrument cluster of an electric vehicle (any vehicle) shows user information from the battery management system.

The most important function of BMS is listed below

  • Overcurrent protection
  • Over voltage protection
  • Undervoltage protection
  • Overheat protection
  • Under thermal protection
  • Overpressure protection
  • Leakage current detection

A BMS acts as a intermediate device which connects the battery load to avoid excessive load changes to protect the battery.

Cell Balancing improving battery performance is another function of BMS. All cells in a battery are maintained at an equal state of charge, dissipating excess charge in some cells, regulating cell charging, and turning off some shells.

Components of a battery management system (BMS)

A battery management system is an electronic control circuit that monitors the battery. Many BMS integrated circuits are available on the market. For instance. Texas Instruments has the TPS65011 BMS IC

What are the components of a battery management system?

BMS block diagram

current sensor

The total current and the currents in the individual cells of the battery are detected to calculate the total energy stored and used from the battery.

The current draw over time would give you the remaining power in the battery.

Voltage sensor

Monitoring cell voltage and battery voltage are the functions of a battery management system in an electric vehicle. Voltage sensors connected to the battery monitor open circuit voltage and the potential difference when the battery is charged.

Battery health is highly dependent on cell voltage. The voltage range in which the battery is allowed to charge and discharge must be strictly followed to obtain the best performance from the battery. For example, a lithium-ion battery with a nominal voltage of 3.6 V has maximum and minimum nominal voltages of 2.8 V and 4.2 V respectively. [Reference].

A method used to check if the battery is fully charged by measuring the open circuit voltage. Thus, the battery charge can be regulated.

Static switches and controllers

Each cell contained in an electric vehicle battery is individually monitored using a battery management system. The static switch such as FET and the controller together connect and disconnect the cells from the load and the load circuit as needed.

Temperature sensor

A Li-ion battery, which is the common battery technology in a modern electric vehicle, is temperature sensitive. A short circuit results in subsequent heat generation and thermal runaway of the battery.

Thermal management is very important in a battery. Temperature also affects battery efficiency.

Active cooling can be triggered by measuring the temperature of a battery’s cells to protect them from fire.

Battery state estimation algorithm

Two important battery parameters are the State of charge and the Health. SOC indicates the charge level of a battery (%).

There are two types of SOC

  • Absolute State of Charge (ASOC) – when the battery is new
  • Relative state of charge (RSOC) – available charge taking into account capacitance fading

Generally, SOC refers to RSOC.

The state of health indicates the capacity of a battery to function as if it were new. The ratio between the current energy capacity and the capacity of the battery when it was new is defined as SOH.

State of Charge (SOC) calculation uses different algorithms such as

  • Coulomb counting method
  • Open circuit voltage method
  • Impedance measurement method
  • Fuzzy logic of artificial neural networks
  • State space model with Kalman filter

The health estimate includes the number of charge-discharge cycle calculations.

User interface

The user interface shows battery status and other relevant data to the user. All user input would also be sent to the battery management control unit if required from the user interface.

Battery mathematical model

A battery model is needed to verify the operation of a battery management system. The model developed using mathematical equations can be used to analyze a BMS.

You can read: How to develop a battery model for electric vehicle simulation?

real time clock

To timestamp measured data, lifetime estimation, energy estimation, etc. require a time signal. A real time clock in BMS helps here.

Memory

The data collected and processed will be stored for later analysis. Estimated lifespan, estimated state of health, etc. requires the initial data stored in memory.

Factors to consider when designing a BMS

Desirable properties of a battery management system are listed below

  • BMS power consumption should be too minimum as possible
  • Energy consumption at idle should be minimum
  • The BMS must operate using power from the battery it is monitoring

Conclusion

A battery management system for an electric and hybrid electric vehicle is necessary to protect and manage the high voltage battery. The Li-ion battery used in electric vehicles is easily damaged if not properly maintained.

Charge, uncontrolled discharge, high temperature, etc. damage cells. A battery management system helps protect the battery from this damage.

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Battery management system

Key points

  • The BMS is essential in an electric vehicle battery system
  • Protection, energy management, cell balancing, etc. are the functions of a battery management system

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