Battery management system explained. BMS thermal runaway protection (TRP)


The electrical power system is one of the only supply networks where the product— electricity—is consumed instantaneously after it is generated. It is mainly because a safe and reliable means to store electrical energy has been missing. The evolving global landscape for electrical distribution and use created a need for energy storage systems (ESSs), making them among the fastest-growing electrical power system products.

The maturity of electrical energy storage technologies can be divided into three categories: deployed, demonstrated, and early-stage technologies. Pumped hydro, compressed air energy storage, battery, and flywheel are examples of the deployed electric energy storage system. The demonstrated energy storage technologies include flow batteries and advanced Pb-acid, superconducting magnetic energy storage, and electrochemical capacitor. The early stage energy storage technologies are adiabatic compressed air energy storage (CAES), hydrogen, and synthetic natural gas. Among all the above-mentioned technologies, batteries and capacitors are susceptible to risks and safety issues [1].

A battery is an electrical energy storage system that can store a considerable amount of energy for a long duration. A battery management system (BMS) is a system control unit that is modeled to confirm the operational safety of the system battery pack [2] [3] [4]. The primary operation of a BMS is to safeguard the battery. Due to safety reasons, cell balancing, and aging issues, supervision of each cell is indispensable. over, BMS ensures the preset corrective measures against any abnormal condition at the system infrastructure. Besides, since the system temperature affects the power consumption profile, BMS also confirms the proper procedure to control the system temperature.

In [5]. authors discussed the battery management systems in electric and hybrid vehicles. The paper addresses concerns and challenges related to current BMSs. State evaluation of a battery, including state of charge, state of health, and state of life, is a critical task for a BMS. By reviewing the latest methodologies for the state evaluation of batteries, the future challenges for BMSs are presented, and possible solutions are proposed. In [6]. authors discussed the battery management system hardware concepts. It focuses on the hardware aspects of battery management systems (BMS) for electric vehicles and stationary applications. In [7]. it presented an enhanced multicell-to-multicell battery equalizer based on bipolar-resonant LC converter. Mathematical analysis and comparison with typical equalizers are provided to illustrate its high balancing speed and good efficiency.

In [8]. it dealt with the susceptibility to electromagnetic interference (EMI) of battery management systems (BMSs) for Li-ion and lithium-polymer (LiPo) battery packs employed in emerging electric and hybrid electric vehicles. A specific test board was developed to experimentally assess the EMI susceptibility of a BMS front-end integrated circuit by direct power injection (DPI) and radiated susceptibility measurements in an anechoic chamber. In [9]. the paper proposed a novel method for accurate hysteresis modeling, which can significantly improve the accuracy of the SOC estimation compared with the conventional methods. The SOC estimation is performed by using an extended Kalman filter (EKF), and the parameters of the battery are estimated by using an auto regressive exogenous (ARX) model and the recursive least square (RLS) filter.

In [10]. it presented the battery management system demonstrator board design using EMC system simulation. The paper explains how EMC system simulation is used to find the root cause and optimize the board design quickly. In [11]. it illustrated a specific test board developed to experimentally assess the EMI susceptibility of a BMS front-end integrated circuit by direct power injection (DPI) and radiated susceptibility measurements. Experimental results are discussed by highlighting different EMI-induced failure mechanisms observed during the tests.

Battery Management System

The definition of BMS varies from application to application. In general, BMS refers to a management scheme that monitors, controls, and optimizes an individual’s performance or multiple battery modules in an energy storage system. BMS can control the disconnection of the module(s) from the system in the event of abnormal conditions. It is used to improve battery performance with proper safety measures within a system. In a power system application, BMS is introduced to monitor, control, and deliver the battery’s power at its maximum efficiency (battery life is also considered here). In automobile applications, BMS is used for energy management in different system interfaces and ensures the system’s safety from various hazards. BMS consists of distinct functional blocks. The functional blocks of BMS are connected to batteries and all other units related to the structured system as controllers, a grid, or other distributed resources, presented in Figure 1. Proper architecture, functional blocks, and advanced circuitry can extend the system battery life. Several commercial BMSs are available in markets. For example, NUVATION Energy provides a flexible, module, reliable, and UL 1973 recognized BMS for mobile and stationary energy storage applications [12].

Figure 1. Battery Management System (BMS) connections and integrations [5].

2.1. Components and Topology

A BMS cannot be used as a standalone within a system infrastructure. It is integrated with other system modules to accomplish the system objectives. For example, an intelligent energy automation system includes a battery management module (BMM), battery interface module (BIM), battery units, and battery supervisory control. The system protects the battery pack, extends the battery lifetime, manages the power demand, and interfaces with the different network [13].

There are three implementation topologies—centralized, distributed, and modular —available in the BMS market. In a centralized topology, a single control unit and battery cells are put together through multiple wires. For distributed topology, each control unit is dedicated to each battery cell by a single communication cable. Lastly, in modular topology, multiple numbers of control units deal with a particular battery cell, but the control units are interconnected [14]. The centralized BMS is the most economical and least expandable. The distributed BMS is the costliest, but it is the easiest among the three to install and offers the cleanest assembly. The modular BMS includes more hardware and programming effort and makes a confrontation between the features and problems of the other two topologies. Figure 2 shows BMS implementation topology.

Figure 2. BMS implementation topology: (a) centralized, (b) distributed.

2.2. Software Architecture

BMS software architecture offers multi-tasking capabilities. Previously, it was not possible to continue different tasks simultaneously; one task was postponed to carry on the other task. Now, in new BMS software architecture, various tasks can be carried out together without any interruption. A BMS software architect’s initial tasks, such as voltage/current measurement, over current/voltage protection, temperature measurement, and protective relay actuation, must be performed promptly to ensure BMS safety. The real-time operating system (RTOS) is introduced in BMS software architecture to perform the real-time functionalities. Figure 3 shows the architecture of BMS software [15].

Figure 3. BMS software architecture.

2.3. Functionalities

BMS deals with battery packs that are connected internally or externally. It calculates the battery quantities, with typical measurements performed for cell voltages, pack current, pack voltage, and pack temperature. BMS uses these measurements to estimate state of charge (SOC), state of health (SOH), depth of discharge (DOD), and the operational key parameters of the cells/battery packs. The measurements also help to increase battery life and keep pace with the demand requirements of the original power network.

BMS is built using functional unit blocks and design techniques. Battery requirements for different applications will help to indicate the appropriate architecture, functional unit blocks, and related electronic circuitry to design a BMS and BMS charging scheme. Battery life can be optimized based on the following features [16] :

Introduction: Battery Management System: Introduction to Hardware

About: Student of Electrical Engineering at Pune University. Having more interest in Electric vehicle and Battery Management System. About Aakash Borse »

Battery Management Systems (BMS) are the key to the safe, reliable and efficient functioning of the lithium-ion batteries. It is an electronic supervisory system that manages the battery pack by measuring and monitoring the cell parameters, estimating the state of the cells and protecting the cells by operating them in the Safe Operating Area (SOA).Battery management systems are an essential component of all lithium-ion battery packs. These battery packs can be classified into Low Voltage (LV) or High Voltage (HV). In automotive engineering, “high voltage” is defined to be within a range of 30 – 1000 VAC or 60 – 1500 VDC (UNECE 2013). Voltages under 30 VAC and 60 VDC are defined as “low voltage.” LV 112-1 presents three voltage classes, which are based on ISO 6469-3 class A and B: Low voltage class 1: ≤ 30 VAC and ≤ 60 VDC; High voltage class 2: ≤ 600 VAC and ≤ 900 VDC; High voltage class 3: ≤ 1000 VAC and ≤ 1500 VDC. LV battery packs are typically used in light electric and hybrid vehicles, two and three wheelers. HV battery packs are typically used in traction applications for electric automotive and stationary applications in Energy Storage Systems (ESS). HV battery packs have a large number of lithium ion cells connected in series and parallel to build up the total voltage and capacity of the pack. For example, a HV battery pack of a hybrid bus rated for 600V, 100kWh built of 18650 NMC cells will have about 160 cells in series and 55 cells in parallel, taking the total cell count to 8800. Irrespective of the voltage, all lithium ion battery packs require a BMS. Several off-the-shelf BMSs are available in the market and they are of different topologies. Selection of the right type for the battery pack is important. These BMSs can be fundamentally classified into – Centralized and Decentralized.

Step 1: Architecture: Centralized Battery Management Systems

Centralized BMS is one central pack controller that monitors, balances, and controls all the cells. The entire unit is housed in a single assembly, from which the wire harness (N 1 wires for N cells in series and temperature sense wires ) goes to the cells of the battery. These wires are used for cell voltage, temperature measurements and balancing.

The board is commonly powered from the battery output and does not require an external power supply. It consists of multiple Analog to Digital Converters (ADC) channels as part of the cell monitoring circuitry. The voltage on each cell is referenced to the BMS ground and this voltage grows with the number of cells and provides high voltage at the ADC channels that are measuring the top most cells in the stack. The cell monitoring circuitry is also coupled with an intelligence circuitry. The intelligence circuitry is responsible for internal communication with the cell monitoring circuitry for data acquisition, computing the battery’s State of Charge (SoC) and State of Health (SoH), controlling the Power Distribution Unit (PDU) and for external communication.

Step 2: Architecture: Decentralized Battery Management Systems

A decentralized BMS, fundamentally, does not have the entire cell monitoring and intelligence circuitry on a single assembly. This architecture can be implemented through various topologies as explained below:

Modular: The battery management system is divided into multiple, identical modules, each with its bundle of wires going to one of the batteries in the pack. Typically, one of the modules is designated as a master, as it is the one that manages the entire pack and communicates with the rest of the system, while the other modules act as simple remote measuring devices. Readings from the other modules to the master module are transferred via a communication link.

Master-Slave: This architecture comprises the Master and Slave BMS units. The slave unit monitors, balances and controls a group of battery cells within the battery module. It communicates with the master unit through a communication interface. The Master unit is responsible for state estimation, control of Power Distribution Unit (PDU) and external communication. A master-slave BMS is similar to a modular system, in the sense that it uses multiple identical modules (the slaves), each measuring the voltage of a few cells. However, the master is different from the modules and does not measure voltages. It only handles computation and communications.

Distributed: A distributed BMS is significantly different from the other topologies. While the electronics are grouped and housed separately from the cells in other topologies, a distributed BMS has the electronics contained on cell boards that are placed directly on the cells being measured.

Instead of many tap wires between cells and electronics, a distributed BMS uses just a few communication wires between the cell boards and a BMS controller, which handles computation and communications.

Importance of Decentralized BMS Architecture for HV Battery Systems:

HV battery systems consist of a large number of cells. This implies that there are also a large number of wires originating from these cells to the BMS. This makes the assembly, management, and maintenance of these HV battery packs more complex. Decentralized BMS architecture offers the following advantages in this context:

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Measurement Precision – The quality of measurement of fundamental parameters such as voltage, current and temperature is critical to the functioning of the BMS. These measurements are made over the cell monitoring wires and are prone to perturbations from disturbances such as noise. The short wires have high electromagnetic compatibility which decreases the noise susceptance.

Connection Reliability – It enables the cell monitoring circuitry to be placed in close proximity to the cells. As a result, the wires from the cells to the BMS are of short length. Short wires are less prone to be affected by mechanical shocks and vibrations that can cause a disconnection. Hence, having short wires ensures better connection reliability.

Expansion Versatility – Typically, the number of cell monitoring inputs in centralized battery management systems is fixed. But decentralized battery management systems allow for multiple cell monitoring units to be stacked. Therefore a decentralized BMS is more versatile in the sense that it can be used even if the number of cells in the pack is increased or decreased, just by changing the number of cell monitoring units.

The definition from Bureau Veritas:

Battery Management System (BMS): Electronic system associated with a battery pack which monitors and/or manages in a safe manner its electric and thermal state by controlling its environment, and which provides communication between the battery system and other macro-system controllers (e.g.: Battery Support System (BSS), Energy Management System (EMS) and Vehicle Management System (VMS).

For “off-grid” applications we can replace “VMS” with all the charge and load equipment of the installation

Based on that document, a BMS should be able to:

  • monitor and manage electric and thermal state
  • estimate the potential need for battery pack (dis)connection
  • provide an environment where the battery, the installation and the people are safe
  • protect against equipment faults, human errors, environmental incidents…
  • communicate with external systems and operators
  • optimise battery life-time and energy availability
  • diagnostic – record battery life history log

A BMS needs to protect the battery itself against the failure modes described in the article “What will kill your battery“. It needs also to implement a multi-level protection as explained in the article “Think multi-level protection

Basic functionalities of a BMS

Cell voltage and temperature measure

  • voltage and temperature are the main factors that lead to battery failure
  • a small difference in voltage or temperature between cells may be an indicator of a serious problem
  • the cell voltage being quite flat in the range of utilisation, it is important to have an accuracy of 1mV
  • all cells voltage must be measured with the same reference (which excludes BMS with a separate measuring board for each cell)

Warning in case of abnormal conditions (level 3 protection)

You should be made aware of any situation that is outside of optimal conditions for the battery

  • difference in voltage or temperature across cells
  • voltage above the bulk / absorption settings of the chargers
  • voltage below your standard discharge cycle
  • pack and cell temperature outside of optimal range

A warning must be visible and audible from the main living area and they must give an indication on the nature of the fault.A warning can be reset automatically if conditions return to normal (with hysteresis)

Communication with external equipment (level 4 protection)

At a minimum the BMS should be able to communicate 3 types or command to (1) disable charging, (2) disconnect loads and (3) isolate the battery. In addition it should be able to command external systems like generators, heaters, coolers…

Each command may have multiple forms:

  • activate a switching devices (relay or circuit-breaker)
  • directly control certain equipment like battery chargers, converters, regulators…

It is important to note that one command like “disable charging” may need to have different forms depending on the type of chargers you use (cut alternator field, directly control a charger or regulator, activate a relay…)

battery, management, system, explained, thermal, protection

Cells balancing

This feature, often overlooked for an off-grid energy storage installation, is one of the most important to optimise battery life-time and energy availability.

It is true that in off-grid storage conditions the cells will not rapidly get out of balance. But it is inevitable that it will happen over time over time. If nothing is done about it, the battery capacity will be reduced and some of the cells will age faster… which in turn accelerates the unbalance…

Alarm in case of extreme conditions (level 5 protection)

Extreme conditions are situations where the battery starts ageing more rapidly or there is a risk for external equipment or people.

Must be audible and visible from the main living area and give an indication on the nature of the fault

Must be persistant – meaning that an operator intervention is necessary to reset them

Activation of safeguard procedures (level 6 protection)

Procedures that are triggered by an alarm (with an eventual time delay):

These procedures can only be deactivated manually after the fault has been repaired

Internal and external self-test

This is critical to ensure that the whole protection system is operational. It includes checking:

  • BMS internal components operation (measuring components, measure references, outputs…)
  • proper settings of warnings and alarms
  • connection and communication between system components
  • external equipment operation

Without these basic functionalities your BMS may be useless the day you need it!

TAO BMS has all these functionalities, andTAO Monitor takes it a few steps further…

BMS advanced functionalities

These functionalities put you in control of your installation and give you a true Energy Management System (EMS)

Personalised events

In addition to warnings and alarms you can define situations that you want to be made aware of, and can optionally trigger some actions:

  • based on voltage, temperature, SOC or current conditions
  • can be set to command BMS outputs and / or send CAN messages to external equipment
  • turn on the water-heater or water-maker when SOC is above a certain level
  • disconnect non essential equipment when SOC or voltage is below a certain level
  • activate a led when discharge current is over a set level
  • activate a fan or Pelletier cooling system when temperature is above a set level

Lifetime history log

Keep records for the lifetime of the system:

  • measures, including cell voltage and temperature, current, SOC, cell internal resistance…
  • warnings and alarms when they are activated and de-activated
  • all personalised events
  • standard events like BMS power-up, SOC reset, and many more…

In addition to fault analysis and diagnostic capabilities it enables for on site or remote support, this could save you a lot of money if your battery fails prematurely and you need to demonstrate your battery has been used within the manufacturer’s specifications

Monitor screen

  • View battery and cell status (voltage, temperature, current, SOC, cell internal resistance, cycle count, SOH)
  • Edit BMS parameters
  • Review events log
  • Statistical data and graphs

Remote access

  • Access all BMS functions remotely with a web browser (phone, tablet, computer)
  • Run diagnostics and simulation
  • connect to a Wi-Fi network (or create an access point)
  • Access historical data and events on the Cloud

This enables you to monitor your installation when you are not on site. It also give the ability for a technicien to diagnose some system dysfunctions and faults.

BMS cell balancing protection

When using a lithium-ion battery, it is important to make sure that the cells are balanced. This means that all of the cells in the battery pack have approximately the same voltage. If one or more cells have a higher voltage than the others, it can cause damage to the battery.

BMS cell balancing protection is the process of ensuring that all cells in a battery pack are at or near the same state of charge. This is important to maintain healthy cells and to extend battery lifespan

Cell balancing protection is usually done by the BMS when it senses that one or more cells have reached a higher state of charge than others. The BMS will then send a charge or discharge current to the affected cells until they reach the same level of charge as the other cells in the pack.

Lithium batteries used in electric vehicles come with built-in cell balancers, which take care of this task without needing input from the BMS. However, most lithium batteries do not have such built-in cell balancing capabilities and will require the BMS to perform this function.

If the BMS is not able to properly balance the cells in a battery pack, it can cause cell damage and even failure. It’s therefore important to ensure that your battery pack’s BMS has adequate BMS cell balancing protection capabilities. Make sure your BMS is enabled and perform this function properly to get the most out of your battery pack.

BMS overcurrent protection (OCP)

The over-current protection function is a key safety feature of the BMS. The OCP will cut off the current if it exceeds the programmed limit, which helps protect the battery and its surrounding components from damage. Most BMSs have an adjustable BMS overcurrent protection threshold, so you can configure it to meet your application’s needs.

When setting the OCP threshold, be sure to consider both the maximum current draw of your load and also the short-circuit current of your battery. The BMS overcurrent protection should be set slightly below these values to allow for some headroom.

If the OCP is set too close to the max discharge current, it could trigger unnecessarily and cause premature shutoff of the battery.

It’s also worth noting that some BMS has a “hysteresis” feature, which prevents false triggering of the BMS overcurrent protection due to small fluctuations in current. This can be particularly helpful in applications where there are large current spikes, such as when starting an electric motor.

The short-circuit current is the amount of current that flows through a battery when it is shorted out. For example, if you have a 12V battery with a 0.05-ohm resistor, its short-circuit current would be 240 amps (12 x 0.05 = 0.60A).

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So, when selecting a BMS overcurrent protection threshold for your application, make sure it’s greater than the maximum current draw of your load multiplied by the safety factor (usually around two or three). This will help ensure that the OCP will always cut off before any damage can occur.

In addition to setting an appropriate BMS overcurrent protection threshold, you should also monitor the battery temperature to avoid any false positives caused by the battery heating up. A false positive can occur if the OCP threshold is set too low, and will cause the battery to cut off unnecessarily.

So be sure to take into account both the current draw of your load and the thermal characteristics of your battery when selecting a BMS overcurrent protection threshold.

BMS overcharge protection

BMS overcharge protection is a common battery management system (BMS) protection setting for lithium batteries. If the voltage of a lithium battery exceeds the maximum safe level, overcharge protection will activate and stop current from flowing into or out of the battery. This prevents further damage to the battery and helps ensure safety.

Most BMS overcharge protection settings are adjustable, so you can set the voltage at which protection will activate. It’s important to consult your battery’s datasheet or manufacturer to determine the recommended overcharge voltage for your specific battery.

Overcharging can cause permanent damage to a lithium battery, so it’s important to be aware of your BMS protection settings and to ensure that your battery isn’t being overcharged.

How to choose a high-quality battery with intelligent BMS?

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Thank you for reading our blog post on the BMS protection settings of the lithium battery.

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