Battery Management System

Battery Management System

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Encyclopedia
A battery management system (BMS) is any electronic system that manages a rechargeable battery
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...

 (cell
Electrochemical cell
An electrochemical cell is a device capable of either deriving electrical energy from chemical reactions, or facilitating chemical reactions through the introduction of electrical energy. A common example of an electrochemical cell is a standard 1.5-volt "battery"...

 or battery pack
Battery pack
A battery pack is a set of any number of identical batteries or individual battery cells. They may be configured in a series, parallel or a mixture of both to deliver the desired voltage, capacity, or power density...

), such as by monitoring its state, calculating secondary data, reporting that data, protecting the battery, controlling its environment, and / or balancing
Battery balancing
Battery balancing and battery redistribution refer to techniques that maximize a battery's capacity to make all of its energy available for use and increase the battery's lifetime. A battery balancer or battery regulator is a device in a battery pack that performs battery balancing...

 it.

Monitor


A BMS may monitor the state of the battery as represented by various items, such as:
  • Voltage
    Voltage
    Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

    : total voltage, voltage of periodic taps, or voltages of individual cells
  • Temperature
    Temperature
    Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

    : average temperature, coolant intake temperature, coolant output temperature, or temperatures of individual cells
  • State of charge
    State of charge
    State of charge is the equivalent of a fuel gauge for the battery pack in a battery electric vehicle , hybrid vehicle , or plug-in hybrid electric vehicle...

     (SOC) or depth of discharge
    Depth of discharge
    Depth of discharge is an alternate method to indicate a battery's state of charge . The DOD is the inverse of SOC: as one increases, the other decreases. While the SOC units are percent points , the units for DOD can be Ah or percent points...

     (DOD): to indicate the charge level of the battery
  • State of health
    State Of Health
    State of health is a figure of merit of the condition of a battery , compared to its ideal conditions. The units of SOH are percent points ....

     (SOH), a variously-defined measurement of the overall condition of the battery
  • Coolant flow: for air or fluid cooled batteries
  • Current
    Electric current
    Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...

    : current in or out of the battery

Computation


Additionally, a BMS may calculate values based on the above items, such as:
  • Maximum charge current as a charge current limit (CCL)
  • Maximum discharge current as a discharge current limit (DCL)
  • Energy delivered since last charge or charge cycle
  • Total energy delivered since first use
  • Total operating time since first use

Communication


A BMS may report all the above data to an external device, using communication links such as:
  • Serial communications
    Serial communications
    In telecommunication and computer science, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are sent as a whole, on a link with several parallel channels...

    . A CAN bus is one particular implementation of a serial link most commonly used in automotive environments
  • Direct wiring
  • DC-BUS - Serial communication over power-line
  • Wireless communications

Protection


A BMS may protect its battery by preventing it from operating outside its safe operating area
Safe operating area
For power semiconductor devices , the safe operating area is defined as the voltage and current conditions over which the device can be expected to operate without self-damage....

, such as:
  • Over-current
  • Over-voltage (during charging)
  • Under-voltage (during discharging), especially important for lead–acid and Li-ion cells
  • Over-temperature
  • Under-temperature
  • Over-pressure (NiMH
    NIMH
    NIMH or NiMH may refer to:*Nickel-metal hydride battery, a type of rechargeable battery*National Institute of Mental Health, a part of the United States National Institutes of Health...

     batteries)


The BMS may prevent operation outside the battery's safe operating area by:
  • Including an internal switch
    Switch
    In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....

     (such as a relay
    Relay
    A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal , or where several circuits must be controlled...

     or solid state device) which is opened if the battery is operated outside its safe operating area
  • Requesting the devices to which the battery is connected to reduce or even terminate using the battery.
  • Actively controlling the environment, such as through heaters, fans, air conditioning or liquid cooling

Optimization


In order to maximize the battery's capacity, and to prevent localized under-charging or over-charging, the BMS may actively ensure that all the cells that compose the battery are kept at the same State Of Charge, through balancing. The BMS can balance the cells by:
  • Wasting energy
    Energy
    In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

     from the most charged cells by connecting them to a load (such as through passive regulators)
  • Shuffling energy from the most charged cells to the least charged cells (balancers)
  • Reducing the charging current to a sufficiently low level that will not damage fully charged cells, while less charged cells may continue to charge (does not apply to Lithium chemistry cells)
  • Modular charging

Topologies


BMS technology varies in complexity and performance:
  • Simple passive regulators achieve balancing across batteries or cells by bypassing charging current when the cell's voltage reaches a certain level. The cell voltage is a poor indicator of the cell's SOC (and for certain Lithium chemistries such as LiFePO4 it is no indicator at all), thus, making cell voltages equal using passive regulators does not balance SOC, which is the goal of a BMS. Therefore, such devices, while certainly beneficial, have severe limitations in their effectiveness.
  • Active regulators intelligently turning on and off a load when appropriate, again to achieve balancing. If only the cell voltage is used as a parameter to enable the active regulators, the same constraints noted above for passive regulators apply.
  • A complete BMS also reports the state of the battery to a display, and protects the battery.


BMS topologies fall in 3 categories:
  • Centralized: a single controller is connected to the battery cells through a multitude of wires
  • Distributed: a BMS board is installed at each cell, with just a single communication cable between the battery and a controller
  • Modular: a few controllers, each handing a certain number of cells, with communication between the controllers

Centralized BMSs are most economical, least expandable, and are plagued by a multitude of wires.
Distributed BMSs are the most expensive, simplest to install, and offer the cleanest assembly.
Modular BMSs offer a compromise of the features and problems of the other two topologies.

The requirements for a BMS in mobile applications (such as electric vehicles) and stationary applications (like stand-by UPSs in a server room) are quite different, especially from the space and weight constraint requirements, so the hardware and software implementations must be tailored to the specific use. In the case of electric or hybrid vehicles, the BMS is only a subsystem and cannot work as a standalone device. It must communicate with at least a charger (or charging infrastructure), a load, thermal management and emergency shutdown subsystems. Therefore, in a good vehicle design the BMS is tightly integrated with those subsystems. Some small mobile applications (such as medical equipment carts, motorized wheelchairs, scooters, and fork lifts) often have external charging hardware, however the on-board BMS must still have tight design integration with the external charger.

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