How does battery management system work?

Electronic system that manages a rechargeable battery (cell or battery pack)

A battery management system (BMS) is any electronic system that manages a rechargeable battery (cell or battery pack), such as by protecting the battery from operating outside its safe operating area[clarification needed], monitoring its state, calculating secondary data, reporting that data, controlling its environment, authenticating it and / or balancing it.[1] Protection circuit module (PCM) is a simpler alternative to BMS.[2] A battery pack built together with a battery management system with an external communication data bus is a smart battery pack. A smart battery pack must be charged by a smart battery charger.[citation needed]

Functions

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Safety circuit for four-cell LiFePO4 batteries with a balancer

Monitor

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A BMS may monitor the state of the battery as represented by various items, such as:

• Voltage: total voltage, voltages of individual cells, or voltage of periodic taps
• Temperature: average temperature, coolant intake temperature, coolant output temperature, or temperatures of individual cells
• Coolant flow: for liquid cooled batteries
• Current: current in or out of the battery
• Health of individual cells
• State of balance of cells

Electric vehicle systems: energy recovery

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• The BMS will also control the recharging of the battery by redirecting the recovered energy (i.e., from regenerative braking) back into the battery pack (typically composed of a number of battery modules, each composed of a number of cells).

Battery thermal management systems can be either passive or active, and the cooling medium can either be air, liquid, or some form of phase change. Air cooling is advantageous in its simplicity. Such systems can be passive, relying only on the convection of the surrounding air, or active, using fans for airflow. Commercially, the Honda Insight and Toyota Prius both use active air cooling of their battery systems.[3] The major disadvantage of air cooling is its inefficiency. Large amounts of power must be used to operate the cooling mechanism, far more than active liquid cooling.[4] The additional components of the cooling mechanism also add weight to the BMS, reducing the efficiency of batteries used for transportation.

Liquid cooling has a higher natural cooling potential than air cooling as liquid coolants tend to have higher thermal conductivities than air. The batteries can either be directly submerged in the coolant or coolant can flow through the BMS without directly contacting the battery. Indirect cooling has the potential to create large thermal gradients across the BMS due to the increased length of the cooling channels. This can be reduced by pumping the coolant faster through the system, creating a tradeoff between pumping speed and thermal consistency.[4]

Computation

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Additionally, a BMS may calculate values based on the items listed below, such as:[citation needed]

• Voltage: minimum and maximum cell voltage
• State of charge (SoC) or depth of discharge (DoD), to indicate the charge level of the battery
• State of health (SoH), a variously defined measurement of the remaining capacity of the battery as a fraction of the original capacity
• State of power (SoP), the amount of power available for a defined time interval given the current power usage, temperature and other conditions
• State of safety (SOS)
• 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
• Internal impedance of a cell (to determine open circuit voltage)
• Charge delivered or stored (sometimes this feature is called coulomb counting)
• Total operating time since first use
• Total number of cycles
• Temperature monitoring
• Coolant flow for air or liquid cooled batteries

Communication

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The central controller of a BMS communicates internally with its hardware operating at a cell level, or externally with high level hardware such as laptops or an HMI.[clarification needed]

High level external communication are simple and use several methods:[citation needed]

Low-voltage centralized BMSes mostly do not have any internal communications.

Distributed or modular BMSes must use some low-level internal cell–controller (modular architecture) or controller–controller (distributed architecture) communication. These types of communications are difficult, especially for high-voltage systems. The problem is voltage shift between cells. The first cell ground signal may be hundreds of volts higher than the other cell ground signal. Apart from software protocols, there are two known ways of hardware communication for voltage shifting systems, optical-isolator and wireless communication. Another restriction for internal communications is the maximum number of cells. For modular architecture most hardware is limited to maximum 255 nodes. For high-voltage systems the seeking time of all cells is another restriction, limiting minimum bus speeds and losing some hardware options. Cost of modular systems is important, because it may be comparable to the cell price.[6] Combination of hardware and software restrictions results in a few options for internal communication:

• Isolated serial communications
• Wireless serial communications

To bypass power limitations of existing USB cables due to heat from electric current, communication protocols implemented in mobile phone chargers for negotiating an elevated voltage have been developed, the most widely used of which are Qualcomm Quick Charge and MediaTek Pump Express. "VOOC" by Oppo (also branded as "Dash Charge" with "OnePlus") increases the current instead of voltage with the aim to reduce heat produced in the device from internally converting an elevated voltage down to the battery's terminal charging voltage, which however makes it incompatible with existing USB cables and relies on special high-current USB cables with accordingly thicker copper wires. More recently, the USB Power Delivery standard aims for a universal negotiation protocol across devices of up to 240 watts.[7]

Protection

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BMS main controller

A BMS may protect its battery by preventing it from operating outside its safe operating area, such as:[citation needed]

• Over-charging
• Over-discharging
• Over-current during charging
• Over-current during discharging
• Over-voltage during charging, especially important for lead–acid, Li-ion and LiFePO4 cells
• Under-voltage during discharging, especially important for Li-ion and LiFePO4 cells
• Over-temperature
• Under-temperature
• Over-pressure (NiMH batteries)
• Ground fault or leakage current detection (system monitoring that the high voltage battery is electrically disconnected from any conductive object touchable to use like vehicle body)

The BMS may prevent operation outside the battery's safe operating area by:

• Including an internal switch (such as a relay or mosfet) which is opened if the battery is operated outside its safe operating area
• Asking the devices to reduce or even stop using or charging the battery.
• Actively controlling the environment, such as through heaters, fans, air conditioning or liquid cooling
• Reduce processor speed to reduce heats.

Battery connection to load circuit

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A BMS may also feature a precharge system allowing a safe way to connect the battery to different loads and eliminating the excessive inrush currents to load capacitors.

The connection to loads is normally controlled through electromagnetic relays called contactors. The precharge circuit can be either power resistors connected in series with the loads until the capacitors are charged. Alternatively, a switched mode power supply connected in parallel to loads can be used to charge the voltage of the load circuit up to a level close enough to battery voltage in order to allow closing the contactors between battery and load circuit. A BMS may have a circuit that can check whether a relay is already closed before precharging (due to welding for example) to prevent inrush currents to occur.

Balancing

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Distributed battery management system

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 voltage or State of Charge, through balancing. The BMS can balance the cells by:

• Wasting energy 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)

Topologies

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Cable data transfer module BMS wireless communication

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

  LiFePO

  
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  , 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 three 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 handling 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 BMSes 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 UPSes 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 stand-alone 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.

Various battery balancing methods are in use, some of them based on state of charge theory.

See also

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References

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Lithium-ion batteries have a lot of advantages over their lead-acid counterparts. They’re lighter, more efficient, charge faster, and have a longer lifespan. However, they’re susceptible to conditions that can damage the battery pack. Tapping into all of this potential requires lithium-ion batteries to be more complex and include components to help avoid these damaging conditions. In fact, this is the primary purpose of the BMS, which means a battery management system.

What is a Battery Management System? 

A battery management system (BMS) is said to be the brain of a battery pack. The BMS is a set of electronics that monitors and manages all of the battery’s performance. Most importantly, it keeps the battery from operating outside of its safety margins.

The battery management system is critical to the battery’s safe operation, overall performance, and longevity. Moreover, it protects whatever the lithium battery is installed in (boat, RV, etc.) and the people who are using it.

What is the Function of a Battery Management System? 

The primary function of the BMS is to protect the battery cells from damage caused by being overcharged or over-discharged. Additionally, the BMS calculates the remaining charge, monitors the battery’s temperature, monitors the battery’s health and safety by checking for loose connections and internal shorts. The BMS also balances the charge across the cells to keep each cell functioning at maximum capacity.

If it detects any unsafe conditions, the BMS shuts the battery down to protect the lithium-ion cells and the user.

How Does a Battery Management System Work? 

The battery management system monitors individual cells in the battery pack. It then calculates how much current can safely go in (charge) and come out (discharge) without damaging the battery.

The current limits prevent the source (usually a battery charger) and the load (such as an inverter) from overdrawing or overcharging the battery. This protects the battery pack from cell voltages getting too high or low, which helps increase the battery’s longevity.

The BMS also monitors the remaining charge in the battery. It continually tracks the amount of energy entering and exiting the battery pack and monitors cell voltages. It uses this data to know when the battery is drained and shut the battery down. This is why lithium-ion batteries don’t show signs of dying like a lead-acid, but just shut off.   

Why a BMS is Important

Battery management systems are critical in protecting the battery’s health and longevity but even more important from a safety perspective. The liquid electrolyte in lithium-ion batteries is highly flammable.

So, these batteries need to be operating optimally and within safety limits at all times to prevent a fire.   

Battle Born Built-In Battery Management System

All Battle Born Batteries have a built-in BMS. This protects against all of the most common causes of battery failures and dangers.

These include protecting the cells against short circuits, high currents, excessive heat, cold, and high or low voltages. Battle Born’s built-in BMS also protects against faults.  

Learn All About Battle Born’s Battery Management System here.

Protections Offered By a Battery Management System

Let’s review the protections of a battery management system:

Under and Over-Voltage

Damage occurs if you overcharge (cell voltage getting too high) or over-discharge (cell voltage gets too low) a lithium-ion battery cell. The BMS helps protect from under and over-voltage situations so that damage to the battery’s cells does not occur.

Temperature Extremes

The safety and stability of lithium-ion battery cells depend on temperature maintenance within certain limits. If the temperature exceeds the critical level on either end, thermal runaway can occur. Consequently, this can lead to an inextinguishable fire.

The BMS monitors the temperature and sometimes controls cooling fans (in the case of an electric vehicle) to help maintain proper conditions. It will even shut down cells if needed to protect the battery.

Protection from Shorts

Internal and external shorts can also lead to thermal runaway. For this reason, protection from shorts is another critical component of a battery management system.

Keep Your Batteries and Your Family Safe With a BMS

There are many benefits to lithium-ion battery technology. But lithium-ion battery cells and conditions must be monitored, managed, and balanced to ensure safety and optimal longevity and efficiency.

The battery management system is the primary component in the battery pack that monitors all of these conditions. Above all, it keeps your batteries operating safely and optimally so you can get out there and stay out there with peace of mind.

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