WHAT IS BMS/PCM of Lithium Battery

Battery Management System (BMS) monitors and protects the rechargeable Lithium batteries.  BMS is an essential component of Lithium battery packs as it ensures its efficiency, reliability, and safety. Since they monitor and control the vital parameters of a Lithium battery, they are becoming an important component in various advanced current-day technologies that utilize battery power.

One best example is automobiles where the rapid development of battery system integration into vehicles is gaining traction these days. BMS not only protects your battery when it reaches an alarming state but also detects the potential dangers in the very initial stages that help in the longevity of the battery.

Why do you need BMS?

They help in avoiding erratic or abrupt interruption of power distribution. Such arrangement safeguards the battery pack from severe damages. The major functionalities of BMS for Lithium ion batteries are to protect them from overcurrent, over-discharge, overcharge, thermal runaway, under-voltage, and over-voltage.

The BMS estimates and balances State of Charge (SOC) and State of Health (SOH) ensuring that the battery operates within the Safe Operating Area (SOA).Protection Circuit Module (PMC) and BMS have similar basic functionality.

They help in optimizing the battery life by monitoring the important battery parameters. BMS is useful for protecting rechargeable batteries such as cells and battery packs of NMC, LiFePo4, LTO, and Lithium Metal batteries.

What does Battery Management System (BMS) monitor?

BMS monitors the Voltage i.e., the voltage of individual cells, total voltage, or the voltage of periodic taps. Coming to temperature it checks the coolant intake and output temperatures as well as the temperatures of individual cells and average temperature.

BMS also takes care of the current flowing in or out of the battery. For fluid or air-cooled batteries, it also monitors the coolant flow.

How Does BMS work?

BMS works by sensing the discrepancies in the nominal battery parameters. If it finds any parameter crossing the designated value, it activates the internal circuits. These internal circuits limit the parameters to a point where all the cells stay protected.

The cell level and battery level performance are monitored with the help of processors, controllers, and the sense amplifiers present in a BMS. The controller and sensor, measure and compute the data to check if it is well within the Safe Operating Area (SOA).

Functions of Battery Management System (BMS/PMC)

Let us see the functions of BMS and how it helps in improving the life of rechargeable batteries.

• Monitorscell voltage – BMS identifies the over-current which may vary in charging or discharging modes. Checks for over-voltage during charging and under-voltage when discharging.
• Monitorscelltemperature–BMS helps in measuring the temperature to protect it from thermal runaway ensuring battery safetyas well as indicating ideal conditions for charging or discharging a battery.
• Balances State Of Charge (SOC) –BMS balances the SOC by wasting the energy of most charged cells through load connection, shuffling energy to least charged cells from the most charged cells,and by reducing the charge to a low levelso that the fully charged cells are not damaged.
• Limits power input and output – BMS limits the power input and output for thermal, overcharge, and under discharge protection.
• Estimates SOC and SOH – BMS uses various algorithms to estimate SOC and SOH which help in improving the reliability, durability, and system performance.
• Isolatesthe battery pack – BMS electrically disconnects the high voltage battery detected through systematic monitoring for leakage current and ground fault. It ensures that the conductive object such as a vehicle body stays safe even if the battery pack is faulty.
• Protectsthe battery from load shorts – BMS eliminates the excess inrush currents that cause shorts by using a precharge system that safely connects different loads and the battery.

Architecture of BMS

BMS comprises several functional blocks and is manufactured using various design techniques. Here we look at the overall features of a BMS and the common architecture model.

IC

When the BMS is designed using ICs capable of high voltage processes, it stays protected from transient events but is expensive. However, when designed with low voltage process ICs ensuring that the device never goes above the process rating, it is cheaper.

In this case, the transient event is suppressed from reaching the pin using circuitry made from diodes, resistors, and capacitors.

SAR ADC

The number of cells for scanning and the load behavior are key factors in determining the voltage cell’s acquisition time. When monitoring a cell’s out-of-bounds behaviors or erratic load conditions, fast scan time is important. A SAR ADC helps in quickly procuring these measurements. However, they have less resolution and consume more power.

Thermistors

A temperature sensor monitors each cell or group of cells. Such monitoring is vital for energy storage systems or other portable applications. This job is done by thermistors which are powered by internal ADC voltage. Since the internal voltage is referenced, possibilities of inaccuracies in temperature measurement are reduced despite varying environmental temperatures.

Microcontroller Unit

An MCU processes the data captured by the sensors using various peripherals. Based on the battery pack configuration file and the data processed, an MCU makes appropriate decisions.

Other Building Blocks

Most BMS also contains a real-time clock, battery authentication, daisy chain, and memory. For instance, the black box application uses memory for data storage and a real-time clock for time stamps which allow the user to understand the behavior of the battery pack in case of any mishaps.

The battery authentication block does not allow any third-party battery pack to access the electronics of BMS. Daisy chain circuitry simplifies the stacked device connections as it eliminates the need for level shifting circuitry.

VariousBMS Configurations

Another important thing about BMS is its configuration. When purchasing a BMS for a certain project or any other integration, checking its configuration is mandatory.

Fixed Configuration

Fixed configuration BMS is available for multiple chemistries such as for LiFePo4, NMC, Lithium Metal, or LTO cells. Users may not be able to change any of the BMS parameters in fixed configuration BMS.

Cell voltage levels, sense wire configuration, max input charging voltage, and max load current are preconfigured in fixed configuration BMS.

For example, 7S fixed-configuration BMS can be used only for a 7S battery of respective cell chemistry.  It cannot be used for any other chemistry or battery configuration.

Multiple Interface Configuration

Smart BMS is also available with multiple interface configurations such as UART, Bluetooth, RS485, etc. Smart BMS can be used for any Lithium chemistry by configuring cells parameters inside BMS via Bluetooth, RS485, CAN, or UART interfaces.  There are some limitations of smart BMS too. User hasto go through BMS specifications based on their requirement.

Role of BMS in end-user Safety

BMS ensures end-user safetyby protecting the battery atthe component level, battery level, and integrationlevel.The battery safety challenges are addressed by pre-emption, detection, intervention, and containment.

These processes involve avoiding the thermal runaway, warning of any potential failure, stopping the thermal runaway, and minimizing the damage.

Manufacturers of BMS

Many manufacturers are making BMS for rechargeable Lithium batteries. DALY BMS are popular for waterproof battery applications.JIOBIADA and XIANXING also make a variety of BMS. But BMS from Xinaxing or JBD (JIOBIADA) are not waterproof.

Conclusion

A wide variety of Battery Management Systems are available in the market. When an appropriate BMS is utilized, it enhances the durability, dependability, and safety of the battery pack. Battery life goals and battery requirements are important things to consider when choosing the architecture of BMS.