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Introduction to BMS

BMS is the abbreviation of Battery Management System. The function of BMS is mainly to ensure the safety of batteries and extend the service life of batteries. All lithium batteries must be equipped with a BMS system before they can be used.

BMS composition

The main components of BMS for electric vehicle, robot, energy storage system and other applications are as follows:

Basic functions of BMS

The basic function of lithium battery is mainly five functions: measurement, protection, control, charge SOC and communication.

1.Measurement function

Test battery voltage, temperature, and charge and discharge current to provide relevant basic data for later protection, control, SOC, etc. The accuracy and precision of its measurement directly determine the quality of the whole system.

  2.Protection function

A sound BMS generally has six basic protection functions, which are as follows:

  a. Overvoltage protection (OV), also known as overcharge protection

When charging, any string of battery voltage of the battery pack appears overcharge, the battery system will occur overcharge protection, close the charge, to ensure the safety of the battery.

 b. Under voltage protection (UV), also known as over discharge protection

When discharging, any string of battery voltage of the battery pack is over discharge, the battery system will have over discharge protection, close the discharge, and extend the battery life.

   c. Overcurrent protection (OC)

When charging or discharging, if the current exceeds the set value of the system, the battery system will shut down the charging or discharging to avoid damaging the battery.

   d.Short Circuit Protection (SC)

When charging or discharging, the positive and negative electrodes of the battery short-circuit, the battery current is very large, the battery system will close the charging or discharging, so as not to damage the battery.

   e.High Temperature Protection (OT)

When the battery temperature exceeds the maximum operating temperature range, the system automatically stops charging or discharging to ensure battery safety.

   f.Low Temperature Protection (UT)

When the battery temperature exceeds the minimum operating temperature range, the system automatically shuts down the charging or discharging to avoid battery damage.

  3.Control function

Control function mainly refers to charge and discharge control, power balance function.

Charge/discharge control is to turn off the control of charge or discharge switch once protection occurs, and the circuit is to turn on or off the control function of the power device MOS. (Shenzhen IYPower Co., LTD /www.iypower.com)

Power balance is divided into passive balance and active balance, the mature technology is generally passive balance, through the resistance discharge to adjust different series of voltage function. In popular terms, the high voltage is released a little, and the low voltage is added. The general strategy of equalization is charge equalization.

The consistency of the battery is mainly achieved by the consistency of the battery cell, the balance of the current is very small, generally about 10~50mA, just adjust the difference of the self-discharge of the battery, to the consistency of the battery is very poor balance to the consistent is difficult to achieve.

  4.Power SOC

Power SOC generally refers to the percentage of remaining power, SOC reminds the user of power endurance. There are many strategies for SOC. The more mature technologies are ampere integration method, ampere integration plus voltage correction method, Kalman filter method and so on. (Shenzhen IYPower Co., LTD /www.iypower.com)

The SOC algorithm generally needs to be adjusted according to the characteristics of the battery itself, because the charging and discharge curves of different batteries are different to some extent, in addition, different temperatures, different discharge rates are also different, as well as differences in internal resistance.

  5.Communication function

Communication is to transfer the collected battery information to the host through the communication interface. The main communication interfaces are wired interfaces, which are UART/RS232/RS485/CAN, etc. Among them, RS485 and CAN are the most widely used because they adopt differential transmission and have long transmission distance and high reliability.

With the popularity of mobile Internet, wireless interfaces are becoming more and more common. For example, Bluetooth, GPRS/4G/5G are also increasingly applied to lithium battery systems.

The above are only the most common functions of conventional BMS, and there are certain differences in the actual application of different system requirements of BMS. For example, the functional requirements of BMS of electric vehicles will be very different.

(Shenzhen IYPower Co., LTD /www.iypower.com)