I. BMS function
First, we'll detail its four main functions.
(1) Perception and measurement Measurement is the perception of the state of the battery
This is the basic function of BMS, including the measurement and calculation of some index parameters, including voltage, current, temperature, power, SOC (state of charge), SOH (state of health), SOP (state of power), SOE (state of 能源). SOC can be commonly understood as how much power is left in the battery, and its value is between 0-100%, which is the most important parameter in BMS; SOH refers to the state of health of the battery (or the degree of battery deterioration), which is the actual capacity of the current battery The ratio of the rated capacity to the rated capacity, when the SOH is lower than 80%, the battery cannot be used in a power environment.
(2) Alarm and protection
When the battery is in an abnormal state, the BMS can send an alarm to the platform to protect the battery and take corresponding measures. At the same time, it will send the abnormal alarm information to the monitoring and management platform and generate alarm information of different levels. For example, when the temperature is overheated, the BMS will directly disconnect the charging and discharging circuit, perform overheating protection, and send an alarm to the background.
Lithium batteries mainly issue alarms for the following problems: overcharge: single overvoltage, total voltage overvoltage, charging overcurrent; overdischarge: single undervoltage, total voltage undervoltage, discharge overcurrent; temperature: cell temperature over High, the ambient temperature is too high, the MOS temperature is too high, the battery temperature is too low, the ambient temperature is too low; status: flooding, collision, inversion, etc.
(3) Balanced management
The necessity of balanced management comes from the inconsistency in the production and use of batteries. From the perspective of production, each battery has its own life cycle and characteristics. There are no two identical batteries. Due to the inconsistency of materials such as separators, cathodes, and anodes, the capacities of different batteries cannot be exactly the same. For example, each battery cell that forms a 48V/20AH battery pack has a certain range of differences in its consistency indicators such as voltage difference and internal resistance. From the point of view of use, in the process of battery charging and discharging, the process of electrochemical reaction can never be consistent. Even if it is the same battery pack, the charge and discharge capacity of the battery will be different due to different temperature and impact, resulting in inconsistent cell capacity. Therefore, the battery needs both passive equalization and active equalization. That is to set a pair of thresholds for starting and ending equalization: for example, in a group of batteries, when the difference between the extreme value of the individual voltage and the average value of the voltage of this group reaches 50mV, the equalization is started, and the equalization is ended at 5mV.
(4) Communication and positioning
BMS has a separate communication module, which is used for data transmission and battery positioning respectively, and can transmit relevant data sensed and measured to the operation management platform in real time.
II. Working principle of BMS protection
BMS includes control IC, MOS switch, fuse Fuse, NTC thermistor, TVS transient voltage suppressor, capacitor and memory, etc. Its specific form is shown in the figure:

In the above figure, the control IC controls the MOS switch to turn on and off the circuit to protect the circuit, and FUSE realizes secondary protection on this basis; TH is temperature detection, and the inside is a 10K NTC; NTC mainly realizes temperature detection; TVS Mainly to suppress the surge.
(1) Primary protection circuit
Control IC The control IC in the figure above is responsible for monitoring the battery voltage and loop current, and controlling the switches of two MOSs. The control IC can be divided into AFE and MCU: AFE (Active Front End, analog front-end chip) is the sampling chip of the battery, which is mainly used to collect the voltage and current of the battery cell. MCU ((Microcontroller Unit, microcontroller chip) mainly calculates and controls the information collected by AFE.
The relationship between the two is shown in the figure:

1. AFE
AFE is generally a 6-pin chip, CO, DO, VDD, VSS, DP and VM, the introduction is as follows:
CO: charge output (charge control);
DO: discharge output (discharge control);
VDD: power supply voltage, also known as output voltage, is the place with the highest voltage;
VSS: reference voltage, which is the place with the lowest voltage;
VM: Monitor the voltage value across the MOS.
When BMS is normal, CO, DO, VDD are high level, VSS, VM are low level, when any parameter of VDD, VSS, VM changes, the level of CO or DO terminal will change.
2. MCU
MCU refers to a micro control unit, also known as a single-chip microcomputer, which has the advantages of high performance, low power consumption, programmable, and high flexibility. It is widely used in consumer electronics, automobiles, industry, communications, computing, home appliances, medical equipment and other fields. In a BMS, the MCU acts as the brain, capturing all the data from the sensors through its peripherals and processing the data to make appropriate decisions based on the profile of the battery pack. The MCU chip processes the information collected by the AFE chip, and plays the role of calculation (such as SOC, SOP, etc.) and control (MOS off, on, etc.), so the battery management system has high requirements on the performance of the MCU chip. AFE and MCU realize the protection to the circuit by controlling MOS.
3.MOS
MOS is the abbreviation of Metal-Oxide-Semiconductor Field-Effect Transistor, referred to as field effect transistor, which acts as a switch in the circuit and controls the on and off of the charging circuit and the discharging circuit respectively. Its on-resistance is very small, so its on-resistance has little effect on the performance of the circuit. Under normal conditions, the consumption current of the protection circuit is μA level, usually less than 7μA.
4. Realization of BMS primary protection: linkage between control IC and MOS
If the lithium battery is overcharged, overdischarged or overcurrented, it will cause chemical side reactions inside the battery, which will seriously affect the performance and service life of the battery, and may generate a large amount of gas, which will rapidly increase the internal pressure of the battery and eventually lead to pressure release. The valve opens and the electrolyte is ejected to cause thermal runaway.
When the above situation occurs, the BMS will activate the protection mechanism and execute as follows:

(1) Normal state
In normal state, both "CO" and "DO" pins in the circuit output high level, both MOSs are in conduction state, and the battery can be charged and discharged freely.
(2) Overcharge protection
When charging, AFE will always monitor the voltage between pin 5 VDD and pin 6 VSS. When this voltage is greater than the overcharge cut-off voltage, MCU will control pin 3 CO (CO pin changes from high level to low level) Ping) to close the MOS tube M2, at this time the charging circuit is cut off, and the battery can only be discharged. At this time, due to the existence of the body diode V2 of the M2 tube, the battery can discharge the external load through this diode.
(3) Over-discharge protection
When discharging, AFE always monitors the voltage between pin 5 VDD and pin 6 VSS. When this voltage is lower than the over-discharge cut-off voltage, MCU will pass pin 1 DO (DO pin changes from high level to low level) Turn off the MOS tube M1, then the discharge circuit is cut off, and the battery can only be charged. At this time, due to the existence of the body diode V1 of the MOS transistor M1, the charger can charge the battery through the diode.
(4) Overcurrent protection
During the normal discharge process of the battery, when the discharge current passes through two MOSs in series, a voltage will be generated at both ends due to the on-resistance of the MOS. The voltage value U=2IR, and R is the on-resistance of a single MOS. AFE pin 2 VM will monitor the voltage value all the time. When the loop current is so large that the voltage U is greater than the overcurrent threshold, the MCU will turn off the MOS transistor M1 through the first pin DO (DO pin changes from high level to low level), and the discharge loop is cut off, so that the current in the loop is zero. , to play the role of overcurrent protection.
(5) Short circuit protection
Similar to the working principle of overcurrent protection, when the loop current is so large that the voltage U instantly reaches the short-circuit threshold, the MCU will turn off the MOS tube M1 through the first pin DO (DO pin changes from high level to low level), and cut off The discharge circuit acts as short circuit protection. The delay time of short-circuit protection is very short, usually less than 7 microseconds.

The above can be briefly described as:

Circuit status	MOS1	MOS2	Charge and discharge status
Normal status	ON	ON	Rechargeable and dischargeable
Over charge protection	ON	OFF	Dischargeable and non-rechargeable
Over discharge protection	OFF	ON	rechargeable non-dischargeable
Over current protection	OFF	ON	When the overcurrent is released, it can be charged and discharged
Short circuit protection	OFF	ON	When the short circuit is released, it can be charged and discharged

(2) Secondary protection circuit: three-terminal fuse Fuse
For security reasons, a secondary protection mechanism still needs to be added. At the current stage, REP (Resistor Embedded Protector, built-in resistance protector) is highly applied, while the three-terminal fuse Fuse is more cost-effective in comparison.
When the current is too large, the Fuse will be blown in the same principle as the ordinary fuse; and when the MOS is in an abnormal operating state, the main control will automatically blow the three-terminal fuse. The main advantages of this security protection mechanism are low power consumption, fast response speed, and good protection effect. At this stage, it has high applicability and has been widely used in electric vehicles, mobile phones and other equipment.

Three-level protection circuit: NTC and TVS1.NTC thermistor Thermistor, which is extremely sensitive to heat, is a kind of variable resistor, mainly divided into PTC and NTC. PTC (Positive Temperature Coefficient, positive temperature coefficient thermistor), the higher the temperature, the greater the resistance, mainly used in mosquito killers, heaters and other products. NTC (Negative Temperature Coefficient, Negative Temperature Coefficient Thermistor) is the opposite of PTC. The higher the temperature, the lower the resistance. It is mainly used as a resistance temperature sensor and a current limiting device.

The BMS of lithium batteries generally uses NTC. In comparison, this product consumes less power, has high accuracy and quick response, and has three main functions.

(1) Temperature measurement
Using the characteristics of this resistor, the following three temperature categories can be measured: Cell temperature: Place the NTC thermistor between the cells to measure the cell temperature, and the number of cells covered by each NTC needs to be considered. Power temperature: Place the NTC thermistor between the MOS to measure the power temperature. It is necessary to ensure that the NTC is in close contact with the MOS device during installation. Ambient temperature: place the NTC thermistor on the BMS board to measure the ambient temperature, and the installation location is required to be far away from the power device.
(2) Temperature compensation
The resistance of most components will increase as the temperature rises. At this time, NTC needs to be used for compensation to offset the error caused by temperature.
(3) Suppress inrush current
Surge (electrical surge), also known as surge, is the momentary peak value beyond the stable value, including surge voltage and surge current. When the electronic circuit is turned on, it will generate a large surge current, which is easy to cause damage to the components. Using NTC can prevent this from happening and ensure the normal operation of the circuit. For surge protection, TVS is needed.
2. TVS transient voltage suppressor
TVS (Transient Voltage Suppressors) are transient voltage suppressors, which respond quickly and are suitable for port protection. The specific implementation is as follows: