Interpretation of the core functions and key technologies of battery management system (BMS)
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This post was last edited by Hot Ximixiu on 2024-9-12 09:11
BMS is the Chinese name for battery management system, also known as battery nanny or battery steward. It is mainly used to monitor and manage battery energy storage units and is an indispensable and important part of energy storage batteries. BMS mainly adopts a three-level architecture, including battery master control unit (System), battery master control unit (Master), and battery information monitoring unit (Slave).
1. Popularization of basic knowledge
1. Open circuit voltage: The difference in electrode potential between the two poles of a battery when the battery is disconnected. The open circuit voltage is an actual measured value. For example, the open circuit voltage of a lithium-ion battery is 4.1V, and that of a lead-acid battery is 2.1V. Electromotive force > open circuit voltage. The electromotive force or open circuit voltage of a battery depends on the activity of the electrode materials and electrolytes that make up the battery and the temperature of discharge, and has nothing to do with the geometric shape and size of the battery.
2. Rated voltage: The standard voltage of a battery under specified conditions. Used to distinguish battery systems. For example: lead-acid battery: 2.0V nickel-cadmium battery: 1.2V nickel-hydrogen battery: 1.2V zinc-manganese battery: 1.5V lithium-ion battery: 3.6-3.8V.
3. Discharge termination voltage: refers to the lowest working voltage value when the voltage drops to the point where it is not suitable to continue discharging during discharge. It is a value set artificially. For example, when charging a lithium-ion battery, the termination voltage is 4.2V, and when discharging it is 3.0V or 2.75V.
4. Working voltage: Also known as discharge voltage or load voltage, it refers to the potential difference between the two poles of the battery when the battery outputs current. The working voltage is always lower than the open circuit voltage.
The change of battery discharge voltage is related to the discharge system, that is, the change of discharge curve is also affected by the discharge system, including: discharge current, discharge temperature, discharge termination voltage; intermittent or continuous discharge.
The greater the discharge current, the faster the working voltage drops; as the discharge temperature increases, the discharge curve changes more slowly; for secondary batteries, a discharge voltage lower than the specified termination voltage is called over-discharge, which often affects the cycle life of the battery. Constant resistance discharge Constant current discharge Constant current discharge Continuous discharge Intermittent discharge.
5. Energy and specific energy: The electrical energy that a battery can output when doing external work under certain conditions is called the energy of the battery, and the unit is generally expressed in Wh.
a. Theoretical energy The discharge process of the battery is in a state of equilibrium, the discharge voltage maintains the value of the electromotive force (E), and the utilization rate of the active material is 100%. Under this condition, the output energy of the battery is the theoretical energy (W0), which is the maximum non-expansion work done by the reversible battery at constant temperature and pressure (W0=C0E).
b. Actual energy The energy actually output when the battery is discharged is called actual energy. The energy actually output when the battery is discharged is called actual energy.
c. Specific energy The energy given by a battery per unit mass and unit volume is called mass specific energy or volume specific energy, also known as energy density. The unit of specific energy is wh/kg or wh/L.
6. Battery capacity: refers to the amount of electricity a battery can produce under a certain discharge regime (at certain I discharge, T discharge, V final). It characterizes the battery's ability to store energy, and its unit is Ah or C. The capacity is affected by many factors, such as discharge current, discharge temperature, etc. The capacity is determined by the amount of active substances in the positive and negative electrodes.
Theoretical capacity: the capacity given by all active substances participating in the reaction. Actual capacity: the capacity actually released under a certain discharge system.
Rated capacity: also known as nominal capacity, refers to the minimum amount of power that the battery guarantees to give under the designed discharge conditions. In practical applications, battery capacity = positive electrode capacity Specific capacity: In order to compare different batteries, the concept of specific capacity is introduced.
Specific capacity refers to the capacity given by the battery per unit mass or unit volume, which is called mass specific capacity or volume specific capacity. The usual calculation method is:
Battery first discharge capacity/(active material mass*active material utilization rate)
Factors affecting battery capacity:
a. Battery discharge rate (usually expressed in mA): the greater the current, the smaller the output capacity;
b. Battery discharge temperature: As the temperature decreases, the output capacity decreases;
c. Battery discharge termination voltage: It is set by the limitations of the electrical appliance and the battery reaction itself. For example, when charging, the termination voltage is 4.2V, and when discharging, it is 3.0V or 2.75V.
d. Battery storage time: After the battery has been stored for a long time, the discharge capacity of the battery will decrease accordingly.
7. Power and specific power: The energy output per unit time by the battery under certain discharge conditions is represented by P, and the unit is W. Theoretical power Actual power Actual specific energy Mass specific energy Specific energy Theoretical specific energy Volume specific energy Power is an important performance of the battery, which indicates the size of the battery discharge rate. The greater the power of the battery, the larger the current or high speed the battery can discharge.
8. Battery self-discharge: refers to the phenomenon that the battery capacity decreases automatically when there is no load on the battery. It is mainly due to the spontaneous redox reaction of the electrode material. Among the two electrodes, the self-discharge of the negative electrode is the main one, and the self-discharge consumes the active material in vain. Battery self-discharge is closely related to battery storage performance.
The storage performance of the battery must be good during storage. The self-discharge must be small during storage, and no leakage or alkali creep should occur. Storage methods of various batteries: full charge storage, partial charge storage, and discharge state storage. The relationship curve between self-discharge and temperature: The relationship curve between self-discharge and temperature
9. SOC and DOD: SOC (State of Charge) - is the state of charge, which indicates the percentage of the remaining capacity of the battery to the total capacity. DOD (Depth of Discharge) - is the depth of discharge, which is a measure of the degree of discharge, which is the percentage of the discharge capacity to the total discharge capacity. The depth of discharge has a great relationship with the life of the secondary battery: the deeper the depth of discharge, the shorter its life.
10. Discharge rate and discharge multiple: a method of expressing the discharge current of a battery. Discharge rate: refers to the discharge of the battery's full rated capacity within the specified discharge time. Discharge multiple: refers to the discharge current being a multiple of the battery's rated capacity.
Discharge rate × discharge rate = 11C5 - the capacity of the battery at a 5-hour rate, that is, the total capacity of the battery discharged in 5 hours. Unit: Ah or mAh 0.5C - the battery is discharged at a current of 0.5 times the capacity, unit: A or mA For example: the rated capacity of a battery is 1Ah, and the current when it is discharged at 0.5C is 0.5A
2. BMS (Battery Management System)
The battery management system is the link between the battery and the user, and its main target is the secondary battery.
Secondary batteries have some disadvantages, such as low storage energy, short life, problems with series and parallel use, safety of use, difficulty in estimating battery power, etc. The performance of batteries is very complex, and the characteristics of different types of batteries vary greatly.
The main purpose of the battery management system (BMS) is to improve the utilization rate of the battery, prevent the battery from overcharging and over-discharging, extend the battery life, and monitor the battery status.
2. Mainly covers the following functions:
① Battery working status monitoring: mainly refers to the real-time monitoring or calculation of a series of battery-related parameters such as battery voltage, temperature, working current, battery power, etc. during the battery's working process, and judging the current battery status based on these parameters to perform corresponding operations to prevent the battery from overcharging or over-discharging.
② Battery charge and discharge management: During the battery charging or discharging process, manage the battery charging or discharging according to relevant parameters such as environmental conditions and battery conditions, and set the optimal battery charging or discharging curve (such as charging current, charging upper limit voltage value, discharging lower limit voltage value, etc.)
③ Balancing between single cells: that is, charging single cells evenly so that each cell in the battery pack reaches a balanced and consistent state. The equalizer is the core component of the battery management system, but the technology in this area is not yet mature in China.
Note: Currently, many electric vehicles specifically distinguish between BMS and BBS (BATTERY BALANCE SYSTEM), which can easily lead to a misunderstanding that they are two independent components, but in fact they are subordinate to each other. In addition, the functions of charge and discharge management and equalizer in domestic vehicles are relatively weak. In fact, BMS only calculates the power and implements an over-voltage and under-voltage (group and single cell) protection and communication function.
3. The battery management system mainly includes the following parts:
① Signal acquisition module: mainly used to collect battery pack voltage, charging current, discharging current, single cell voltage, battery temperature and other parameters. Usually isolation processing is adopted. (Except temperature signal)
② Battery protection circuit module: This part is usually implemented by using software to control some external devices. For example, the on and off of the relay is controlled by a signal to allow or prohibit the operation of the charging and discharging equipment or the battery to achieve battery protection.
③ Balancing circuit module: mainly used to collect the voltage of the battery pack cells, and to balance the charging between the cells so that the batteries in the group reach a balanced and consistent state. Currently, there are two main balancing methods: active balancing and passive balancing. (I really can't think of a third one?) It can also be called lossless balancing and lossy balancing.
④ Lower computer module: signal processing, control, and communication.
4. Charging characteristics
Charging characteristic curve
Lithium Iron Discharge Curve
8. The battery life dilemma
9. Comparison of the advantages and disadvantages of each balancing solution
① Passive balancing method
- Advantages: simple circuit structure and low cost.
- Disadvantages: It can only do charge balancing. At the same time, during the charge balancing process, the excess energy is released as heat, making the efficiency of the entire system low, the power consumption high, and the balancing current 50mA.
- BMS applications: electric bicycles and electric motorcycles.
- Advantages: low cost, simple structure, high active energy utilization.
- Disadvantages: The balancing efficiency is limited, and the capacitor is used as a carrier for energy transfer. This solution can realize the direct transfer of energy between any two cells in the battery pack. Since the balancing current is limited by the difference between the capacitor voltage and the cell voltage in the battery pack, the balancing speed will become slower and slower as the balancing process proceeds, and the mass production balancing current is about 300mA.
③ Energy storage inductor solution
- Advantages: Active energy utilization is high, the balancing effect is greater than the capacitor solution, and the mass production balancing current is 5A.
- Disadvantages: can only transfer energy between two adjacent cells; high cost and complex structure. DC/DC unidirectional balancing: limited balancing performance, currently the mass-produced balancing current can reach 1A. DC/DC bidirectional balancing: ideal balancing effect, high cost, complex structure, suitable for large power batteries or energy storage station batteries, currently the mass-produced balancing current can reach 5A.
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