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Introduction to Cell Balancing System – Semco university

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Popularization of Basic Knowledge | Introduction to Cell Balancing System.

Open Circuit Voltage: The difference between the electrode potentials of the two poles of the 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 is4.1V, the lead-acid battery is2.1V Electromotive force > open circuit voltage the electromotive force or open circuit voltage value of a battery depends on the electrode material and the activity of the electrolyte and the discharge temperature of the battery, and has nothing to do with the geometry and size of the battery.

Rated Voltage: The standard voltage at which the battery operates under specified conditions. Used to distinguish battery systems.3. Rated voltage: The standard voltage at which the battery operates under specified conditions. Used to distinguish battery systems. Such as: lead-acid battery:2.0VNiCd batteries:1.2VNiMH battery:1.2VZinc-manganese battery:1.5VLithium Ion Battery:3.6-3.8V.

Discharge Termination Voltage: Refers to the discharge, the voltage drops to the minimum working voltage value that should not continue to discharge. is an artificial value. For example: when a lithium-ion battery is charged, the termination voltage is4.2V, when discharging is3.0Vor2.75V.

Operating 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 to the outside. The operating voltage is always lower than the open circuit voltage. The change of the battery discharge voltage is related to the discharge system, that is, the change of the discharge curve is also affected by the discharge system, including: discharge current, discharge temperature, discharge termination voltage; intermittent or continuous discharge. The larger the discharge current is, the faster the working voltage drops; with the increase of the discharge temperature, the discharge curve changes more smoothly; for the secondary battery, the discharge voltage lower than the specified termination voltage is called over-discharge, and the over-discharge often affects the cycle life of the battery. Constant resistance discharge, constant current discharge, constant current discharge, continuous discharge and intermittent discharge.

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Energy and Specific Energy: The electric energy that the battery can output under certain conditions is called the energy of the battery, and the unit is generally used Express.

a. The discharge process of the theoretical energy battery is in a state of equilibrium, and the discharge voltage maintains the electromotive force (E) value, and the active substance utilization rate is 100%, under this condition, the output energy of the battery is the theoretical energy (W0), the maximum non-expansion work done by the reversible battery at constant temperature and pressure (W0=C0E).

b. The actual energy output when the actual energy battery is discharged is called actual energy. The energy actually output when the battery is discharged is called actual energy. 

W = V Work TV

Work = V open circuit – IRi

Specific Energy: The energy given by the 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/kgorwh/L.

Battery Capacity: Refers to a certain discharge system (under a certain Tput, Vend), the power given by the battery. Characterizes the ability of a battery to store energy in units ofAhorC. The capacity is affected by many factors, such as: discharge current, discharge temperature, etc. The capacity is determined by the number of active materials 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 regime. Rated capacity: Also known as nominal capacity, it refers to the minimum power guaranteed by the battery 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 unit mass or unit volume of battery, which is called mass specific capacity or volume specific capacity.

Usually the calculation method is:

Battery first discharge capacity/ (amount of active substance*active substance utilization) Factors affecting battery capacity:

a. Battery discharge rate (Usually the amperage Mato represent): The larger the current, the smaller the output capacity;
b. The discharge temperature of the battery: the temperature decreases, the output capacity decreases;
c. The discharge termination voltage of the battery: it is set by the limitation of the electrical appliance and the battery reaction itself, for example: when charging, the termination voltage is4.2V, when discharging is3.0Vor2.75V.
d. Storage time of the battery: After the battery is stored for a long time, the discharge capacity of the battery will decrease accordingly.

Power and Specific Power: The energy output per unit time by the battery under certain discharge conditions, with means that the unit is W. Theoretical power, actual power, actual power, actual power, mass ratio, energy ratio, theoretical specific energy volume, and energy ratio, are an important performance of the battery. It indicates the size of the battery discharge rate.

Self-Discharge of the Battery: It refers to the phenomenon that the battery capacity decreases by itself when there is no load on the battery. The main reason is that the redox reaction occurs spontaneously in the electrode material; in the two electrodes, the self-discharge of the negative electrode is the main one, and the self-discharge causes the active material to be consumed in vain. Battery self-discharge is closely related to battery storage performance. The storage performance of the battery must be good during storage, and the self-discharge should be small during storage, and there should be no leakage or alkali creeping. Various battery storage methods: full charge storage, partial charge storage, and discharge state storage. Self-discharge versus temperature Self-discharge versus temperature

SOC and DOD: SOC (State of Charge) – Is the state of charge, indicating the percentage of the remaining capacity of the battery to the total capacity (Depth of Discharge) – is the depth of discharge, a measure of the degree of discharge, which is the percentage of the discharge capacity and discharge rate. It is a way to express the magnitude of the battery discharge current. Discharge rate refers to the battery discharges its full rated capacity within the specified discharge time. Discharge rate: It means that the discharge current is a certain multiple of the rated capacity of the battery. discharge time rate Discharge rate =11C5—Battery5hourly rate capacity, i.e., battery5full 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 the life.

Discharge Time Rate capacity in hours of discharge. unitAhormAh0.5C—battery with0.5Double capacity current discharge, unit: Aorm A. For example: the rated capacity of a battery is1Ah,use0.5CThe current during discharge is 0.5A

BMS: The battery management system is the link between the battery and the user. The main object is the secondary battery.

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Secondary batteries have the following disadvantages, such as less energy storage, short lifespan, problems in series and parallel use, safety in use, and difficulty in estimating battery power. The performance of batteries is very complex, and the characteristics of different types of batteries are also very different. Battery management system (BMS) is mainly to improve the utilization rate of the battery, prevent the battery from overcharging and over discharging, prolong the service life of the battery, and monitor the status of the battery.

BMS Mainly Cover 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, operating current, and battery power during the working process of the battery, and judging the current battery status according to these parameters. Perform corresponding operations to prevent overcharge or over discharge of the battery.

2) Battery charge and discharge management: In the process of charging or discharging the battery, manage the charging or discharging of the battery according to the environmental status, battery status and other related parameters, and set the best charging or discharging curve of the battery (such as charging current, charging upper limit) voltage value, discharge lower limit voltage value, etc.)

3) Balance between single cells: that is, the balance charging of single cells, so that each battery in the battery pack reaches a balanced state. The equalizer is the core component of the battery management system, but the current domestic technology in this area is still immature.

Note: At present, many electric vehicles will be specially distinguished BMS and BBS (BATTERY BALANCE SYSTEM), it is easy for people to have a misunderstanding that they are two independent parts, but they are actually a subordinate relationship. Moreover, the functions of charge and discharge management and equalizer in domestic automobiles are relatively weak BMS In fact, it only calculates the amount of electricity and realizes the functions of overvoltage and undervoltage (group and single) protection and communication

The Battery Management System Mainly Includes the following Parts:

1) Signal acquisition module: mainly used to collect battery pack voltage, charging current, discharging current, cell voltage, battery temperature, and other parameters. Usually in isolation. (Except temperature signal)

2) Battery protection circuit module: Usually this part is realized by using software to control some external devices. For example, the on-off of the relay is controlled by the signal to allow or prohibit the operation of the charging and discharging equipment or the battery to realize the protection of the battery.

3) Balance circuit module: It is mainly used to collect the voltage of the cells of the battery pack, and perform balanced charging between the cells to achieve a balanced and consistent state of each battery in the group. At present, there are mainly two types of balancing methods: active balancing and passive balancing. (Can’t think of a third type?) It can also be called lossless equalization and lossy equalization.

4) Lower computer module: signal processing, control. communication.

Charging Characteristics

System Block Diagram

Energy Flow Diagram

Common Way

Common Way

Comparison of the Advantages and Disadvantage of each Equalization Scheme

Passive balancing method

Advantages: The circuit structure is simple and the cost is low.
Disadvantage: Only Charging equalization can be done, At the same time, in the process of charging equalization, the excess energy is used as the heat is realized, which makes the efficiency of the entire system low, the power consumption is high, and the equalization current is 50mA. BMS application, electric bicycle, electric motorcycle.

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Fit Capacitor scheme

Advantages: Low Cost, Simple structure, high active energy utilization rate.
Disadvantages: The equalization efficiency is limited, and the capacitor is used as the carrier of energy transfer. The program can be realized. The direct transfer of energy between any two monomers of the battery pack. Since the equalization current is affected by the capacitance voltage. The limit of the difference from the voltage of the monomer in the battery pack, as the equalization process progresses, the equalization speed will increase. The slower it comes, the mass production equalization current is about 30 a.

Question & Answer
  • Phone battery charging problem,
  • Why balance,
  • So, calculate,
  • Under what conditions is the battery damaged?
  • Fast charging principle,
  • power factor and efficiency,
  • Master-slave equilibrium bidirectional equilibrium problem.

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BMS Battery Management System,
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Analysis and summary of lithium battery,
IEC Battery Safety Standard for Power Batteries,
POWER BATTERY SHELL WATERPROOF DESIGN,
BATTERY SAFETY PERFORMANCE TEST,

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