Introduction to the application of high-efficiency battery equalizer technology in ladder energy storage batteries -Lithium - Ion Battery Equipment
Battery balancing technology can improve the service life of the battery pack and extend the use time of the battery pack. It is suitable for large-capacity nickel metal hydride, 2V lead-acid batteries, lithium-ion batteries, 6V lead-acid, 12V lead-acid and other battery packs, as well as supercapacitor packs.
Ladder batteries and selection
Ladder batteries refer to batteries that have been used and have reached their original design life, and their capacity has been fully or partially restored through other methods for continued use.
Generally, the effective capacity of a battery after 5 years of use is about 80%. The natural attenuation of the battery has entered a stable period, and it can be used as a small-capacity battery. By using a certain number of batteries in parallel, the available capacity can be increased several times, fully meeting the energy storage and power requirements. This is consistent with the purpose of increasing the battery life of electric vehicles. mileage, the reason for using a large number of parallel batteries to increase battery capacity is the same.(Lithium - Ion Battery Equipment)
After a battery pack has been used for 5 years, the available capacity and battery life are significantly shortened. Users and dealers usually replace the entire battery pack. However, not all batteries in a battery pack need to be replaced, but the capacity of one or a few of the batteries has seriously diminished. It affects the entire battery pack. If there are multiple such battery packs, the severely attenuated batteries can be eliminated through detection, and other batteries can be reused in series through capacity classification and internal resistance detection. The sequential utilization of power lithium batteries can significantly extend the efficiency and life cycle of batteries and reduce the environmental pollution caused by batteries. It is hailed as a key development target at present and in the future.
The reuse of power lithium batteries is a key link in the formation of a closed loop in the power lithium battery industry chain. It has important value in environmental protection, resource recycling and improving the full life cycle value of power lithium batteries. After being tested, screened, and reorganized, retired power lithium batteries are still capable of being used in areas such as low-speed electric vehicles, backup power supplies, and electric energy storage that have relatively good operating conditions and have low battery performance requirements.
As the promotion and application of new energy vehicles continues to increase, a large number of retired batteries will appear every year, and the concept of echelon utilization of power lithium batteries has emerged and has received widespread attention.
The use of cascade batteries can improve the utilization rate of the battery and extend the life cycle of the battery. It is of great significance in terms of energy saving and environmental protection. However, there are some things that must be paid attention to when using cascade batteries:
1. Use basic unit batteries (cells) as much as possible, such as 2V single lead-acid batteries, various lithium-ion batteries, including lithium iron phosphate batteries, lithium titanate batteries, ternary lithium-ion batteries, and lithium cobalt oxide batteries. , lithium manganate batteries, etc. Batteries that are packaged with multiple units connected in series, such as 6V lead-acid batteries (three 2V units) and 12V lead-acid batteries (six 2V units), are not suitable for echelon use, mainly because these batteries have multiple strings inside. The battery itself has imbalance problems that cannot be solved externally.
2. The principle of grouping batteries of the same type must be followed. Batteries in a group must be of the same type, that is, the batteries must have the same operating voltage range. Batteries with different working voltage ranges cannot appear in the same battery pack, and cannot be mixed even if they have the same capacity.
3. If possible, the capacity, voltage and internal resistance of the battery pack should be measured before assembly, and batteries with similar capacity and internal resistance should be selected as much as possible to reduce the expansion of consistency differences during reuse. Since the capacity of ladder batteries is generally lower than the nominal capacity, in order to obtain sufficient capacity, a larger number of batteries must be used to achieve the design capacity through appropriate series and parallel connections, so they must be assembled according to technical conditions.
Assembly method one: first in parallel and then in series. For example, this method is used in battery packs for electric vehicles.
Assembly method two: serial first and then parallel, often used in data centers or computer rooms.
Both assembly methods have their own advantages and disadvantages and are suitable for different environments:
Disadvantages of paralleling first and then series: The selection of unit cell connecting wires and busbars is very important, otherwise it will cause differences in battery charge and discharge. Individual battery leakage current (or failure) will affect a parallel unit, which will have a greater impact on capacity and directly Affects battery life (mileage); Advantages: Easy to manage, if a new battery equalizer is added, only one set (set) is enough.
Advantages of stringing first and then paralleling: easy connection, easy maintenance, quick detection and treatment of faulty batteries, easy maintenance, unit battery capacity in each string can be different, high battery utilization, capacity (power) can be expanded arbitrarily, new Increase backup time and improve reliability, especially suitable for data centers; Disadvantage: If a new battery equalizer is added, multiple sets (sets) are required.
4. The following batteries cannot be reused: first, batteries with large leakage current (or high self-discharge rate); second, batteries with deformed appearance, such as expansion of the shell; third, batteries with leakage.
Ladder cell balancing
Even if the screening of ladder batteries is very strict, it is difficult to ensure the consistency of the batteries. Even if the batteries with the best consistency are assembled together, differences will still occur to varying degrees after dozens of charge and discharge cycles, and this difference will increase with use. As time goes by, the consistency will become worse and worse, which is obviously manifested in the fact that the voltage difference between batteries gradually increases, and the effective charge and discharge time becomes shorter and shorter. A large amount of testing data found that battery packs with poor consistency have the following characteristics:
1. The voltage of the unit battery shows obvious high and low levels and irregular distribution;
2. The remaining capacity of the unit battery presents an irregular and discrete distribution;
3. The internal resistance of the unit cells also presents an irregular and discrete distribution.
Through further statistics on the detection data, it was found that the biggest killer of battery imbalance is:
1. Battery temperature differences, battery packs are usually installed densely, and the battery temperatures in each part are different, which affects the consistent performance of the battery and accelerates differences between batteries;
2. Vigorous charging and discharging accelerate the expansion of differences between batteries.
The capacity of energy storage battery packs is very large. Taking the nominal 500Ah battery pack as an example, assuming that the difference between the maximum capacity and the minimum capacity of the battery is 50Ah, and the difference between other batteries ranges from 5 to 10Ah, the maximum effective discharge of the system The capacity is 450Ah (tentatively designated as D battery, the same below), assuming the discharge current is 50A, the theoretical maximum discharge time is about 9h. Beyond this time, the D battery will reach the discharge cut-off voltage and enter an over-discharge state. If it continues to discharge, it will seriously damage the D battery and its maximum effective capacity will be sharply reduced, thereby further reducing the maximum effective capacity of the battery pack.
This also involves a problem of discharge rate. The discharge rate of the maximum capacity battery is 0.1C, the discharge rate of D battery is 0.11C, and the discharge rate of other batteries is between 0.1C and 0.11C. The difference in discharge rate makes The degree of attenuation of each battery is different, which will lead to the gradual expansion and accelerating trend of battery differences and uniformity.
Similarly, during charging, charge at a rate of 0.1C. The charging rate of D battery reaches 0.11C, which is at the maximum. It reaches the charging limit voltage first. If it continues to charge, it will enter the overcharge state, causing further damage to D battery. The charging rate of other batteries It is between 0.1C and 0.11C. The difference in charging rate will aggravate the difference and consistency of the battery, and it will show an accelerating trend.
After repeated charging and discharging of such a battery pack, the effective capacity will eventually become smaller and smaller, and the effective discharge time will become shorter and shorter. There is another serious problem in large-capacity energy storage battery packs, that is, the risk of thermal runaway. Regarding this battery pack, if effective prevention and control cannot be carried out, the D battery may become the battery with the highest temperature during the charging and discharging process of the battery pack, which can easily If a thermal runaway failure occurs, the battery may be completely scrapped, or even cause battery pack failure, or more serious associated problems may occur, which is unimaginable. If the battery pack can keep each battery from being overcharged or overdischarged during operation, then the effective capacity and discharge time of the battery pack can be guaranteed and always in a state of natural attenuation. It can be seen that battery balancing is related to the battery pack. How critical is normal and safe operation.
Regarding the D battery in this example, if its discharge current can be automatically reduced to less than 50A, such as 47~48A, and the insufficient 2~3A current is automatically supplied by other batteries with large capacity, then the overall discharge time can exceed 9h, and Other batteries reach the end of discharge together, and no over-discharge will occur; similarly, if its charging current can be automatically reduced to below 50A, such as 47~48A, the remaining 2~3A current will automatically be transferred to other batteries with large capacity, and the current will automatically increase. When the charging current of a large-capacity battery reaches the charging limit voltage together with other batteries, overdischarge will not occur. It can be seen that the balancing current must reach above 5A to meet the requirements, especially at the end of charge and discharge. From the balancing principle, only a transfer battery equalizer can be competent.
The current progress of effective battery balancing technology is very uneven, especially in terms of balancing current and balancing efficiency. Although some methods have adopted synchronous rectification technology, the maximum balancing current is mostly limited to 5A, and the continuous balancing current is only 1~3A, which is satisfactory. No need. Since bidirectional balancing must be supported, the current conversion efficiency is usually not high. The problem of self-heating under larger balancing currents is still prominent. Another important obstacle is the equipment cost. Since most of them use synchronous rectifier chips, the cost increases a lot.
Efficient cell balancing technology
At present, a high-power, high-efficiency, real-time, dynamic transfer battery equalizer technology has been successfully developed by Comrade Zhou Baolin of the Daqing Municipal Transportation Bureau after many years of development. It is based on national patented technology (patent numbers 201220153997.0 and 201520061849. 201710799424.2), this is a bidirectional synchronous rectification technology that does not require a synchronous rectification chip. It not only greatly reduces the equipment cost, but also greatly improves the balance current and balance efficiency. It has achieved a breakthrough in balanced technical indicators and has the following characteristics:
1. The balanced current range is large. A large balancing current means that the balancing speed is very fast, see the attached table. At present, the enhanced version of the lithium-ion battery equalizer has realized that the relationship between the balancing current and the voltage difference is about 1A/13mV. For example, when the voltage difference reaches 130mV, the balancing current can reach about 10A, which is especially beneficial for high-speed balancing.
2. High balanced efficiency. High equilibrium efficiency means less power loss, higher utilization, and lower temperature rise of the equipment.
3. Real-time dynamic equalization. When the battery pack is at rest, the maximum voltage difference within the pack can be controlled within 10mV or even less (depending on the setting of the reference voltage difference), and it enters the micro-power standby detection state. Whether the battery pack is charging or In the discharge state, once the voltage difference is detected to be greater than the reference voltage difference, it immediately enters the high-speed equalization state. The biggest benefit of real-time dynamic equalization is that the effective equalization time is long, the equalizer has the highest efficiency, and its unique pulse technology has good maintenance and capacity for the battery. The improvement effect has been tested by application.
The use of high-current, high-efficiency battery equalizers can minimize overcharging, over-discharging, and thermal runaway failures of attenuated batteries. Even if the capacity of the battery pack has declined and the consistency has become worse, the decay rate can be very well reduced. By automatically forcing the voltage to maintain consistency, it can also increase the effective capacity of the battery pack to a certain extent and extend the life of the battery pack. Cycle life, especially significantly reduced repair and maintenance costs.
Actual use effect: Used on 24-cell 2V170Ah lead-acid battery pack returned by the customer. Using standard 17A current charging and discharging, without an equalizer, the maximum discharge time after full charge is about 3 hours. During the discharge period, the three batteries generated serious heat and were seriously over-discharged. The voltage value was lower than 0.5V, and one of the batteries was -0.1 V, there is polarity reversal, the voltage of 21 batteries ranges from 1.8 to 2.0V, and there is still a lot of power that has not been released; after using the battery equalizer prototype in this article, under standard charge and discharge parameters, after several charge and discharge cycles , the discharge time was gradually extended to about 5.5h, and the efficiency was improved by more than 80%. The voltages of the three worst batteries were all above 1.5V after discharge, and the discharge voltage gradually increased, especially when the heat was serious at the beginning. Great improvement, the temperature drop is very obvious, the voltage of only 4 batteries is around 1.9V, and the rest of the batteries are around 1.8V, the battery power is fully and effectively released.
Ladder batteries and selection
Ladder batteries refer to batteries that have been used and have reached their original design life, and their capacity has been fully or partially restored through other methods for continued use.
Generally, the effective capacity of a battery after 5 years of use is about 80%. The natural attenuation of the battery has entered a stable period, and it can be used as a small-capacity battery. By using a certain number of batteries in parallel, the available capacity can be increased several times, fully meeting the energy storage and power requirements. This is consistent with the purpose of increasing the battery life of electric vehicles. mileage, the reason for using a large number of parallel batteries to increase battery capacity is the same.(Lithium - Ion Battery Equipment)
After a battery pack has been used for 5 years, the available capacity and battery life are significantly shortened. Users and dealers usually replace the entire battery pack. However, not all batteries in a battery pack need to be replaced, but the capacity of one or a few of the batteries has seriously diminished. It affects the entire battery pack. If there are multiple such battery packs, the severely attenuated batteries can be eliminated through detection, and other batteries can be reused in series through capacity classification and internal resistance detection. The sequential utilization of power lithium batteries can significantly extend the efficiency and life cycle of batteries and reduce the environmental pollution caused by batteries. It is hailed as a key development target at present and in the future.
The reuse of power lithium batteries is a key link in the formation of a closed loop in the power lithium battery industry chain. It has important value in environmental protection, resource recycling and improving the full life cycle value of power lithium batteries. After being tested, screened, and reorganized, retired power lithium batteries are still capable of being used in areas such as low-speed electric vehicles, backup power supplies, and electric energy storage that have relatively good operating conditions and have low battery performance requirements.
As the promotion and application of new energy vehicles continues to increase, a large number of retired batteries will appear every year, and the concept of echelon utilization of power lithium batteries has emerged and has received widespread attention.
The use of cascade batteries can improve the utilization rate of the battery and extend the life cycle of the battery. It is of great significance in terms of energy saving and environmental protection. However, there are some things that must be paid attention to when using cascade batteries:
1. Use basic unit batteries (cells) as much as possible, such as 2V single lead-acid batteries, various lithium-ion batteries, including lithium iron phosphate batteries, lithium titanate batteries, ternary lithium-ion batteries, and lithium cobalt oxide batteries. , lithium manganate batteries, etc. Batteries that are packaged with multiple units connected in series, such as 6V lead-acid batteries (three 2V units) and 12V lead-acid batteries (six 2V units), are not suitable for echelon use, mainly because these batteries have multiple strings inside. The battery itself has imbalance problems that cannot be solved externally.
2. The principle of grouping batteries of the same type must be followed. Batteries in a group must be of the same type, that is, the batteries must have the same operating voltage range. Batteries with different working voltage ranges cannot appear in the same battery pack, and cannot be mixed even if they have the same capacity.
3. If possible, the capacity, voltage and internal resistance of the battery pack should be measured before assembly, and batteries with similar capacity and internal resistance should be selected as much as possible to reduce the expansion of consistency differences during reuse. Since the capacity of ladder batteries is generally lower than the nominal capacity, in order to obtain sufficient capacity, a larger number of batteries must be used to achieve the design capacity through appropriate series and parallel connections, so they must be assembled according to technical conditions.
Assembly method one: first in parallel and then in series. For example, this method is used in battery packs for electric vehicles.
Assembly method two: serial first and then parallel, often used in data centers or computer rooms.
Both assembly methods have their own advantages and disadvantages and are suitable for different environments:
Disadvantages of paralleling first and then series: The selection of unit cell connecting wires and busbars is very important, otherwise it will cause differences in battery charge and discharge. Individual battery leakage current (or failure) will affect a parallel unit, which will have a greater impact on capacity and directly Affects battery life (mileage); Advantages: Easy to manage, if a new battery equalizer is added, only one set (set) is enough.
Advantages of stringing first and then paralleling: easy connection, easy maintenance, quick detection and treatment of faulty batteries, easy maintenance, unit battery capacity in each string can be different, high battery utilization, capacity (power) can be expanded arbitrarily, new Increase backup time and improve reliability, especially suitable for data centers; Disadvantage: If a new battery equalizer is added, multiple sets (sets) are required.
4. The following batteries cannot be reused: first, batteries with large leakage current (or high self-discharge rate); second, batteries with deformed appearance, such as expansion of the shell; third, batteries with leakage.
Ladder cell balancing
Even if the screening of ladder batteries is very strict, it is difficult to ensure the consistency of the batteries. Even if the batteries with the best consistency are assembled together, differences will still occur to varying degrees after dozens of charge and discharge cycles, and this difference will increase with use. As time goes by, the consistency will become worse and worse, which is obviously manifested in the fact that the voltage difference between batteries gradually increases, and the effective charge and discharge time becomes shorter and shorter. A large amount of testing data found that battery packs with poor consistency have the following characteristics:
1. The voltage of the unit battery shows obvious high and low levels and irregular distribution;
2. The remaining capacity of the unit battery presents an irregular and discrete distribution;
3. The internal resistance of the unit cells also presents an irregular and discrete distribution.
Through further statistics on the detection data, it was found that the biggest killer of battery imbalance is:
1. Battery temperature differences, battery packs are usually installed densely, and the battery temperatures in each part are different, which affects the consistent performance of the battery and accelerates differences between batteries;
2. Vigorous charging and discharging accelerate the expansion of differences between batteries.
The capacity of energy storage battery packs is very large. Taking the nominal 500Ah battery pack as an example, assuming that the difference between the maximum capacity and the minimum capacity of the battery is 50Ah, and the difference between other batteries ranges from 5 to 10Ah, the maximum effective discharge of the system The capacity is 450Ah (tentatively designated as D battery, the same below), assuming the discharge current is 50A, the theoretical maximum discharge time is about 9h. Beyond this time, the D battery will reach the discharge cut-off voltage and enter an over-discharge state. If it continues to discharge, it will seriously damage the D battery and its maximum effective capacity will be sharply reduced, thereby further reducing the maximum effective capacity of the battery pack.
This also involves a problem of discharge rate. The discharge rate of the maximum capacity battery is 0.1C, the discharge rate of D battery is 0.11C, and the discharge rate of other batteries is between 0.1C and 0.11C. The difference in discharge rate makes The degree of attenuation of each battery is different, which will lead to the gradual expansion and accelerating trend of battery differences and uniformity.
Similarly, during charging, charge at a rate of 0.1C. The charging rate of D battery reaches 0.11C, which is at the maximum. It reaches the charging limit voltage first. If it continues to charge, it will enter the overcharge state, causing further damage to D battery. The charging rate of other batteries It is between 0.1C and 0.11C. The difference in charging rate will aggravate the difference and consistency of the battery, and it will show an accelerating trend.
After repeated charging and discharging of such a battery pack, the effective capacity will eventually become smaller and smaller, and the effective discharge time will become shorter and shorter. There is another serious problem in large-capacity energy storage battery packs, that is, the risk of thermal runaway. Regarding this battery pack, if effective prevention and control cannot be carried out, the D battery may become the battery with the highest temperature during the charging and discharging process of the battery pack, which can easily If a thermal runaway failure occurs, the battery may be completely scrapped, or even cause battery pack failure, or more serious associated problems may occur, which is unimaginable. If the battery pack can keep each battery from being overcharged or overdischarged during operation, then the effective capacity and discharge time of the battery pack can be guaranteed and always in a state of natural attenuation. It can be seen that battery balancing is related to the battery pack. How critical is normal and safe operation.
Regarding the D battery in this example, if its discharge current can be automatically reduced to less than 50A, such as 47~48A, and the insufficient 2~3A current is automatically supplied by other batteries with large capacity, then the overall discharge time can exceed 9h, and Other batteries reach the end of discharge together, and no over-discharge will occur; similarly, if its charging current can be automatically reduced to below 50A, such as 47~48A, the remaining 2~3A current will automatically be transferred to other batteries with large capacity, and the current will automatically increase. When the charging current of a large-capacity battery reaches the charging limit voltage together with other batteries, overdischarge will not occur. It can be seen that the balancing current must reach above 5A to meet the requirements, especially at the end of charge and discharge. From the balancing principle, only a transfer battery equalizer can be competent.
The current progress of effective battery balancing technology is very uneven, especially in terms of balancing current and balancing efficiency. Although some methods have adopted synchronous rectification technology, the maximum balancing current is mostly limited to 5A, and the continuous balancing current is only 1~3A, which is satisfactory. No need. Since bidirectional balancing must be supported, the current conversion efficiency is usually not high. The problem of self-heating under larger balancing currents is still prominent. Another important obstacle is the equipment cost. Since most of them use synchronous rectifier chips, the cost increases a lot.
Efficient cell balancing technology
At present, a high-power, high-efficiency, real-time, dynamic transfer battery equalizer technology has been successfully developed by Comrade Zhou Baolin of the Daqing Municipal Transportation Bureau after many years of development. It is based on national patented technology (patent numbers 201220153997.0 and 201520061849. 201710799424.2), this is a bidirectional synchronous rectification technology that does not require a synchronous rectification chip. It not only greatly reduces the equipment cost, but also greatly improves the balance current and balance efficiency. It has achieved a breakthrough in balanced technical indicators and has the following characteristics:
1. The balanced current range is large. A large balancing current means that the balancing speed is very fast, see the attached table. At present, the enhanced version of the lithium-ion battery equalizer has realized that the relationship between the balancing current and the voltage difference is about 1A/13mV. For example, when the voltage difference reaches 130mV, the balancing current can reach about 10A, which is especially beneficial for high-speed balancing.
2. High balanced efficiency. High equilibrium efficiency means less power loss, higher utilization, and lower temperature rise of the equipment.
3. Real-time dynamic equalization. When the battery pack is at rest, the maximum voltage difference within the pack can be controlled within 10mV or even less (depending on the setting of the reference voltage difference), and it enters the micro-power standby detection state. Whether the battery pack is charging or In the discharge state, once the voltage difference is detected to be greater than the reference voltage difference, it immediately enters the high-speed equalization state. The biggest benefit of real-time dynamic equalization is that the effective equalization time is long, the equalizer has the highest efficiency, and its unique pulse technology has good maintenance and capacity for the battery. The improvement effect has been tested by application.
The use of high-current, high-efficiency battery equalizers can minimize overcharging, over-discharging, and thermal runaway failures of attenuated batteries. Even if the capacity of the battery pack has declined and the consistency has become worse, the decay rate can be very well reduced. By automatically forcing the voltage to maintain consistency, it can also increase the effective capacity of the battery pack to a certain extent and extend the life of the battery pack. Cycle life, especially significantly reduced repair and maintenance costs.
Actual use effect: Used on 24-cell 2V170Ah lead-acid battery pack returned by the customer. Using standard 17A current charging and discharging, without an equalizer, the maximum discharge time after full charge is about 3 hours. During the discharge period, the three batteries generated serious heat and were seriously over-discharged. The voltage value was lower than 0.5V, and one of the batteries was -0.1 V, there is polarity reversal, the voltage of 21 batteries ranges from 1.8 to 2.0V, and there is still a lot of power that has not been released; after using the battery equalizer prototype in this article, under standard charge and discharge parameters, after several charge and discharge cycles , the discharge time was gradually extended to about 5.5h, and the efficiency was improved by more than 80%. The voltages of the three worst batteries were all above 1.5V after discharge, and the discharge voltage gradually increased, especially when the heat was serious at the beginning. Great improvement, the temperature drop is very obvious, the voltage of only 4 batteries is around 1.9V, and the rest of the batteries are around 1.8V, the battery power is fully and effectively released.
