The relationship between high-efficiency battery equalizer technology and cascade energy storage batteries
The battery balancing technology can improve the service life of the battery pack and prolong the service time of the battery pack. It is suitable for large-capacity nickel-metal hydride, 2V lead-acid batteries, lithium batteries, 6V lead-acid, 12V lead-acid battery packs and supercapacitor packs.
Ladder battery and selection
A secondary battery refers to a battery that has been used and has reached its original design life, and its capacity has been restored in full or in part by other methods.
Generally, the effective capacity of the battery after 5 years of use is about 80%. The natural decay of the battery has entered a stable period, and it can be used as a small-capacity battery. Through the parallel use of a certain number of batteries, the available capacity can be increased several times, which fully meets the needs of energy storage and power. , the reason for using a large number of parallel batteries to increase the battery capacity is the same.
After a battery pack has been used for 5 years, the usable capacity and battery life are significantly shortened. Users and dealers usually replace it as a whole. As everyone knows, not all batteries in a battery pack need to be replaced, but one or several of the batteries have a serious capacity degradation. It affects the entire battery pack. If there are multiple such battery packs, the severely attenuated batteries are removed by detection, and other batteries can be reused in a cascade through capacity division and internal resistance detection. The cascade utilization of power batteries obviously prolongs the use efficiency and life cycle of batteries, and reduces the environmental pollution caused by batteries. It is known as the key development object at present and in the future.
Power battery reuse is a key link in the formation of a closed-loop power battery industry chain, and has important value in environmental protection, resource recovery, and improving the full life cycle value of power batteries. After decommissioning, power batteries are still capable of being used in low-speed electric vehicles, backup power sources, power storage and other fields with relatively good operating conditions and low battery performance requirements after testing, screening, and reorganization.
With the increasing promotion and application of new energy vehicles, a large number of retired batteries will be produced every year, and the concept of cascade utilization of power batteries has emerged and attracted widespread attention.
The utilization of echelon batteries can improve the utilization rate of batteries and prolong the life cycle of batteries, which is of great significance in terms of energy saving and environmental protection, but the utilization of echelon batteries must pay attention to some matters:
1. Use basic unit cells as much as possible, such as 2V single lead-acid batteries, various lithium batteries, including lithium iron phosphate batteries, lithium titanate batteries, ternary lithium batteries, lithium cobalt oxide batteries, and lithium manganate batteries. Wait. Batteries that are packaged in series with multiple units, such as 6V lead-acid batteries (3 2V units) and 12V lead-acid batteries (6 2V units), are not suitable for cascade utilization, mainly because the interior of these batteries is multi-string The battery itself has the problem of imbalance, which cannot be solved externally.
2. The principle of grouping batteries of the same type must be followed. The batteries in the group must be of the same type, that is, the working voltage range of the batteries must be the same. Batteries with different working voltage ranges cannot appear in the same battery pack, and they cannot be mixed even if they have the same capacity.
3. If conditions permit, the capacity, voltage and internal resistance should be measured before the battery pack is assembled, 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 echelon batteries is generally lower than the nominal capacity, in order to obtain sufficient capacity, it is necessary to use a larger number of batteries to achieve the design capacity through suitable series and parallel connection, so it needs to be assembled according to technical conditions.
Assembly method 1: first in parallel and then in series, such as battery packs for electric vehicles using this method.
Assembly method 2: first in series and then in 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 stringing: the selection of unit battery connection lines and bus bars is very important, otherwise it will cause differences in battery charge and discharge, and individual battery leakage current (or fault) will affect a parallel unit, which has a relatively large impact on capacity. Affects battery life (mileage); advantages: easy to manage, if you add a battery equalizer, only one set (set) is needed.
Advantages of serial first and then parallel: easy connection, easy maintenance, quick detection and handling of faulty batteries, easy maintenance, unit battery capacity in each string can be different, high battery utilization rate, capacity (power) can be arbitrarily expanded, increase Backup time, improve reliability, especially suitable for data centers; Disadvantages: If you add battery equalizers, multiple sets (sets) are required.
4. The following batteries cannot be reused: one is a battery with a large leakage current (or a high self-discharge rate); the other is a battery whose appearance is deformed, such as a swollen shell; the third is a battery that leaks.
Echelon Cell Balance
Even if the screening of echelon batteries is very strict, it is difficult to ensure the consistency of batteries. Even if batteries with excellent consistency are assembled together, there will still be differences to varying degrees after dozens of charge and discharge cycles, and this difference will change with use. The prolongation of time gradually increases, and the consistency will become worse and worse. It is obvious that the voltage difference between the batteries gradually increases, and the effective charge and discharge time becomes shorter and shorter. A large number of test data found that the battery pack with poor consistency has the following characteristics:
1. The voltage of the unit cell is obviously uneven and irregularly distributed;
2. The residual capacity of the unit battery presents an irregular discrete distribution;
3. The internal resistance of the unit cell also presents an irregular discrete distribution.
Through further statistics on the detection data, it is found that the biggest killer of battery imbalance is:
1. The temperature difference of the battery, the installation of the battery pack is usually dense, and the battery temperature of each part is different, which affects the consistency of the battery and accelerates the difference between the batteries;
2. Severe charge and discharge to accelerate the expansion of differences between batteries;
The capacity of the energy storage battery pack is very large. Take 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 numbered as D battery, the same below), assuming the discharge current is 50A, the theoretical maximum discharge time is about 9h. After this time, the D battery will reach the discharge cut-off voltage and enter the over-discharge state. If it continues to discharge, it will seriously damage the D battery, and its maximum effective capacity will decrease sharply, thereby further reducing the maximum effective capacity of the battery pack. There is also a problem of discharge rate. The discharge rate of the largest 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. Each battery has a different degree of attenuation, which will lead to a gradual expansion and acceleration of the differences and uniformity of the batteries. Similarly, during charging, charge at a rate of 0.1C, the charging rate of the D battery reaches 0.11C, which is at the maximum, and the charging limit voltage is reached first. Continuing to charge will enter the overcharge state, causing further damage to the D battery. The charging rate of other batteries It is between 0.1C and 0.11C, and the difference in charging rate will aggravate the difference and consistency of the battery, and it will accelerate. Such a battery pack will eventually lead to smaller and smaller effective capacity and shorter effective discharge time after repeated charging and discharging. There is also a serious problem with the large-capacity energy storage battery pack, which is the risk of thermal runaway. For 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. If a thermal runaway failure occurs, the battery will be completely scrapped, or even cause the battery pack to fail. If the battery pack can maintain each battery without overcharging and overdischarging during operation, the effective capacity and discharge time of the battery pack can be guaranteed, and it is always in a state of natural decay. How critical it is to operate properly and safely.
For the D battery in this example, if the discharge current can be automatically reduced to below 50A, such as 47-48A, and the insufficient 2-3A current is automatically provided by other large-capacity batteries, then the overall discharge time can exceed 9h. Other batteries reach the end of discharge together, and no over-discharge occurs; similarly, if the charging current can be automatically reduced to below 50A, such as 47-48A, the remaining 2-3A current will be automatically transferred to other batteries with large capacity, and automatically increase The charging current of the large-capacity battery reaches the charging limit voltage together with other batteries, so that overdischarge will not occur. It can be seen that the equalizing current must reach more than 5A to meet the requirements, especially at the end of charging and discharging. From the principle of equalization, only the transfer battery equalizer can be competent.
At present, the progress of effective battery balancing technology is very unbalanced, especially in terms of balancing current and balancing efficiency. Although some solutions have adopted synchronous rectification technology, the maximum balancing current is mostly limited to less than 5A, and the continuous balancing current is only 1-3A. No need. Since it is necessary to support bidirectional equalization, the current conversion efficiency is usually not high, and the self-heating problem under large equalization current is still relatively prominent. Another important obstacle is the cost of equipment. Since most of them use synchronous rectifier chips, the cost increases a lot.
High-efficiency 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 Daqing Transportation Bureau after many years. It takes the national patent technology (patent number 201220153997.0 and 201520061849.X) as the core, and integrates the self-invented bidirectional synchronous rectification technology (patent applied for: a transfer type real-time battery equalizer with bidirectional synchronous rectification function, application number: 201710799424.2), which is a bidirectional synchronous rectification technology that does not require a synchronous rectifier chip, which not only greatly reduces the equipment cost, but also greatly improves the balance current and balance efficiency. Achieved breakthroughs in balanced technical indicators, with the following characteristics:
1. The balance current range is large. A large equalization current means that the equalization speed is very fast, see the attached table. At present, the enhanced lithium battery equalizer has realized that the relationship between the equalizing current and the voltage difference is about 1A/13mV. For example, when the voltage difference reaches 130mV, the equalizing current can reach about 10A, which is especially conducive to high-speed equalization.
2. High balance efficiency. High equilibrium efficiency means less power loss, higher utilization, and lower temperature rise of equipment, see Table 1.
3. Real-time dynamic equalization. In the static state of the battery pack, the maximum voltage difference in the pack can be controlled within 10mV or even smaller (depending on the setting of the reference voltage difference), and enter the micro-power standby detection state, whether the battery pack is in the charging state or in the In the discharge state, once the voltage difference is detected to be greater than the reference voltage difference, it will enter the high-speed equalization state immediately. The biggest advantage 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 the application.
Using a high-current, high-efficiency cell equalizer can minimize battery overcharge, overdischarge, and thermal runaway failures. Even if the capacity decay of the battery pack has become the fact that the consistency has become worse, it can reduce the decay speed very well. By automatically forcing the voltage to maintain consistency, it can also improve the effective capacity of the battery pack to a certain extent and prolong the battery pack. Cycle life in particular significantly reduces repair and maintenance costs.
Actual use effect: used on 24 strings of single 2V170Ah lead-acid battery packs returned by customers. The standard 17A current is used for charging and discharging. In the case of no equalizer, the maximum discharge time after full charge is about 3h. During the discharge of 3 batteries, the heat is serious, and the voltage is severely overdischarged. The voltage value is lower than 0.5V, and one battery is -0.1 V, there is a 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 the standard charge and discharge parameters, after several charge and discharge cycles , the discharge time is gradually extended to about 5.5h, and the efficiency is improved by more than 80%. For the three worst batteries, the voltage after discharge is all above 1.5V, and the discharge voltage gradually rises, especially the problem of serious heat at the beginning. Great improvement, the temperature drop is very obvious, only the voltage of 4 batteries is around 1.9V, the rest of the batteries are around 1.8V, the battery power is fully and effectively released.
