18650 lithium battery 3000mah

18650 lithium battery 3000mah equalizer design

The 18650 lithium battery 3000mah equalizer design introduced in this article is applicable to any state of the battery pack, not only limited to static balancing, but also during charging, discharging, and recovery period after charging and discharging. As long as the voltage difference between adjacent batteries is greater than the voltage difference accuracy set by the device, the equalizer automatically adjusts the balancing current according to the voltage difference, balancing on demand, and improving the balancing efficiency.

Abstract: The consistency problem of power battery packs not only affects the power output and endurance performance of the battery pack, but also seriously threatens the operation safety of the battery pack. Under the existing technical conditions, the recognized ideal solution is battery balancing technology, especially high-speed, efficient, real-time, and transfer battery balancing technology, which has the best comprehensive performance, especially in terms of balancing current, balancing efficiency, and adaptability. The introduction of bidirectional synchronous rectification technology has made the balancing performance qualitatively improved, and the safety, power, endurance, and cycle life of power battery packs have been significantly improved.

Keywords: consistency, battery balancing, bidirectional synchronous rectification, thermal runaway

01

Consistency problem of power battery packs

Since the large-scale application of batteries, the consistency problem of battery packs has been plaguing the majority of users and scientific and technological personnel, and they have been actively seeking the best solutions and technologies. The consistency problem of the battery pack appears to occur in the battery pack itself, but the impact and consequences it brings far exceed the battery pack itself, especially the risk of "thermal runaway". The power and capacity of the power battery pack are very large. Once thermal runaway occurs, the harm caused is usually very large. Therefore, preventing and controlling the consistency problem of the battery pack has become a safety management problem of the battery pack.

Application and practice of bidirectional synchronous rectification technology in 18650 lithium battery 3000mah equalizer of power battery pack

The consistency problem of power battery needs to be solved urgently

Generally speaking, there are many reasons for the consistency problem of battery packs, including the reasons of the battery pack itself, which are related to the production process and quality control of the battery, referred to as internal factors; and external factors during use, such as temperature, charge and discharge current, charge and discharge voltage, charge and discharge rate, etc., referred to as external factors. A large amount of test data shows that external factors are the main reason for the rapid deterioration of battery pack consistency.

02

The impact of equalization on consistency problems

The most obvious parameters of battery pack consistency change are the voltage and capacity of the single cell. The easiest data to measure is voltage. The change characteristics of voltage actually reflect the change characteristics of capacity. For example, the capacity of "lagging" batteries is usually small, and the obvious performance characteristics of voltage parameters are that the voltage rises fastest during charging and drops fastest during discharging, which are obviously greater than the average value and fluctuate greatly.

For normal batteries, the voltage performance characteristics are relatively stable during both charging and discharging, and will not change drastically. Since external factors are the main reason for the rapid degradation of battery pack consistency, we need to adjust the voltage of the "lagging" battery in time according to the changes in external factors, so that its voltage change curve is close to the average value of the entire battery group. This is battery balancing. If this goal can be achieved, the working voltage of the "lagging" battery will keep pace with the voltage of the normal battery.

When charging, the voltage rises at the same rate; when discharging, the voltage drops at the same rate. The role of battery balancing is to intervene in the voltage change of the "lagging" battery by automatically adjusting the power between batteries and the different charging and discharging currents of batteries of different capacities. If the voltage of the "lagging" battery always changes synchronously with the voltage of the normal battery, then the performance of the battery pack in voltage and capacity distribution must be balanced, and the consistency performance of such a battery pack must be good.

03

Choice of balancing strategy

From the previous analysis, it can be seen that the essence of balancing is to achieve voltage balancing by adjusting the power and current. The purpose is to prevent the voltage of the "lagging" battery from being too low or too high, as well as overcharging and over-discharging. When the difference between the "lagging" battery and the normal battery is not large, a smaller balancing current can meet the needs. When the difference between the "lagging" battery and the normal battery is large, the required balancing current also needs to be increased synchronously.

It can be seen that a very important indicator of balancing technology is the size or range of the supported balancing current. The existing battery balancing technology mainly includes three categories, namely resistance discharge balancing technology, charging balancing technology and transfer battery balancing technology. Among them, resistance discharge balancing technology is a typical passive balancing technology, which is only applicable to the charging stage and the battery pack with small battery differences. It is usually only started when the battery is close to full.

Application and practice of bidirectional synchronous rectification technology in 18650 lithium battery 3000mah equalizer of power battery pack

Common lithium battery equalization protection board

There is also a method of early intervention through software control technology to extend the equalization time and speed up the equalization speed, but this solution needs to solve the effective control of heat. The equalization current is usually very small, generally at the milliampere level, usually within 100 milliamperes. When the difference of battery packs is large, this solution is powerless and difficult to support. Its inherent defects and biggest weakness are that it cannot solve the problem of over-discharge of small-capacity batteries.

Charging equalization technology and resistance discharge equalization technology belong to the category of charging equalization technology. The advantage is that the equalization current is large, usually reaching the ampere level, the efficiency is also high, the heat generated is less, and the temperature rise of the battery pack is relatively small, but its inherent defects and weaknesses are also unable to solve the problem of over-discharge of small-capacity batteries.

The transfer battery equalization technology is a more complete solution. It not only meets the needs of charge equalization, discharge equalization and static equalization, but also has a higher equalization current and equalization efficiency. It has a significant effect on stabilizing the consistency of battery pack operation. Although the technical implementation is very difficult and the cost is relatively high, from the perspective of the key requirements of battery packs for operational safety, it effectively solves the "thermal runaway" risk and safety hazards caused by consistency problems in battery packs, and is the future development direction.

04

Design and implementation of high-speed equalization

From the technical principle point of view, the transfer battery equalization technology can quickly complete the voltage balance of the battery pack and quickly achieve the operational consistency of the battery pack. However, from the perspective of practical applications and various comparison tests, the actual supported equalization current size and the level of equalization efficiency are directly related to the speed of equalization and the improvement of voltage consistency.

For conventional series battery packs, the current flowing through each battery is the same, so when charging, the voltage of the battery with a small capacity rises quickly and will be fully charged first. If there is no special control, other batteries cannot be fully charged. In this case, the total voltage of the battery pack will be lower than the charging control voltage, and the charger will continue to charge, which will eventually cause the battery with a small capacity to be overcharged and damaged, and the damage is irreversible.

Similarly, when discharging, the voltage of the battery with small capacity drops quickly, and it will definitely be discharged first. If there is no special control, other batteries will still have residual power that has not been released. In this case, the total voltage of the battery pack will definitely be higher than the discharge cut-off voltage, and the battery pack will continue to discharge, which will eventually cause the battery with small capacity to be over-discharged and damaged, and the damage is irreversible. After several repeated charge and discharge, the capacity of the battery with small capacity will decay faster, and eventually the battery pack will be scrapped in advance.

Therefore, the design of the equalizer must support a larger balancing current to balance the voltage difference of batteries with different capacities. For battery packs with small capacity, such as within tens of ampere hours, the charge and discharge current is usually relatively small, and a smaller balancing current can usually meet the needs. However, for large-capacity battery packs, such as hundreds of ampere hours or high-power battery packs, the working current is usually relatively large, which puts higher requirements on the balancing current and balancing speed, and high-speed balancing is required to meet the needs.

The high-speed balancing mentioned here has two meanings: one is that a higher balancing current can be achieved under a smaller voltage difference, and the voltage difference increases, and the balancing current increases synchronously; the second is that it must have a higher power conversion efficiency to prevent the heat generated by the balancing equipment from affecting the battery pack.

Based on this principle, higher requirements are put forward for the design of high-speed equalizers. First of all, the equalization current must be large enough. Traditional transfer equalization design usually uses diodes for freewheeling. Considering the voltage drop and loss, Schottky diodes with low voltage drop are mostly used. Despite this, the loss at high current is still very large, which not only causes a large temperature rise of the equipment, but also brings additional temperature rise to the battery pack, and also brings danger to its own safe and stable operation.

It can be seen that for high-speed equalization schemes, the use of freewheeling diode design can not only fail to achieve large equalization current, but also fail to achieve high power conversion efficiency. A new energy conversion design scheme must be adopted. We all know that in DC power supplies with high current output, synchronous rectification design schemes are usually adopted, using the characteristics of small conduction voltage drop of power field effect tubes to replace freewheeling diodes to obtain high current output capacity under high efficiency.

Therefore, if the synchronous rectification design ideas and technologies can be transplanted into battery equalization design, both the equalization current and conversion efficiency will be greatly improved. To this end, according to the requirements of high-speed equalization, the author has spent many years of intensive research and developed a unique patented bidirectional synchronous rectification technology, which is applied to the design of single-cell 2V lead-acid battery high-speed equalizer and lithium battery high-speed equalizer respectively. The continuous maximum equalization current reaches more than 20 amperes, far exceeding the equalization current of known domestic battery equalizers, and the temperature rise of the equipment is still within a reasonable range under full load.

A large number of experimental and application measurement data show that whether it is dynamic reaction speed, voltage difference control, or power conversion efficiency, it fully meets the design requirements of high-speed equalization.

05

High-speed equalization experimental test

Below, the static equalization application experimental measurement data of the single-cell 2V lead-acid battery high-speed equalizer in a base station 24 strings of single-cell 2V600Ah battery pack is analyzed. The experimental battery pack is shown in the figure. The additional device on the battery pack is a 2V lead-acid battery dedicated equalizer prototype. The designed continuous maximum equalization current is 10-12 amperes, which can reach more than 20 amperes in a short time.

Before the battery balancer was connected, the maximum voltage difference was close to 0.14V, with poor consistency, far exceeding the industry standard of 30mV voltage difference. After the equalizer prototype was connected, the voltage difference immediately began to shrink under the strong equalization current, and the voltage consistency was rapidly improved. As time went on, the maximum voltage difference further narrowed, and the equalization current gradually decreased. The measurement data after 24 hours showed that the maximum voltage difference was only 10mV, which was better than the standard, and the voltage consistency problem was eliminated. For specific measurement data, see the attached table.

The actual measurement data showed that in the battery pack under floating charge, the voltage of 9# battery was the highest, 79mv higher than the average voltage, and the voltage of 20# battery was the lowest, 60mv lower than the average voltage. With the intervention of the equalizer, the voltage of all batteries quickly approached the average voltage, among which the voltage change rate of 9# and 20# batteries was the largest. This is because the voltage difference between 9# and 20# batteries and adjacent batteries is large, the equalization current is large, and the adjustment speed of charge and power is fast.

The 18650 lithium battery 3000mah equalizer design introduced in this article is applicable to any state of the battery pack, not only limited to static balancing, but also during charging, discharging, and recovery after charging and discharging. As long as the voltage difference between adjacent batteries is greater than the voltage difference accuracy set by the device, the equalizer automatically adjusts the balancing current according to the voltage difference, balancing on demand, and improving the balancing efficiency. Due to space limitations, the charge and discharge balancing data of this special 18650 lithium battery 3000mah equalizer on this battery pack is omitted.

Application and practice of bidirectional synchronous rectification technology in 18650 lithium battery 3000mah equalizer for power battery pack

Experimental battery pack

Application and practice of bidirectional synchronous rectification technology in 18650 lithium battery 3000mah equalizer for power battery pack

Experimental battery pack static balancing voltage table

06

Significance and prospects of high-speed balancing

The role of power battery packs is self-evident. For a long time, achieving its safe and efficient operation and management has always been the focus of people's attention. For power battery packs, whether the battery pack is safe to operate, and whether the safe discharge time or endurance of the battery pack is stable has always been the focus of people's attention.

The biggest risk of battery pack operation is thermal runaway. Thermal runaway, safe discharge time, and whether the endurance is stable are directly related to the consistency of the battery pack. It can be seen that consistency problem management is the difficulty and key to battery pack management. The functional design purpose of the 18650 lithium battery 3000mah equalizer described in this article is to solve the consistency problem of the battery pack.

In the future, with the expansion of the application scope of power battery packs, especially the scaled utilization of retired batteries in high-power, large-capacity energy storage, base stations, etc., the consistency problem will be further highlighted, and the safety problem will be put in front of people again, especially the safety problem caused by the consistency problem of the cascade battery utilization will be more prominent, and it is urgent to solve it from the automatic control technology. The breakthrough and successful development of 18650 lithium battery 3000mah balancing technology will surely bring new concepts and dawn to the safe operation and management of battery packs.

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