Under the influence of the consistency problem, the actual discharge capacity of the battery pack depends on the battery with the smallest capacity in the battery pack. The greater the number of battery packs connected in series, the greater the impact on the discharge capacity of the battery pack, and the lower the utilization rate of the battery pack. It not only affects the charge and discharge capacity and battery life, but also easily causes thermal runaway and other faults, especially for high-power power and energy storage battery packs. The intervention of a real-time, high-efficiency battery equalizer not only intelligently adjusts the charge and discharge current and charge and discharge rate of batteries of different capacities, but also significantly improves the capacity utilization of the battery, and the effect of controlling the attenuation of the temperature rise of the battery is also very obvious. This article fully proves the importance of a high-efficiency battery equalizer in stabilizing the battery life and capacity by comparing and analyzing the experimental data of conventional discharge and equalization discharge of a set of 13 strings of 48-volt scrapped lithium batteries with seriously deteriorated consistency.

　　1 Causes of battery pack consistency problems

　　Ideally, the battery pack should have the following characteristics: when charging or discharging, the voltages of all batteries rise or fall simultaneously, and the capacity, voltage, self-discharge rate and internal resistance between batteries are very close, that is, the performance of all batteries is basically the same and consistent. The performance is very good, all batteries can be fully charged or discharged almost at the same time, and there will be no problem of overcharge or overdischarge of the battery.

　　However, in reality, most battery packs perform very poorly and have prominent consistency issues. The cycle life of the battery pack is usually only 1/3 to 1/5 of the design life of a single battery, which greatly affects the service life and endurance of the device. In serious cases, thermal runaway failure may occur, causing damage to equipment or personnel.

　　Through a large amount of experimental research and operation data analysis, it can be found that there are two main reasons for consistency problems in conventional battery packs: One reason is caused by differences in battery production processes, referred to as internal causes. Once the battery packaging is completed, the differences between batteries There are differences in capacity, self-discharge rate and internal resistance parameters, but the degree of difference is different.

　　The second reason can be called external factors, which are mainly caused by differences and fluctuations in charging and discharging voltage parameters, current parameters, and ambient temperature. These external factors will gradually accumulate and amplify the differences caused by internal factors, and the amplification of this difference will It exhibits exponential amplification characteristics, which is why consistency problems in the battery pack will quickly worsen once they occur.

　　2 Common Ways to Solve Consistency Causes

　　In terms of solving the consistency problem of the battery pack, there are two main technical solutions based on the main reasons for the battery consistency problem. One solution is to make a fuss about the battery production process and improve the battery by improving the production process level. Consistency at the factory, this solution has a certain effect and can slow down and delay the occurrence of consistency problems to the greatest extent, but it cannot be eradicated;

　　Another solution is to use a battery equalizer to intervene. Battery balancing includes passive balancing and active balancing. Passive balancing is also called energy consumption balancing. The balancing current is small and the balancing efficiency is zero. It is only suitable for applications with good consistency and uniform heat dissipation. , and the battery pack capacity is small; the typical representative of active balancing is transfer battery balancing, the balancing efficiency and balancing current are much higher than passive balancing.

　　It is foreseeable that even if active balancing is the mainstream of future development, the design architecture and implementation methods are diverse. This article will not discuss it, but one thing is certain. All design goals are to support larger balancing currents and balanced Develop in the direction of high efficiency and balanced speed.

　　3Real-time, high-speed battery balancing technology and shunt characteristics

　　In battery balancing technology, the most difficult thing to solve is the matching and balancing of balancing current and balancing efficiency. It must be able to provide a larger balancing current and have a higher balancing efficiency. The reason for this requirement is mainly because of the existence of The balancing equipment generates heat under high current balancing conditions and affects the temperature rise of the battery pack.

　　In order to solve this contradictory problem, the author has developed a unique bidirectional synchronous rectification technology [1] after years of research, which not only supports large current balancing, but also has high balancing efficiency. Under full load operation, the temperature of the equipment is reduced. The temperature rise is also low and will hardly increase the temperature rise of the battery pack.

　　Reasonable distribution and optimization of power is achieved through high-speed voltage equalization [2]. In terms of discharge equalization, this technology automatically analyzes and determines the capacity of the battery by detecting the relative voltage difference between adjacent batteries in real time, and automatically adjusts the battery capacity. Batteries with high voltage (high voltage during discharge usually have large capacity) increase the discharge current, and the increased discharge current is transported to both ends of the low-voltage battery through efficient conversion by the equalizer. For low-voltage batteries (low voltage during discharge usually has small capacity) batteries Reduce the discharge current to make up for the lack of discharge capacity of small-capacity batteries, so that batteries with different capacities can be discharged at approximately the same rate.

　　In terms of charge balancing, the charging current is automatically reduced for batteries with high voltage (high voltage during charging means small capacity). The reduced charging current is efficiently converted by the equalizer and transported to both ends of the low-voltage battery. Low voltage during the period means large capacity) The battery increases the charging current, so that batteries with different capacities can be charged at approximately the same rate; this technology can also support high-speed static equalization at the same time, increasing the effective equalization time. The unique pulse technology is very useful for stably attenuating battery capacity. Obviously, the battery equalizer used in the example of this article adopts the newly developed bidirectional synchronous rectification technology [3].

　　4 Thirteen-string lithium battery pack discharge experiment

　　The experimental battery pack is shown in Figure 1. It is assembled from scrapped lithium battery packs after disassembly, selection and echelon utilization. They are all 18650 model lithium batteries. The longest storage time is more than 8 years. The initial voltage of the disassembled battery is only The voltage ranges from a few tenths of a volt to a few tenths of a volt, and most of them are in a scrapped state. The original design capacity of a single cell is between 2200mAh and 2500mAh. Most of the batteries have heavy leakage and generate serious heat during charging.

　　When the capacity is tested for 2 hours after being fully charged, the actual remaining capacity of the 1A discharge test is only between 550mAh and 2350mAh. See the remaining capacity column in Table 1. It can be seen from the detected capacity that the capacity difference is very large, with the maximum difference reaching 1.8Ah, there are 13 experimental batteries in total, and the remaining power histogram of all batteries is shown in Figure 2.

　　The battery with the words "meter power supply" on the far right side of the experimental bench is only responsible for powering the high-precision voltmeters under the 7# and 13# batteries, and does not participate in the charge and discharge experiment. The other voltmeters are powered in cascade mode. The bottom of each battery corresponds to a high-precision voltmeter, which displays the current voltage of the upper battery in real time. The experimental platform (the platform has been modified to facilitate battery replacement) supports up to 2 and 14 strings of 18650 battery experiments. The example in this article only connected 13 strings. Battery, nominal voltage 48 volts.

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