The battery system is the core of the entire energy storage system, consisting of hundreds of cylindrical cells or prismatic cells in series and parallel. The inconsistency of the energy storage batteries mainly refers to the inconsistency of parameters such as battery capacity, internal resistance, and temperature. When batteries with inconsistencies are used in series and parallel, the following problems will occur:

1. Loss of available capacity

In the energy storage system, the single cells are connected in series and parallel to form a battery box, the battery boxes are connected in series and parallel to form a battery cluster, and multiple battery clusters are directly connected to the same DC busbar in parallel. The causes of battery inconsistency leading to loss of usable capacity include series inconsistency and parallel inconsistency.

•Battery series inconsistency loss
According to the barrel principle, the series capacity of the battery system depends on the single battery with the smallest capacity. Due to the inconsistency of the single battery itself, temperature difference and other inconsistencies, the usable capacity of each single battery will be different. The single battery with small capacity is fully charged when charging and emptied when discharging, which restricts the charging of other single batteries in the battery system. Discharge capacity, resulting in a decrease in the available capacity of the battery system. Without effective balanced management, with the increase of operating time, the attenuation and differentiation of single battery capacity will be intensified, and the available capacity of the battery system will further accelerate the decline.

•Battery cluster parallel inconsistency loss

When the battery clusters are directly connected in parallel, there will be a circulating current phenomenon after charging and discharging, and the voltages of each battery cluster will be forced to balance. Dissatisfaction and inexhaustible discharge will cause battery capacity loss and temperature rise, accelerate battery decay, and reduce the available capacity of the battery system.

In addition, due to the small internal resistance of the battery, even if the voltage difference between clusters caused by inconsistency is only a few volts, the uneven current between clusters will be large. As shown in the measured data of a power station in the table below, the difference in charging current reaches 75A ( Compared with the theoretical average, the deviation is 42%), and the deviation current will lead to overcharge and overdischarge in some battery clusters; it will greatly affect the charging and discharging efficiency, battery life, and even lead to serious safety accidents.

2.Accelerated differentiation and shortened life of single cells caused by inconsistent temperature

Temperature is the most critical factor affecting the life of the energy storage system. When the internal temperature of the energy storage system increases by 15°C, the life of the system will be shortened by more than half. The lithium battery will generate a lot of heat during the charging and discharging process, and the temperature difference of the single battery will further increase the inconsistency of internal resistance and capacity, which will lead to the accelerated differentiation of the single battery, shorten the cycle life of the battery system, and even cause safety hazards.

How to deal with the inconsistency of energy storage batteries?

Battery inconsistency is the root cause of many problems in current energy storage systems. Although battery inconsistency is difficult to eradicate due to the chemical characteristics of batteries and the impact of the application environment, digital technology, power electronics technology and energy storage technology can be integrated to use electricity. The controllability of electronic technology minimizes the impact of lithium battery inconsistencies, which can greatly increase the usable capacity of energy storage systems and improve system safety.

•Active balancing technology monitors the voltage and temperature of each single battery in real time, maximally eliminates the inconsistency of battery series connection, and increases the available capacity of the energy storage system by more than 20% in the entire life cycle.

•In the electrical design of the energy storage system, the charge and discharge management of each cluster of batteries is carried out separately, and the battery clusters are not connected in parallel, which avoids the circulation problem caused by the parallel connection of DC, and effectively improves the available capacity of the system.

•Precise temperature control to extend the life of the energy storage system

The temperature of each single cell is collected and monitored in real time. Through three-level CFD thermal simulation and a large amount of experimental data, the thermal design of the battery system is optimized, so that the maximum temperature difference between the single cells of the battery system is less than 5 °C, and the problem of single cell differentiation caused by temperature inconsistency is solved.

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