How important is the consistency of lithium-ion batteries?

Recently, the National Energy Administration of China issued a public solicitation of opinions on the “New Energy Storage Project Management Specifications (Interim) (Draft for Comments)”. In this draft for comments, one point is particularly emphasized: in principle, no new large-scale power battery cascade utilization energy storage projects shall be built before key breakthroughs are made in battery consistency management technology and the power battery performance monitoring and evaluation system is sound.

Why is battery consistency specifically mentioned in the draft?

The inconsistency of energy storage batteries mainly refers to the inconsistency of parameters such as battery capacity, internal resistance, and temperature. Our daily experience is that when two dry batteries are connected positively and negatively, the flashlight will light up, and consistency will not be considered. However, once batteries are used on a large scale in energy storage systems, the situation is not so simple. When inconsistent batteries are used in series and parallel, the following problems will occur:

1) Loss of available capacity

In energy storage systems, battery cells (i.e. battery monomers) are connected in series to form battery packs, battery packs are connected in series to form battery clusters, and multiple battery clusters are directly connected in parallel to the same DC busbar. The reasons for the loss of available capacity due to cell inconsistency include series inconsistency and parallel inconsistency;

Battery pack series inconsistency loss:

Due to the differences in the cells themselves and temperature differences between battery packs, the SOC (remaining power) of each battery pack will be different. As long as one battery pack is full/empty, all battery packs in the cluster will stop charging and discharging.

Battery cluster parallel inconsistency loss:

After the battery packs are directly connected in parallel to form a battery cluster, the voltage of each battery cluster is forced to be balanced. When the battery cluster with smaller internal resistance is fully charged or discharged, the other battery clusters must stop charging and discharging, resulting in insufficient charging and discharging between battery clusters.

In addition, due to the small internal resistance of the battery, even if the voltage difference between each cluster 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 charging current difference reaches 75A (the deviation is 42% compared with the theoretical average value). The deviation current will cause overcharge and over-discharge in some battery clusters; it greatly affects the charging and discharging efficiency, battery life, and even leads to serious safety accidents.

2) The life of the energy storage system will be shortened

Temperature is the most critical factor affecting the life of energy storage. When the internal temperature of the energy storage system rises by 15°C, the energy storage life will be shortened by more than half. Lithium-ion batteries generate a lot of heat during the charging and discharging process. Due to the inconsistent internal resistance of the single cells, the temperature distribution inside the energy storage system will be uneven, the battery aging and attenuation rate will increase, and ultimately the life of the energy storage system will be shortened.

It can be seen that the temperature inconsistency of the battery in the energy storage system is an important factor affecting the performance of the energy storage system. It will reduce the available capacity of the energy storage system, shorten the cycle life of the energy storage system, and even cause safety hazards.

How to deal with the inconsistency of energy storage batteries?

The inconsistency of battery cells is formed during the production process and deepened during use. The weaker the battery cells in the same battery pack, the weaker they are, and they will become weaker faster. However, although there are no completely consistent battery cells, digital technology, power electronics technology and energy storage technology can be integrated to minimize the impact of lithium battery inconsistency with the controllability of power electronics technology. In response to the problems caused by the inconsistency analyzed in the previous article, some manufacturers on the market have launched string energy storage systems with the characteristics of refined energy management and distributed temperature control, which can be used to treat the symptoms:

1) Refined management to increase available capacity

Compared with the traditional PCS managing more than 1000-2000 battery cells, the string energy storage system increases the accuracy of battery cell management to more than a dozen, which is about 100 times higher. For the series mismatch between battery packs, the optimizer design is used to achieve separate charge and discharge management of each battery pack. When a battery pack reaches the set threshold, the battery pack is bypassed, and other battery packs can continue to charge and discharge without affecting each other, maximizing the use of battery capacity.

At the same time, each battery cluster is equipped with an intelligent cluster controller to avoid the impact of battery inconsistency caused by direct parallel connection, so that the charge and discharge current of each cluster can be accurately controlled with an error of less than 1%. This avoids the mismatch between clusters in parallel connection, truly realizes independent charge and discharge management between battery clusters, eliminates the generation of circulating current, and further improves the capacity and safety of the system.

2) Distributed temperature control to extend the life of the energy storage system

Traditional energy storage containers are equipped with 1-2 centralized air conditioners, which use longitudinal air ducts for heat dissipation. The length of the air duct is about 6 meters to 12 meters. Due to the long heat dissipation channel, the temperature consistency of each battery pack and battery cluster cannot be guaranteed. String energy storage uses cluster-level distributed heat dissipation, using distributed air conditioners instead of centralized air conditioners. Each battery cluster can dissipate heat independently and evenly. The air duct length is less than 1 meter, which greatly improves the heat dissipation efficiency and avoids the temperature difference caused by the physical location. At the same time, the battery pack cleverly uses a tree-shaped bionic patented heat dissipation duct to adjust the length and distance of each battery cell air duct to make the cold amount passed by each battery cell as consistent as possible, reducing the temperature inconsistency of each surface of each battery cell.

Battery inconsistency is the root cause of many problems in current energy storage systems. However, due to the chemical characteristics of batteries and the influence of the application environment, battery inconsistency is difficult to eradicate. The controllability of power electronics and digital technology greatly weakens the system’s requirements for battery consistency, which can greatly increase the available capacity of the energy storage system and improve system safety.