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 glow without considering the consistency. However, once the battery is applied on a large scale in the energy storage system, the situation is not so simple. If batteries with inconsistency are connected in series and parallel, the following problems will occur:
1.Loss of usable capacity
In the energy storage system, battery cells (that is, battery cells) are connected in series to form a battery pack, and battery packs are connected in series to form a battery cluster, and multiple battery clusters are directly connected in parallel to the same DC busbar. The reasons for the loss of available capacity caused by the inconsistency of the cells include inconsistency in series and inconsistency in parallel;
·Battery pack series inconsistency loss:
The SOC (remaining power) of each battery pack will be different due to inconsistencies such as differences in the battery cells and temperature differences between the battery packs. As long as one battery pack is full/empty, all battery packs in the cluster will stop charging and discharging.
Inconsistency loss of battery cluster parallel connection:
After the battery packs are directly connected in parallel to form a battery cluster, the voltage of each battery cluster is forced to balance. When the battery cluster with a small internal resistance is fully charged or discharged, the other battery clusters must stop charging and discharging, resulting in insufficient charging and discharge of the battery clusters. .
In addition, due to the small internal resistance of the battery, even if the voltage difference between the clusters caused by inconsistency is only a few volts, the uneven current between the clusters will be very large. As shown in the measured data of a power station in the table below, the charging current difference reaches 75A ( Compared with the theoretical average value, the deviation is up to 42%), and the deviation current will cause overcharge and overdischarge in some battery clusters; it will greatly affect the charge and discharge efficiency, battery life, and even lead to serious safety accidents.
2.The life of the energy storage system is shortened
Temperature is the most critical factor affecting the life of energy storage. When the internal temperature of the energy storage system increases by 15°C, the life of the energy storage will be shortened by more than half. Lithium-ion batteries will generate a lot of heat during charging and discharging. Due to the inconsistent internal resistance of individual batteries, the internal temperature distribution of the energy storage system will be unbalanced, and the aging rate of the battery will be accelerated, which will eventually shorten the life of the energy storage system.
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.