For some important facilities' backup power supply, the basic requirement is to ensure timely and stable electricity supply when the power system fails, and to charge the battery pack quickly when the power system returns to normal, so that necessary electricity can be provided in a short period of time when the power system fails again.
The backup power supply for communication is based on the barrel principle and actual situation. Its released capacity depends on the battery with the smallest capacity in the lithium battery packs. Therefore, the balancing method should ensure that each battery in the battery pack is fully charged, so that the discharge capacity of the battery pack can be maximized.
The overall balance time of lithium battery packs is calculated from the moment when the charging current of the first battery decreases. The decreasing part of the charging current is switched to the balanced load of the constant voltage source through the relay switch, and the balanced load current begins to rise. As the charging proceeds, the charging current of other batteries also gradually decreases and enters a fully balanced charging state. When the charging current of all batteries drops to 0.001 C, the balanced charging process ends. The balanced current of each battery in the 0.1 C-rate battery pack is calculated by subtracting the actual charging current of the test from 2 A (0.1 C), and the balanced current curve is shown in the figure.
The method to obtain the balanced current curve of 0.25 C is the same as that of 0.1 C. The balanced current curve is shown in the figure.
The method for obtaining the balanced current curve of 0.5 C-rate is the same as that of 0.1 C-rate. The balanced current curve is shown in the figure.
At the moment when the balancing circuit is cut in, the balancing current quickly rises and then falls back to a relatively stable value, and then increases according to the standard exponential curve, making the front end of the balancing current curve generate a spike. From the monitored charging current data, it can be seen that when the balancing threshold voltage is high and the charging current is large, the balancing current value exceeds the current value set by the constant current charging, and the charged battery has an instant discharge phenomenon. However, this process is very short and can be ignored for the entire charging process. The balancing energy consumption of the lithium battery packs is calculated by software, and the balancing time and energy consumption of the battery pack for balanced charging experiments are shown in Table 1.