Due to differences in internal functions, operating environments, and cycle times, each individual battery in a battery exhibits different operational characteristics, resulting in inconsistencies between individual batteries. This is primarily manifested in inconsistent terminal voltages and capacities of individual batteries.
(1) Terminal voltage refers to the voltage at both ends of a single battery during charging and discharging. Due to the internal resistance of a single battery, the terminal voltage is higher than the open circuit voltage during charging and lower than the open circuit voltage during discharging. In the charging and discharging state, the inconsistency of characteristic parameters such as battery capacity and remaining power will be manifested as the inconsistency of terminal voltage. The inconsistency of terminal voltage can better reflect the inconsistency of charging and discharging depth of individual batteries. During charging, the terminal voltage of a single battery with less rechargeable capacity is higher and will first reach the charging cut-off voltage. Conversely, the terminal voltage of a single battery with less remaining capacity during discharge is lower and will first reach the discharge stop voltage
(2) The actual capacity of a single battery indicates the maximum amount of electricity that can be charged and discharged. During the production process, there is an initial inconsistency in the actual capacity of individual batteries. Due to differences in the raw materials and production processes of individual batteries, the aging rate of each battery cell is different, resulting in increasingly severe capacity inconsistency.
During the charging process, when the terminal voltage of a single battery cell is high or the capacity is low before the other cells are fully charged, and it is in an overcharge state. During the discharge process, when the terminal voltage or capacity of a single battery cell is lower than that of other cells, it is discharged to a low voltage earlier and is in an over discharge state.
Schematic diagram of unbalanced charging and discharging and balanced charging and discharging
Unbalanced charging and discharging can have an impact on the lifespan of the battery and system safety, primarily manifested in the following aspects:
(1) When a single battery cell is fully charged before the other cells and is in an overcharge state:
Overcharging can cause a chemical reaction of electrolyzed water, which produces oxygen and hydrogen. When combustible gases are ignited or arc drawn, there is a risk of explosion;
Overcharging can cause electrolyte overflow from individual cells, leading to corrosion of battery casings or aircraft structures, and even corrosion of wires, posing a safety hazard;
Overcharging can cause a single unit temperature rise, which will trigger the battery overheating alarm system. When the crew recognizes the battery overheating, they need to disconnect the emergency power supply. When the aircraft loses the emergency power supply, it should immediately return, affecting the normal operation of the aircraft.
Frequent overcharging can lead to a decrease in the lifespan of individual cells, accelerating their aging rate and reducing the lifespan of the battery pack.
(2) When a single battery cell is discharged to low voltage earlier than the other cells and is in an overdischarged state:
Overdischarge of a single cell will result in a decrease in the overall voltage holding capacity of the battery, which will prematurely to the discharge stop voltage and shorten the emergency power supply time of the battery;
Overcharging of a single monomer can lead to a chemical reaction of electrolyzed water, which produces oxygen and hydrogen gases. When combustible gases ignite or arc, there is a risk of explosion;
4. Solutions
(1) Process measures
During the production process, the production technology of batteries should be improved, especially the selection of raw materials and components, which should be strictly controlled and handled in the same batch to ensure good consistency in the production process
Select a single battery capacity classification method, and when understanding the discharge capacity of each individual cell in each group of batteries, select a discharge time difference of less than 5 minutes as a category. Each group of batteries is screened and expanded from the same type of individual batteries using a voltage measurement method. When discharging the entire group of batteries to a stop voltage of 21V, the voltage of each individual battery should be greater than 0.9V. Adjust the equipment used for capacity detection of this type of battery, select a single point conversion device to accurately detect the capacity of each battery, and summarize and classify them. (2) Balanced charging and discharging measures for loss type: Add a parallel balancing circuit to each individual battery in the battery pack to achieve the function of diversion. When a single battery reaches full charge first, the balancing device can prevent overcharging and convert the remaining energy into heat energy to continue charging the unfilled battery. Non destructive type: The system is equipped with a set of equalization modules. When the equalization module is started, the overcharged battery will transfer the remaining power (capacitor or transformer) to the single battery that is not fully charged, completing dynamic balancing. Its efficiency is high with minimal loss, and all battery voltages are monitored throughout the entire process by the equalization module.
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