As the main equipment for urban and rural freight, field operations and short-distance heavy-load transportation, electric tricycles have long been operating under high-intensity working conditions such as frequent start-stop, steep slope climbing and all-weather continuous operation, which put forward extremely high requirements for the stability, consistency and durability of energy storage systems. The 72V 180Ah large-capacity battery pack is the mainstream configuration for heavy-duty tricycles. With the characteristics of large capacity, long cruising range and strong power, it adapts to the needs of long-term and high-load operations. This high-voltage and large-capacity battery pack is composed of dozens of single cells in series and parallel. The performance upper limit and service life of the entire system completely depend on the parameter consistency of single cells. Under the influence of long-term cycle use, temperature difference changes and working condition loss, single cells will gradually produce subtle differences in voltage, capacity, internal resistance and self-discharge rate, namely cell parameter deviation. Most common failures encountered by users, such as sudden cruising range drop, weak power, uneven charging, early power-off protection and local battery overheating, are not caused by single cell damage, but overall pack imbalance resulting from continuous expansion of cell parameter deviation. An in-depth understanding of the causes and hazards of deviation and adherence to regular correction and maintenance are the key to ensuring the long-term stable operation of battery packs.

　　Cell parameter deviation is mainly divided into four core types: voltage deviation, capacity deviation, internal resistance deviation and self-discharge deviation. These four types of parameters are interrelated and mutually influential, jointly determining the overall working state of the battery pack. Voltage deviation is the most intuitive manifestation of imbalance, referring to the numerical difference between static voltage and dynamic working voltage of single cells in series-parallel modules. In a healthy state, the voltage difference of the entire cell pack should be controlled within a minimal range. Once the voltage difference exceeds the standard, some cells are fully charged in advance during charging, and some cells are depleted in advance during discharging, directly limiting the available capacity of the entire battery pack. Capacity deviation refers to the numerical difference in actual releasable power of cells. Affected by production processes and later loss, the actual capacity of different cells will diverge. Following the bucket effect, the actual available capacity of the entire battery pack is determined by the weakest cell with the lowest capacity, greatly reducing the cruising mileage. Internal resistance deviation is the most concealed. The difference in cell internal resistance leads to uneven distribution of charging and discharging current. Cells with high internal resistance generate more heat and higher loss during operation, forming a vicious cycle of increased internal resistance, temperature rise and accelerated aging. Self-discharge deviation causes different power loss rates of cells during static storage, and the voltage difference will expand after long-term idling, further aggravating overall imbalance.

　　The generation of cell parameter deviation is divided into factory inherent deviation and later working condition derived deviation, among which improper later use is the main cause of continuous deviation expansion. At the factory stage, even cells from the same batch have tiny initial parameter differences due to the influence of raw material ratio, electrode coating accuracy, electrolyte injection volume and winding process errors, which are inevitable process characteristics. Formal manufacturers control the initial deviation within a compliant range through capacity screening to ensure factory consistency. However, in actual heavy-load working conditions, the deviation will be continuously amplified. The 72V 180Ah large-capacity battery pack has a large number of cells and dense modules. During operation, the internal temperature distribution of the battery pack is uneven. Cells in the center and edge positions have different heat dissipation conditions. Temperature differences lead to differentiated aging rates and gradual expansion of cell parameter deviation. At the same time, frequent heavy-load start-up, high-current discharge during slope climbing and irregular deep charge and discharge in tricycle operation cause uneven current load on each cell. After long-term accumulation, the initial tiny deviation will evolve into obvious parameter imbalance, completely breaking the working balance of the battery pack.

　　Ambient temperature fluctuation and lack of idle maintenance are invisible core factors accelerating the expansion of cell parameter deviation. All parameters of lithium battery cells are highly sensitive to temperature. High temperature accelerates the rise of cell internal resistance and capacity attenuation, while low temperature increases ion migration resistance and reduces cell activity. Most electric tricycles are parked outdoors and operated in open environments. Extreme temperature differences such as summer sun exposure and winter severe cold lead to differentiated aging of cells in different positions of the battery pack. Cells working in high-temperature areas have faster internal resistance growth and greater capacity loss, while cells frequently operating in low-temperature environments are more prone to activity attenuation, eventually leading to continuous expansion of parameter differences among cells. In addition, many users leave vehicles idle for a long time without balanced maintenance. During battery static storage, each cell has a different self-discharge rate and power loss speed. The original tiny parameter deviation will accumulate continuously during idling, resulting in a series of imbalance faults such as sudden power jump, virtual cruising range and intermittent power after reuse, seriously affecting operational efficiency.

　　Continuous expansion of cell parameter deviation will cause irreversible multiple hazards to the 72V 180Ah battery pack and greatly shorten the service life of equipment. Firstly, the cruising ability is greatly reduced. Limited by weak cells, the battery pack cannot fully release its rated capacity, and the original sufficient cruising mileage will continue to decrease, especially under heavy-load operation. Secondly, the dynamic performance continues to decay. The uneven current distribution caused by internal resistance deviation reduces the effective output power, resulting in weak start-up power, insufficient climbing power and heavy-load stuttering, failing to meet the needs of high-intensity freight operations. More seriously, parameter imbalance aggravates local cell overheating. High internal resistance cells accumulate serious heat during operation. Long-term high-temperature operation accelerates electrolyte decomposition and thickening of the negative electrode passivation layer, triggering cell bulging and accelerated aging, and even increasing the risk of thermal runaway. Meanwhile, excessive cell parameter deviation will frequently trigger overvoltage, undervoltage and overtemperature protection of the battery management system, leading to sudden power failure during driving and early charging termination, which not only affects the use experience, but also buries potential operational safety hazards.

　　Irregular daily charge and discharge operations are human-induced core factors that aggravate cell parameter deviation and accelerate premature aging of battery packs. Many users habitually perform deep discharge and recharge only after the power is completely exhausted. Deep power deficit will cause priority damage to weak cells, further deteriorating capacity and internal resistance parameters and widening the gap with high-quality cells. Some users use non-original chargers with mismatched parameters, resulting in unstable charging current and voltage, uneven charging of the entire cell pack, overcharging of some cells and undercharging of others. After long-term repeated cycles, the parameter consistency is completely destroyed. In addition, bad habits such as long-term full-power storage, overnight floating charging and immediate charging in high-temperature environments keep cells in a long-term high-pressure and high-temperature stress state, intensifying the differentiation of loss rates among different cells. Slight initial parameter deviation deteriorates rapidly, eventually leading to premature scrapping of the entire battery pack and greatly increasing equipment operation, maintenance and replacement costs.

　　According to the working condition characteristics of the 72V 180Ah large-capacity battery pack, the establishment of a regular correction and maintenance mechanism can effectively inhibit and repair slight cell parameter deviation and maintain module parameter consistency. In daily operations, avoid deep discharge and replenish power in a timely manner when the remaining power is 20%, adhere to shallow charging and shallow discharging to reduce cell loss under extreme working conditions. Refrain from immediate charging after heavy-load slope climbing or high-temperature sun exposure, and recharge only after the cell temperature drops to normal temperature to reduce parameter differentiation caused by temperature difference. Always use original special chargers matching the charging parameters of high-voltage large-capacity battery packs to ensure balanced constant current and constant voltage charging. Meanwhile, carry out cell balance calibration maintenance every two to three months, and correct single-cell voltage and capacity deviation through low-current complete charge and discharge cycles to balance module parameter consistency. During long-term idling, maintain the battery power at the optimal range of about 50%, store it in a cool and ventilated environment, and perform regular power replenishment and calibration to avoid accumulated deviation during static storage.

　　For battery packs with obvious parameter deviation, professional equipment should be used for active balance repair instead of long-term operation with faults. The passive balance function of the battery management system can only correct slight voltage differences. For capacity and internal resistance deviation caused by aging, professional testing equipment is required to screen weak cells, accurately locate single cells with abnormal parameters, and repair parameter consistency through active balance and capacity calibration. If there are seriously aged or failed cells, timely replacement of matched cells is necessary to avoid dragging down the performance of the entire battery pack by a single weak cell. Adhering to scientific maintenance and regular calibration can continuously control cell parameter deviation within the compliant range, maximize the restoration of the rated capacity and dynamic performance of the 72V 180Ah battery pack, stabilize the cruising range and power output of heavy-load operations, greatly extend the cycle service life of the battery pack, and provide reliable guarantee for the long-term and stable operation of electric tricycles.