Lithium-ion cells have similar methods for determining capacity and testing battery discharge performance, including constant current discharge, constant resistance discharge, constant voltage discharge, fixed voltage, fixed current discharge, continuous discharge, and intermittent discharge. The battery capacity can be calculated based on the discharge time and current.
The advantage of this method is that the current is stable during discharge, making it easy to calculate the capacity. The discharge capacity of the constant current method has a great relationship with the discharge current, and the charging system, discharge system, and idle time all affect the capacity. Under the same discharge system, different charging systems have different charging efficiencies for the battery, therefore, the discharge capacity of lithium-ion cells will also vary.
During the constant resistance method of testing capacity, the discharge current is not constant. The discharge current is initially high before gradually decreasing. The larger the discharge resistance, the smaller the discharge current, and the smaller the voltage drop that is generated. The working voltage decreases slowly and the discharge curve is flatter, resulting in a larger discharge capacity.
Batteries of different types and different ages should not be mixed to avoid overcharging and overdischarging due to mismatched capacities. Additionally, in a battery pack, the overall performance is generally determined by the worst-performing battery. For a primary battery, capacity testing is destructive, so product consistency can only be maintained through strict production control. For secondary batteries, in addition to strict production process control, sorting testing should also be used to ensure battery capacity consistency.
The so-called sorting involves dividing lithium-ion cells into certain capacity ranges. Sorting testing can generally be divided into two types: capacity sorting and characteristic sorting. Characteristic sorting, also known as curve sorting, is an extension of capacity sorting, where batteries in the same capacity range are distinguished based on different voltage characteristics.
Secondary battery sorting is generally achieved through computer control. Most automatic sorting equipment is equipped with corresponding software, making these operations easy to perform. Manual sorting equipment can generally only meet the needs of capacity sorting, but cannot perform characteristic sorting, even in the case of capacity sorting, which requires a lot of manpower and is operationally cumbersome. Nowadays, most secondary battery manufacturers use automatic sorting equipment for sorting operations, and the conditions for sorting are set by the computer and then sent to the equipment, which indicates the batteries that meet the conditions.
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