What Is Cell Performance?
Cell Performance
The cell performance is mainly divided into four aspects: electrical performance, safety performance, mechanical performance, and environmental performance.
Electrical Performance
(1) Rated Capacity Discharge at 0.5C, the discharge time of a single cell shall not be less than 120 min (≥90%), and the discharge time of the battery pack shall not be less than 114 min (≥90%). This indicator is one of the core items in lithium battery rated capacity supplier delivery standard.
(2) 1C Discharge Capacity Discharge at 1C, the discharge time of a single cell shall not be less than 57min (95%), and the discharge time of the battery pack shall not be less than 45min (90%).
(3) Low-Temperature Discharge Capacity Discharge at 0.5C at -20 °C, the discharge time of a single cell or battery pack shall not be less than 72 min (≥60%). Low-temperature performance directly affects the winter range of new energy vehicles and is a key concern in power battery low temperature discharge specification.
(4) High-Temperature Discharge Capacity Discharge at 0.5C at 55°C, the discharge time of a single cell shall not be less than 114min (95%), and the discharge time of the battery pack shall not be less than 108min (90%).
(5) Charge Retention and Recovery Capability After being stored at room temperature at full charge for 28 days, the discharge time for charge retention shall not be less than 96 min (≥80%), and the discharge time for charge recovery shall not be less than 108 min (≥90%). This is an important reference indicator for energy storage battery 28-day charge retention rate.
(6) Storage Performance The single cell or battery pack selected for the storage test shall be less than three months from the date of manufacture, and shall be charged to 50%～60% of its capacity before storage. It shall be stored for 90 days in an environment with a temperature of 40°C ± 5°C and a relative humidity of 45%～75%. After the storage period, the battery pack is taken out, fully charged at 0.2C and allowed to stand for 60min, and then discharged at a constant current of 0.5C to the cut-off voltage. The above experiment can be repeated 3 times, and the discharge time shall not be less than 72min (60%).
(7) Cycle Life The single cell or battery pack is charged at 0.2C and discharged at 0.5C for cycling. The test is stopped when the discharge capacity is lower than 72 min (60%) for two consecutive times. The cycle life of the single cell shall not be less than 600 cycles, and the cycle life of the battery pack shall not be less than 500 cycles. For most car companies and energy storage projects, power battery pack 500-cycle life has become the basic threshold for supplier selection.
(8) High-Temperature Storage Life A single cell less than three months from the date of manufacture shall be selected for the high-temperature storage life test. Before storage, 50% ± 5% of the capacity is charged, and then the cell is stored for 7 days at an ambient temperature of 55 °C ± 2 °C. After 7 days, the battery is taken out and allowed to stand for 120–300 min at an ambient temperature of 20 °C ± 5 °C. First, the battery is discharged to the cut-off voltage at 0.5C, then charged at 0.2C after 30 min, allowed to stand for 30 min, and then discharged at a constant current of 0.5C to the cut-off voltage, and this capacity is taken as the recovery capacity. The above steps constitute a 1-week cycle, and the experiment ends when the discharge time is less than 72 min (60%) in a certain week. The storage life shall not be less than 56 days (8 weekly cycles). This test is listed as a mandatory item in most lithium iron phosphate battery high-temperature storage life supplier audit requirements.
Safety Performance
(1) Continuous Charging The single cell is charged with a constant current of 0.2C. When the terminal voltage of the single cell reaches the charging limit voltage, it is switched to constant voltage charging and maintained for 28 days. After the experiment, the battery shall not leak, vent, rupture, catch fire, or explode (equivalent to full charge float charging).
(2) Overcharge The single cell is charged with a constant current of 3C using a constant current source, and when the voltage reaches 10V, it is switched to constant voltage charging until the battery explodes or catches fire. The charging is stopped when the charging time is 90min or the battery surface temperature stabilizes (temperature difference ≤ 2°C within 45min). The battery shall not catch fire or explode; the battery pack is charged with a constant current of 0.5C using a regulated power supply, and when the voltage reaches 5nV (n is the number of series cells), it is switched to constant voltage charging until the battery pack explodes or catches fire. The charging is stopped when the charging time is 90min or the battery pack surface temperature stabilizes (temperature difference ≤ 2°C within 45min). The battery shall not catch fire or explode.
(3) Forced Discharge (Reverse Charging) The single cell is first discharged at a constant current of 0.2C to the cut-off voltage, and then the cell is reverse charged with a 1C current. The charging time is required to be not less than 90min. The battery shall not catch fire or explode; one single cell in the battery pack is discharged to the cut-off voltage, and the rest are fully charged cells, and then the battery pack is discharged at a constant current of 1C until the voltage of the battery pack is 0V, and the discharge is stopped. The battery shall not catch fire or explode.
(4) Short Circuit Test The single cell is externally short-circuited for 90min, or until the cell surface temperature stabilizes (temperature difference ≤ 2°C within 45min), and then the short circuit is stopped. The external circuit resistance shall be less than 50mΩ, and the battery shall not catch fire or explode; connect the positive and negative poles of the battery pack with a wire with a resistance of less than 0.1Ω until the battery pack voltage is less than 0.2V or the battery pack surface temperature stabilizes (temperature difference ≤ 2°C within 45min), and then the discharge is stopped. The battery shall not catch fire or explode.
Mechanical Performance
(1) Extrusion Place the single cell in the middle of two pressing plates and gradually increase the pressure to 13kN. For cylindrical cells, the extrusion direction shall be perpendicular to the cylinder axis; for prismatic cells, the wide and narrow faces of the cell shall be extruded. Each cell can only be subjected to one extrusion, and the test results shall comply with the provisions of relevant standards. Place a steel rod with a diameter of 15cm on the battery pack to press the wide and narrow faces of the battery pack, extrude it to 85% of the original size of the battery, and maintain for 5min. Each battery pack can only be subjected to one extrusion.
(2) Penetration Place the single cell in a steel fixture, and use a steel nail with a diameter of φ3～8mm to penetrate the cell from a direction perpendicular to the cell plate (the steel nail remains in the cell), continue for 90min, or stop the experiment when the cell surface temperature stabilizes (temperature difference ≤ 2°C within 45min).
(3) Heavy Object Impact Place the single cell on a rigid plane, and place a steel rod with a diameter of 15.8mm flat on the center of the cell, with the longitudinal axis of the steel rod parallel to the plane. A heavy object with a mass of 9.1kg is allowed to free fall from a height of 610mm onto the steel rod in the center of the cell; when the single cell is cylindrical, the impact direction shall be perpendicular to the longitudinal axis of the cylinder; when the single cell is prismatic, the wide and narrow faces of the cell shall be impacted. Each cell can only be subjected to one impact.
(4) Mechanical Shock The battery or battery pack shall be rigidly fixed (the method can support all fixed surfaces of the battery or battery pack) and secured to the experimental equipment. The battery or battery pack shall be subjected to one equivalent shock in three mutually perpendicular directions. At least one direction must be perpendicular to the wide face of the battery or battery pack. Each shock shall be conducted as follows: the minimum average acceleration shall be 725m/s² within the initial 3ms, and the peak acceleration should be between 1225～1715m/s² between.
(5) Vibration The battery or battery pack is directly mounted or installed on the vibration table through a fixture for vibration testing. The test conditions are: frequency 10～55Hz, acceleration 29.4m/s², XYZ three directions, 10 sweep cycles in each direction, and the sweep rate is 1oct/min.
(6) Free Drop The single cell or battery pack is allowed to free drop from a height (lowest point height) of 600mm onto a 20mm thick hard wooden board placed on a cement floor, once from each of the XYZ three directions.
Environmental Performance
(1) High Temperature Baking Place the single cell in a high-temperature explosion-proof box, and raise the temperature to 130°C at a rate of 5°C/min, and keep it at this temperature for 10min.
(2) High Temperature Storage Place the single cell or battery pack in an oven at (75 ± 2)°C for 48h. The battery shall not leak, vent, rupture, catch fire, or explode.
(3) Low Pressure Meet the UL1642 (Underwriters Laboratories Inc.) standard. The battery is stored for 6h under the condition of an absolute pressure of 11.6kPa and a temperature of 20°C ± 3°C. The battery shall not catch fire or explode, and there shall be no perforation or leakage.
