Low temperature environments pose a serious threat to the performance and safety of lithium-ion batteries. This article focuses on the extreme working condition of -20 ℃, and explores the aging and safety characteristics of batteries through cycling, charging and discharging, and thermal runaway experiments. It was found that low temperature caused a 30% increase in polarization resistance and a nonlinear change in internal resistance; There is an optimal range for the influence of charge discharge rate on capacity and internal resistance; Aging batteries still pose a risk of severe thermal runaway, with a maximum temperature of 614.2 ℃. The research results provide important basis for battery management and safety protection in low-temperature scenarios.
1.Overview
In practical application scenarios, batteries are often exposed to complex working conditions, where ambient temperature has a significant impact on lithium-ion activity. When the temperature is below 0 ℃, the viscosity of the electrolyte increases and the ion conductivity weakens, severely restricting the performance of the battery and accelerating aging. The Battery Management System (BMS) requires precise monitoring of battery status, but existing research lacks systematic exploration of extreme low temperature conditions of -20 ℃.
Low temperature not only leads to a sharp decline in electric vehicle range, but also poses safety hazards due to lithium deposition and abnormal growth of SEI film. The polarization problem of high rate charging and discharging also urgently needs experimental data to support optimization solutions. This study focuses on 18650 NCM lithium batteries and analyzes the low-temperature aging law through multivariate experiments (temperature, cycle times, rate, DOD/DOC). Combined with thermal runaway testing, it provides a basis for BMS low-temperature management and life prediction.
2.Battery Test System
2.1 Battery samples and parameters
This experiment uses a commercial 18650 cylindrical lithium battery as the research object, and its positive electrode material is a nickel cobalt manganese (NCM) ternary system. To ensure the reliability of experimental data, 100 batteries from the same production batch were strictly selected, and samples with a capacity deviation controlled within 10mAh were screened through 0.3C rate charge discharge pre-test.
2.2 Experimental Equipment and Environmental Control
(1) Cycle performance testing system
This system relies on four core devices to build a high-precision data acquisition system:
High precision battery tester (CT-4008-5V12A-DB): with ± 0.1% voltage/current measurement accuracy, supporting 8-channel synchronous charging and discharging, covering multiple charging and discharging modes such as constant current constant voltage (CC-CV) and constant current (CC);
Low temperature chamber: The minimum temperature control can reach -40 ℃, with a temperature control accuracy of ± 2 ℃, ensuring the stability of the low-temperature experimental environment;
Data acquisition unit (CA-4008-1U-VT): temperature measurement accuracy ± 0.1 ℃, voltage measurement accuracy ± 0.001V, realizing real-time dynamic monitoring of battery electrical parameters;
Intelligent control terminal: Automated control and data storage of experimental processes are achieved through supporting software.
(2) Thermal safety testing system
The HEL BTC-130 Accelerated Calorimeter (ARC) was used to study the thermal runaway characteristics of batteries. This device has a maximum tracking temperature of 500 ℃, a maximum container pressure of 4 bar, and a temperature control accuracy of 0.1%. Through software construction of an insulated environment, it accurately compensates for heat loss and achieves high-precision capture of temperature and pressure parameters during thermal runaway.
2.3 Experimental Plan Design
(1) Low temperature cyclic aging experiment
At -20 ℃, accelerated aging was carried out using CC-CV charging mode (3C constant current charging to 4.2V, converted to constant voltage until the current dropped to 0.05C) and 3C constant current discharge (discharge to 2.5V cut-off). Divide the batteries into 5 groups (6 cells per group) and set different cycle times of 0, 15, 25, 50, 75, 100, 125, and 150 to systematically study the impact of aging on the electrical performance and safety characteristics of the batteries.
(2) Multivariate Influence Experiment
Charge and discharge rate experiment: Test the effect of 0.33C-3C charge and discharge rate on battery aging process under -20 ℃ conditions, with a focus on analyzing the trends of capacity decay and internal resistance growth.
Multi temperature comparative experiment: Conduct 3C charge discharge cycles at five temperature gradients of -20 ℃, -10 ℃, 0 ℃, 25 ℃, and 45 ℃. Conduct HPPC testing every 25 cycles to dynamically monitor the changes in internal resistance.
DOD/DOC Impact Experiment: Investigating the effects of different charging and discharging voltage windows, such as 2.5V-3.8V and 2.5V-4.2V, on low-temperature aging, and exploring the intrinsic correlation between voltage range and battery life.
(3) Thermal runaway test
Using the “Heating Wait Search” (H-W-S) adiabatic method, with a starting temperature of 40 ℃ and a temperature increase of 5 ℃ each time, the maximum search temperature is 350 ℃. Calibration lasts for 30 minutes and search lasts for 5 minutes. Accurately record the self heating starting temperature T1 (self heating rate ≥ 0.02 ℃/min), thermal runaway starting temperature T2 (heat release rate ≥ 1 ℃/s), and maximum thermal runaway temperature T3 to assess the thermal safety risks of aging batteries.
(4) HPPC internal resistance test
After pre charging the battery to 100% SOC, apply pulses at intervals of 10% SOC (10 seconds 1C discharge → 40 seconds static → 10 seconds 0.75C charge), and quantify the changes in electrochemical activity inside the battery by calculating the ohmic resistance Ro and polarization resistance Rp.