Charge and discharge cut-off voltages are directly linked to battery safety and service life. Their setting requires a balance between material properties and operating conditions. This paper analyzes the voltage threshold design logic for systems such as LFP and NCM, and elaborates on the coordination mechanism between temperature adaptability and multi-stage hierarchical protection of BMS.
1. Material Systems and Electrochemical Properties
The cathode and anode materials define the voltage window, as different cathode-anode combinations feature distinct electrochemical stability windows. Examples:
Lithium Iron Phosphate (LFP) system: Standard charge cut-off voltage = 3.65 V; discharge cut-off voltage = 2.5 V (ambient temperature) or 2.0 V (low temperature).
Ternary Material (NCM/NCA) system: Charge cut-off voltage = 4.2 V; discharge cut-off voltage ranges from 2.75 V to 3.0 V.
Lithium Titanate (LTO) system: Charge cut-off voltage = 2.9 V; discharge cut-off voltage = 1.5 V.
Risks of Overcharging and Over-discharging
Excessively high charging voltage triggers cathode structural degradation, oxygen evolution and electrolyte decomposition.
Excessively low discharging voltage breaks the anode SEI film and causes current collector corrosion.
2. Safety Protection Mechanisms
Hierarchical protection design (taking LFP as an example):
Normal charge termination voltage: 3.65 V - BMS stops charging once this value is reached.
Primary overcharge protection: ≥ 3.8 V - charging is forcibly terminated.
Secondary overcharge protection: ≥ 4.0 V - BMS locks the system to prevent thermal runaway.
Normal discharge termination voltage: 2.5 V - discharge ceases once this value is reached.
Primary over-discharge protection: ≤ 2.0 V - discharge is forcibly terminated.
Secondary over-discharge protection: ≤ 1.8 V - BMS locks the system and manual reset is required.
3. Temperature Adaptability
Adjustment for low-temperature environments: The discharge cut-off voltage drops as temperature decreases. For LFP cells:
Temperature T > 0 °C: 2.5 V
Temperature T ≤ 0 °C: 2.0 V This prevents premature discharge cutoff caused by polarization at low temperatures.
4. Service Life and Performance Optimization
Impacts on cycle life
Raising the charge cut-off voltage (e.g., lifting LFP's limit from 3.65 V to 4.0 V) accelerates capacity fade.
Excessively low discharge cut-off voltage (e.g., below 2.85 V for LFP) leads to loss of active lithium on the anode.
Formation process
The pre-charging cut-off voltage shall be controlled within the SEI film formation range (e.g., 2.8–3.0 V) to avoid side reactions induced by impurities.
Conclusion
The determination of cut-off voltages is a comprehensive outcome of material characteristics, safety requirements, cycle life, temperature adaptability and industry standards. Manufacturers verify voltage thresholds via electrochemical testing, and BMS implements hierarchical protection accordingly to guarantee battery operation within a safe voltage window.
