In daily use, the charging and discharging management of batteries not only affects the endurance mileage, but also directly affects the safe and reliable operation of vehicles.
Among them, the setting of charging and discharging cut-off voltage is the key link in the battery management system (BMS), which is like the “safety valve” of the battery, and needs to find a precise balance between energy release and long-term durability.
We will deeply analyze how to scientifically set these voltage parameters based on the characteristics of battery core materials, system performance objectives and safety bottom line.
Whether you are an automotive engineer, a technology enthusiast, or an ordinary car owner, understanding this topic will help you improve your understanding of electric vehicle and avoid the risks caused by improper use. Let’s start from the basic principle and reveal the mystery of battery cut-off voltage setting step by step.
Cell material characteristics
The basic interval of the charge and discharge cut-off voltage of the battery is determined by the characteristics of the cathode material, the anode material and the electrolyte of the battery core:
1.Charging cut-off voltage (upper voltage limit)
Battery charging is the application of an external power source to the battery, which causes lithium ions to be removed from the positive electrode material, transported through the electrolyte, passed through the separator, and then embedded in the negative electrode material; (electrons “flow” from the positive electrode to the negative electrode through an external circuit).
The charge cut-off voltage is determined by the cathode material, and overcharge will lead to irreversible structural phase transition, lattice collapse, and violent oxygen-producing side reactions in the cathode material.
Batteries made of different materials have different charging cut-off voltages:
1) NCM battery
Charge cutoff voltage is typically: 4.2 V
The high-voltage version of NCM improves the structural stability of the material through element doping and coating modification; (for example, the cut-off voltage of positive battery is 4.4 V; the purpose of high voltage is to improve the energy density)
2) LFP battery
Typical charge cutoff voltage: 3.65 V
2.Discharge cut-off voltage (lower voltage limit)
The anode material determines the discharge cut-off voltage. Over-discharge will lead to the destruction of the layered structure of the anode graphite, and the copper foil of the current collector may dissolve and form deposits during subsequent charging, which will pierce the diaphragm and cause safety risks.
The discharge cut-off voltage of the graphite negative electrode is generally 2.0 V to 2.5 V
To sum up, for the charge-discharge cut-off voltage of the battery, a theoretical voltage window [Vmin, Vmax] is first determined according to the positive and negative electrode material system of the battery core.
Ternary NCM battery, typically: 2.8 ~ 4.2 V
Lithium iron phosphate LFP battery, typically: 2.5 ~ 3.65 V
Battery performance
Setting the cut-off voltage of battery charging and discharging is essentially a trade-off between “how much electricity can be discharged at a time” and “battery cycle life”.
1.High charge cut-off voltage:
1) Advantages: More lithium ions can be released to increase the battery capacity and energy density, so as to increase the range of the vehicle.
2) Disadvantages: Aggravate the side reaction of the positive electrode, accelerate the consumption of electrolyte and the increase of interface impedance, resulting in accelerated capacity decay and shortened life, and there are safety risks.
2.Low discharge cut-off voltage:
1) Advantages: Increase the capacity of the battery.
2) Disadvantages: security risks.
Through a large number of cycle aging tests, the battery development formulates the curve of “cut-off voltage-cycle life”, and selects to meet the capacity attenuation requirements of the vehicle life cycle; (for example, 8 years/160,000 km) capacity attenuation ≤ 20%)
Battery safe
The cut-off voltage of battery charging and discharging is the key line of defense to prevent thermal runaway of the battery core:
1.High charge cut-off voltage:
If the charging voltage is too high, it will cause safety problems to the positive and negative materials and electrolytes, and even thermal runaway.
1) Cathode material:
The structure disintegrates, releases oxygen, and reacts violently with the electrolyte.
2) Cathode material:
Lithium is separated from the negative electrode, and the lithium dendrite pierces the diaphragm, which will cause internal short circuit and thermal runaway.
3) Electrolyte:
Oxidation and decomposition produce a large amount of gas, and the internal pressure rises sharply, resulting in the swelling of the battery.
2.Low discharge cut-off voltage:
If the discharge voltage is too low, it will cause safety risks to the negative electrode and electrolyte:
1) Negative electrode material:
The current collector copper foil of the negative electrode dissolves and precipitates to form copper dendrites in the negative electrode, and the copper dendrites pierce the diaphragm, which will cause internal short circuit thermal runaway.
2) Electrolyte:
The electrolyte is reduced and decomposed to generate a large amount of heat.
Implementation mode and protection mechanism
The setting of cut-off voltage needs to be realized by the cooperation of hardware and software, and equipped with multiple protections.
- Core Hardware:
- Charger management IC:It is responsible for executing the constant current and constant voltage algorithm and controlling the charging process according to the set cut-off voltage and termination conditions (minimum current, timing).
- Battery protection board:It is the last line of hardware defense to prevent overcharge. Its control IC continuously monitors the cell voltage, and when the voltage of any cell reaches the overcharge cut-off voltage (usually slightly higher than charge cut-off voltage, such as 4.25 V) and meets the delay condition, it will turn off the MOS transistor and cut off the charging circuit.
- System collaboration:In complex systems such as electric vehicle, the termination of charging is completed by the vehicle control unit (VCU), the battery management system (BMS) and the charging pile. BMS is responsible for monitoring the voltage and temperature of all cells in real time, and sending instructions to VCU or charger to execute cut-off or protection actions.
- Safety protection:In addition to voltage, temperature is a critical monitoring parameter. The charging system normally sets a temperature threshold (e.g. 0 ° C to 45 ° C) beyond which charging is suspended or terminated. Some schemes also monitor the rate of temperature rise (dT/DT) as a basis for protection.
Scenario-based and dynamic optimization
In practical applications, the set value of cut-off voltage is not fixed, and it needs to be adjusted dynamically according to battery type, usage scenario and battery status.
Application parameters for different battery types:
Lead-acid batteries (often used for automotive starting or traditional energy storage): The end-of-charge (equalized) voltage of a 12 V battery (6 strings) is typically 13.8-14.4 V (about 2.3-2.4 V per cell), followed by a lower float voltage (such as 13.2 V).
Lithium-ion battery pack: The total voltage is calculated based on the number of series connections. For example, a 16-string LiFePO4 battery pack has an end-of-charge voltage of 3.2 V * 16 = 51.2 V. Floating charge is generally not recommended for lithium batteries to avoid overcharge.
Dynamic adjustment factors:
Temperature compensation: When the ambient temperature changes, the cut-off voltage needs to be adjusted accordingly.
For example, the charging voltage at high temperatures (> 35 ° C) needs to be reduced (about 0.003 V/° C for lithium batteries) to prevent accelerated electrolyte decomposition and the risk of thermal runaway.
Battery aging: As the number of cycles increases, the internal impedance of the battery increases and the polarization intensifies. In order to prolong the service life, the full charge voltage of the aging battery can be reduced appropriately (for example, 0.05 V after 500 cycles).
Lifetime vs. capacity tradeoff: Studies have shown that charging a ternary lithium battery to 4.1V instead of 4.2V extends cycle life by about 30%, but sacrifices about 10% of usable capacity. This requires a strategic choice based on product positioning (more life or endurance).
Harm and Prevention of Overcharge and Overdischarge
The core purpose of setting the cut-off voltage is to maximize the use of battery energy within the safety boundary.
- Overcharge risk:Exceeding the charging cut-off voltage will cause damage to the structure of the positive electrode of the lithium-ion battery, decomposition of the electrolyte, generation of gas and a large amount of heat, which may cause thermal runaway or even fire and explosion in serious cases.
- Over-discharge risk:the voltage lower than discharge cut-off voltage will lead to the collapse of the negative electrode structure, the dissolution of the copper foil, and the formation of lithium dendrites during subsequent charging, which will pierce the diaphragm and cause internal short circuit, bringing serious potential safety hazards.
- Depth of discharge (DoD) management: In order to extend the life, the “shallow charge and discharge” strategy is often used in daily use. For example, set the discharge cutoff point at a high level (e.g., corresponding to 20% remaining charge) to avoid deep discharge. The study shows that the cycle life can be significantly extended by reducing the depth of discharge from 100% to 80%.
- Advanced discharge cut-off adjustment method:The latest research technology dynamically determines the optimal discharge cut-off voltage by monitoring the change of negative electrode potential. According to the method, a cathode voltage change curve of the three-electrode battery is obtained, and the characteristic peak deviation of a differential curve of the cathode voltage change curve is analyzed to adjust (improve) the discharge cut-off voltage in advance before the battery capacity is significantly attenuated, so that the attenuation of the battery is effectively delayed, and the service life of the battery is prolonged.
Battery Safety Development:
Battery development involves many professional fields such as battery core, structure, electrical, thermal management, control, etc. To ensure the safety of batteries, it is necessary to conduct comprehensive and meticulous consideration, adequate calculation and analysis, and test verification during battery development.
Other factors
- Temperature effect:
The dynamic adjustment of the battery voltage range with the temperature shall be considered.
1) Low temperature condition
The mobility of lithium ion is poor, and it is easy to separate lithium on the surface of the negative electrode. Therefore, it is necessary to reduce the charging current (rate), and it is also necessary to consider reducing the charging cut-off voltage appropriately to avoid lithium precipitation.
2) High temperature condition
Battery side effects are exacerbated and may require a slight voltage adjustment to preserve life.
- Battery consistency and BMS accuracy:
The battery pack is composed of a large number of cells connected in series and in parallel, and there is a voltage difference between the cells, and there is also a measurement error in the voltage sampling of BMS. The cut-off voltage shall be set in consideration of these worst cases to ensure that the cell with the highest voltage is not overcharged and the cell with the lowest voltage is not overdischarged.
- Customer experience and strategy:
1) SOC calibration of display electric quantity
Full charge cut-off voltage is the key node for BMS to calibrate SOC.
2) Buffer
Due to the electrochemical characteristics of the battery, the power attenuation after charge-discharge cycle and the reduction of low temperature capacity, some of the battery control will retain part of the buffer power, on the one hand, to protect the battery, on the other hand, to reserve margin for future OTA upgrades, battery lifetime warranty, etc.
Through the above analysis, we can see that the setting of the cut-off voltage of battery charging and discharging is not a simple digital choice, but a complex system engineering involving material science, engineering optimization and safety management. It is based on the electrochemical material characteristics of the battery, and releases the energy and power capacity of the battery as far as possible on the premise of ensuring safety and cycle life.
It needs to strike a balance between improving single endurance and guaranteeing long-term life, while putting safety first.
With the continuous evolution of battery technology, such as the application of solid-state batteries and new materials, the setting of cut-off voltage may be more intelligent in the future, combining AI and big data to achieve dynamic adjustment.
For ordinary users, understanding these principles helps to develop scientific charging habits, avoid excessive charging and discharging, and thus prolong battery life.