I. Fundamentals: Capacity, Energy, and Density
Many people’s first confusion about lithium batteries is the misconception that “large capacity = long battery life.” In reality, both capacity and voltage together constitute the battery’s energy.
1. Capacity: Unit: Ah (ampere-hour) or mAh (milliampere-hour), 1 Ah = 1000 mAh.
Defined as the total amount of electricity a battery can output under specific conditions, the physical formula is Q = I × t (current × time). The capacity varies significantly depending on the scenario: mobile phone batteries are mostly 2000-5000mAh, electric bicycle lithium batteries are about 8-15Ah, and electric vehicle single cells can reach 20-200Ah. Capacity is like a water tank; it only refers to volume, not the size of the machine the water can power.
2. Energy: Unit: Wh (watt-hour) or kWh (kilowatt-hour)
The calculation formula is: Energy (Wh) = Capacity (Ah) × Nominal Voltage (V). This is the essential difference between Ah and Wh: For the same 10Ah capacity, NMC (nominal 3.7V) has 37Wh of energy, while LFP (nominal 3.2V) has 32Wh. When comparing battery range, Wh is always more accurate than Ah.
3. Energy Density and Power Density:
Energy Density: The energy stored per unit mass (Wh/kg) or unit volume (Wh/L) determines the upper limit of the battery’s range. At the battery pack level, due to the weight of structural components, BMS, etc., the energy density is usually 15%-30% lower than that of the battery cell.
Power Density: The power output per unit mass (W/kg) or unit volume (W/L) determines the battery’s instantaneous discharge capacity, directly affecting the acceleration performance of the electric vehicle. In simple terms: batteries with high energy density have strong endurance and long battery life; batteries with high power density have strong burst power and fast acceleration.
4. Theoretical Capacity of Materials
The theoretical capacity of an electrode material is the capacity provided assuming all lithium ions in the material participate in the electrochemical reaction. It is calculated from the Faraday constant and the molar mass of the material. The theoretical specific capacity of mainstream materials is as follows:
LiFePO₄: approximately 170 mAh/g
NCM (1:1:1): approximately 278 mAh/g
LiCoO₂: approximately 274 mAh/g
Graphite anode: approximately 372 mAh/g
Silicon anode: theoretical upper limit approximately 4200 mAh/g. In actual production, to ensure the reversibility of the material structure, the lithium ion insertion/extraction coefficient is less than 1. Therefore, the actual specific capacity = lithium ion insertion/extraction coefficient × theoretical capacity. This is the core reason why the performance of mass-produced materials is lower than the theoretical value.
II. Operating Status: C-Rate and SOC/SOH/SOE, the Battery’s Real-Time Dashboard
A battery is not a static “energy storage block,” but a dynamically operating system. Parameters such as rate, state of charge, and health together constitute the battery’s “real-time operating panel.”
1. C-Rate: The Battery’s Speedometer
Unit: C, Calculation Formula: C = Charge/Discharge Current (A) / Rated Capacity (Ah). For a 10Ah battery:
1C Discharge = 10A Current, discharged in 1 hour
0.5C Discharge = 5A Current, discharged in 2 hours
2C Discharge = 20A Current, discharged in 0.5 hours
It should be noted that the higher the rate, the more severe the internal resistance and heat generation of the battery, and the actual usable capacity will decrease slightly—one of the core reasons for the reduced range of electric vehicles at high speeds is the capacity loss caused by high-rate discharge.
2. Four Key State Parameters: SOC, DOD, SOH, SOE
These four parameters correspond to remaining capacity, depth of discharge, battery health, and state of energy, respectively, and are the core monitoring targets of the BMS.
SOC (State of Charge): The percentage of remaining capacity relative to the rated capacity, ranging from 0-100%, essentially the battery’s “fuel gauge.” SOC=0 indicates complete discharge, and 100% indicates full charge. Current mainstream estimation methods include the open-circuit voltage method, ampere-hour metering method, and Kalman filtering method.
DOD (Depth of Discharge): The percentage of actual discharged capacity relative to the rated capacity, i.e., “how much electricity was used.” Under the same operating conditions, SOC + DOD = 100%. The depth of discharge has a significant impact on battery life; shallow charging and discharging (low DOD) can significantly extend cycle life. For example, the cycle life in the 25%-75% range is far higher than that of a full charge and discharge from 0-100%.
SOH (State of Health): The percentage of current battery performance relative to a brand-new battery, with core references for capacity decay and internal resistance increase. Generally, a battery is considered to have reached the end of its lifespan when its State of Energy (SOH) is below 80%. A significant decrease in battery life after two years of use is a typical sign of a declining SOH.
SOE (State of Energy) is the ratio of remaining usable energy to rated energy, measured in Wh (Wh) rather than Ah. Because battery voltage dynamically changes with State of Charge (SOC), SOE is more accurate than SOC in estimating “remaining range,” and high-end BMS systems estimate both.
In short: SOC shows “how much capacity is left,” DOD shows “how much has been used,” SOH shows “the degree of battery aging,” and SOE shows “how much power can still be output.”
III. Electrical Characteristics: Voltage and Internal Resistance
Voltage is a fundamental electrical attribute of a battery, while internal resistance is an implicit performance indicator. Together, they determine the battery’s operational stability, energy efficiency, and heat dissipation.
1. Four Types of Voltage Parameters
الجهد الاسمي: The standard voltage defined by the design, also the voltage value commonly marked. LFP nominal voltage is 3.2V, NMC nominal voltage is 3.6-3.7V.
Open Circuit Voltage (OCV): The static voltage of the battery when it is unloaded and at rest. The basic principle of the power display is to estimate the SOC value using the open circuit voltage.
Operating Voltage: The stable voltage when operating under load. Because internal resistance consumption is always lower than the open circuit voltage, the difference becomes more pronounced with increasing current.
Charge/Discharge Cut-off Voltage: The highest/lowest permissible operating voltage of the battery. Exceeding this voltage will cause irreversible damage. NMC charging cut-off voltage is approximately 4.2V, discharging cut-off voltage is approximately 2.75V; LFP charging cut-off voltage is approximately 3.65V, discharging cut-off voltage is approximately 2.5V.
2. Internal Resistance: The hidden loss of a battery.
Unit: mΩ (milliohms), divided into ohmic internal resistance and polarization internal resistance:
Ohmic internal resistance: Composed of the contact resistance of electrode materials, electrolyte, separator, and components, conforming to Ohm’s law;
Polarization internal resistance: Caused by electrochemical polarization and concentration polarization, related to the reaction rate.
The lower the internal resistance, the more stable the discharge voltage, the less heat generated, and the higher the energy efficiency. Internal resistance is not a fixed value and increases with battery aging, temperature decreases, and SOC decreases. The reduced range of electric vehicles in winter is mainly due to decreased electrolyte activity, but also to increased internal resistance and energy loss caused by low temperatures.
IV. Battery Life Cycle: Cycle, Calendar, and Self-Discharge
Battery life is not a single number, but a comprehensive indicator determined by cycle life, calendar life, and self-discharge characteristics. Usage habits directly affect the actual service life.
1. Cycle Life: The “Upper Limit” of Charge and Discharge Cycles
This refers to the number of complete charge and discharge cycles a battery undergoes under specified conditions (typically 80% DOD, a specific rate, and temperature) before its capacity decays to 80% of its initial value.
Cycle life is strongly correlated with depth of discharge. Shallow charging and discharging can significantly increase the number of cycles. This is why energy storage stations typically limit the depth of discharge, and why automakers recommend charging to 80% daily.
2. Calendar Life and Self-Discharge: Aging Even When Idle
Even when not in use, batteries will slowly degrade due to chemical aging; this is calendar life. High temperatures and high SOC accelerate calendar aging. Therefore, for electric vehicles that are not used for extended periods, it is recommended to store them at 50%-70% SOC in a cool environment. Self-discharge rate refers to the natural loss of electrical energy during idle periods, expressed as %/month:
Lithium-ion batteries: 1%-3% per month, extremely low self-discharge;
Lead-acid batteries: 4%-6% per month;
Ni-MH batteries: 15%-20% per month.
Self-discharge is divided into reversible and irreversible losses. Reversible losses can be recovered through charging, while irreversible losses permanently deplete the active materials.
V. Charging Principles: CC/CV/CP
Lithium-ion battery charging involves a rigorous, phased control logic. Mainstream charging methods include constant current, constant voltage, constant power, and combinations thereof.
1. Constant Current Charging (CC): The current remains constant throughout the charging process, while the battery voltage gradually increases. This is used in the initial charging stage. The advantage is fast charging speed, but the disadvantage is the risk of overcharging later on.
2. Constant Voltage Charging (CV): After the battery reaches its cutoff voltage, the voltage remains constant, and the current gradually decreases as the battery is fully charged. This is used to replenish the charge at the end of the charging process.
3. Constant Current-Constant Voltage Charging (CC-CV): The standard charging method for lithium-ion batteries, divided into four stages:
Pre-charging: When the battery voltage is too low, a small current is used for pre-charging to avoid damage to the cells from a large current surge.
CC Stage: Constant current charging is used, and the voltage rises rapidly.
CV Stage: After reaching the cutoff voltage, constant voltage is switched, and the current gradually decreases.
Charging Termination: Charging is complete when the current drops to the termination threshold (usually 0.05C).
4. Constant Power Charging (CP): Maintains a constant charging power, dynamically reducing the current as the voltage increases (P=U×I). Constant power charging is faster in the initial stage and is one of the mainstream solutions for high-power fast charging. Advanced solutions will use a CP-CV combination mode.
