As the core component of electric vehicle and energy storage system, the performance of power battery directly affects the endurance, safety and service life of the whole vehicle. Understanding the basic parameters of power battery is of great significance for design, selection, use and maintenance. This article will introduce the main parameters of power battery, covering capacity, voltage, internal resistance, cycle life, energy density, safety performance and other aspects, helping readers to fully grasp the basic characteristics of power battery.

1. Capacity (Capacity)

capacity refers to the amount of energy that a battery can store and release, usually in hours (Ah) or hours (mAh). The capacity of the power battery is an important indicator to measure its energy storage capacity, which directly determines the mileage of the vehicle.

• Nominal capacity: the standard capacity value provided by the manufacturer is usually measured under specific conditions.
• Actual capacity: affected by factors such as temperature and discharge rate, the available capacity in actual use is often lower than the nominal capacity.
• Capacity attenuation: With the increase of battery service life and the number of cycles, the capacity will gradually decrease, which is also an important parameter to measure the health status of the battery.

The larger the capacity, the stronger the battery can provide, but it also means an increase in volume and weight.

the rated voltage of the power battery refers to the average voltage value when the battery is working normally, usually in volt (V). Power batteries are generally composed of multiple single cells connected in series, and the overall voltage depends on the number of cells connected in series and the voltage characteristics of single cells.

• Nominal Voltage: single lithium ion batteries are mostly 3.6V or 3.7V, and the voltage of power battery pack is usually between several hundred V and several hundred V.
• Charge/discharge voltage range: When the battery is fully charged, the voltage is the highest (generally about 4.2V per monomer). When discharging to the lowest voltage (usually 2.5V-3.0V), stop using it to avoid overcharging or overdischarging damaging the battery.
• Voltage platform: the voltage variation interval of the battery during discharge directly affects the stability of vehicle power output.

Voltage parameters are not only related to dynamic performance, but also affect the design and safety monitoring of battery management system.

III. Internal Resistance (Internal Resistance)

internal resistance refers to the internal obstruction of current flow in the battery, which is usually measured in milliohm (MΩ). The size of internal resistance directly affects the charging and discharging efficiency and calorific value of the battery.

• DC internal resistance: reflects the impedance of the battery under direct current. The smaller the internal resistance, the higher the charging and discharging efficiency of the battery.
• AC internal resistance: the internal resistance measured under AC conditions reflects the response characteristics of the battery to high frequency signals.
• Internal resistance change: As the service time is prolonged, the internal resistance will gradually increase, resulting in increased battery heating and decreased power.

Internal resistance parameters are very important for judging battery health and preventing thermal runaway accidents.

IV. Cycle Life

cycle Life refers to the number of cycles in which the capacity of the battery remains above a certain proportion (generally 80%) during the charging and discharging process. It is a key indicator to measure battery durability and economy.

• Standard test conditions: the measurement of cycle life is usually carried out under specific charging and discharging rate and temperature conditions.
• Influencing factors: battery material, charging and discharging depth, temperature and charging rate all affect the cycle life.
• Practical significance: The longer the cycle life, the lower the use cost of the battery, and the lower the frequency of vehicle maintenance and replacement.

knowing the cycle life is helpful to arrange the maintenance and replacement plan of the battery reasonably.

V. Energy Density (Energy Density)

the energy density represents the energy that can be stored by the battery per unit weight or volume, which is usually divided weight energy density(Wh/kg) and bulk energy density(Wh/L).

• Weight energy density: Measuring the lightweight level of the battery directly affects the mileage and power performance of the vehicle.
• Bulk energy density: it affects the space utilization rate of battery pack and determines the flexibility of vehicle design.
• High energy density batteries such as ternary lithium batteries are the mainstream choice of current power batteries.

The higher the energy density, the longer the battery life under the same weight or volume, but the high energy density is also accompanied by safety and life challenges.

VI. Safety performance parameters

safety performance is the top priority in power battery design, involving multiple parameters:

thermal stability: the thermal runaway temperature and performance of the battery in overcharge, short circuit or high temperature environment.
• Overcharge protection voltage: prevent the battery from being damaged due to excessive charging voltage.
• Overdischarge protection voltage: avoid irreversible damage of battery due to low discharge voltage.
• Short circuit current: the maximum allowable current when the battery is short-circuited.
• Mechanical strength and explosion-proof performance: battery packaging and structural design ensure the safety of mechanical impact and puncture.

Safety parameters depend not only on battery materials, but also on the effective cooperation of battery management system (BMS) and thermal management system.

VII. Charge/discharge ratio (C-rate)

C- rate indicates the parameter of the charge-discharge rate of the battery, which is defined as the ratio of the charge-discharge current to the rated capacity of the battery within a certain period of time.

• The meaning of charge/discharge ratio: 1C means to fill or empty the battery within 1 hour, 2C means half an hour, 0.5C means two hours.
• Effect of magnification on performance
• Practical application: The power battery design needs to take into account high-rate discharge to meet the power demand of the vehicle and low-rate charging to prolong the service life.