A brief analysis of electric drone power battery 18650 rechargeable technology

　　2 battery 18650 rechargeable principle

　　Polymer lithium batteries, also called polymer lithium batteries, are a new generation of lithium batteries that have the advantages of high energy density, lightweight, bendable, ultra-thin, and can be made into any shape. The capacity of a lithium polymer battery 18650 rechargeable of the same volume can be doubled that of ordinary lithium batteries, and the cost is lower and safer. Generally, polymer lithium batteries are packaged in aluminum plastic or fireproof plastic.

　　The principle of polymer battery 18650 rechargeable is the same as that of liquid lithium. The main difference is that the electrolyte is different from liquid lithium. The main structure of the battery includes three elements: positive electrode, negative electrode and electrolyte. Polymer lithium-ion batteries use polymer materials as the main battery system in at least one or more of the three main structures. In the developed polymer lithium-ion battery system, polymer materials are mainly used in the positive electrode and electrolyte.

　　The cathode material includes conductive polymers or inorganic compounds used in general lithium-ion batteries. The electrolyte can use solid or colloidal polymer electrolytes, or organic electrolytes. Generally, lithium-ion technology uses liquid or colloidal electrolytes. The positive electrode generates The response is:

　　The negative electrode uses lithium-carbon interlayer compound LixC6, etc., and its reaction is:

　　The total reaction expression is:

　　3 Main performance parameters

　　As an electrochemical power source, polymer battery 18650 rechargeable naturally has characteristic parameters such as voltage, internal resistance, capacity, specific energy, and specific power. Battery parameters are measured and evaluated for two purposes. One is to achieve the purpose of active control. For example, the battery cell voltage is inconsistent, which reduces the energy storage capacity of the system. If the cell voltage of the two poles can be actively adjusted, it can have the effect of amplifying the system capacity. Another reason is that for safety reasons, battery parameters have a fixed range. Detecting battery parameters and monitoring their boundaries can play a role in characterizing the safety status of the battery. The main performance parameters of polymer lithium batteries are battery design, an important link in the production process, and play an important role in battery performance evaluation.

　　3.1 Voltage

　　The open circuit voltage (V) of the battery, that is, without any load or power supply connected to the outside of the battery, measuring the potential difference between the positive and negative electrodes of the battery is the open circuit voltage of the battery. The operating voltage corresponds to the open circuit voltage, that is, when the battery is connected to an external load or power supply, current flows through the battery, and the potential difference between the positive and negative electrodes is measured.

　　A single cell refers to a 1S battery with a rated voltage of 3.7V. The actual voltage of a single battery 18650 rechargeable is 2.75~4.2V. The capacity marked on the battery 18650 rechargeable is the amount of electricity obtained by discharging 4.2V to 2.75V. The voltage of each battery 18650 rechargeable is usually 3.7v to 4.2v, which means that the empty voltage of each battery 18650 rechargeable is 3.7v and the full voltage is 4.2v.

　　3.2 Battery capacity

　　The definition of battery capacity: the charge Q that can be accommodated or released, that is, Q=It, battery capacity (Ah) = current (A) x discharge time (h), the unit is generally Ah or mAh. For example, the battery is marked 22000mAh, and when the working current is 1A, it can theoretically be used for 2.2 hours.

　　3.3 Battery energy

　　The energy stored in the battery is measured in Wh (watt hours). Energy (Wh) = voltage (V) × battery capacity (Ah).

　　For example, a 6S battery labeled 3.7V/22000mAh has an energy of 488.4Wh. If two such batteries are connected in series, a battery pack with a voltage of 44.4V and a capacity of 22000mAh is formed. Although the battery capacity is not increased, the total The energy is indeed increased by 2 times.

　　3.4 Energy density

　　The amount of energy released by a battery per unit volume or unit mass. If it is unit volume, it is volume energy density (Wh/L), which is simply called energy density in many places; if it is unit mass, it is mass energy density (Wh/kg), which can also be called specific energy. For example, if a battery 18650 rechargeable weighs 300g, has a rated voltage of 3.7V, and a capacity of 10Ah, its specific energy is 123Wh/kg. The greater the specific energy, the greater the battery life.

　　3.5 Power density

　　Divide energy by time and you get power, measured in W or kW. Similarly, power density refers to the power output by the battery per unit mass or unit volume, in W/kg or W/L, and can also be called specific power. Specific power is an important indicator to evaluate whether the battery meets the acceleration performance of electric drones. What is the difference between specific energy and specific power? A power battery with a high specific energy has good endurance and can work for a long time, ensuring a long cruising range of the drone; a power battery with a high specific power has a fast response speed and can provide a high instantaneous current. , improve the acceleration performance of the drone.

　　3.6 Discharge rate

　　The discharge rate (C) refers to the current value required to discharge its rated capacity (Q) within a specified time, which is numerically equal to a multiple of the battery's rated capacity. The discharge rate determines the discharge current (A) of the battery. For example, for a battery with a capacity of 24Ah and a discharge rate of 5C, its discharge current is 120A. If the discharge rate is 2C, it will be fully discharged in 0.5 hours; if it is charged with 12A, if its charge rate is 0.5C, it will be fully charged in 2 hours;

　　The charge and discharge rate of a battery determines how quickly we can store a certain amount of energy into the battery, or how quickly we can release the energy in the battery.

　　3.7 State of charge

　　SOC, the full name is State of Charge, state of charge, also called remaining capacity, represents the ratio of the remaining capacity of the battery after discharge to its capacity in the fully charged state. Its value range is 0~1. When SOC=0, it means the battery is completely discharged. When SOC=1, it means the battery is fully charged. The battery management system (BMS) mainly ensures the efficient operation of the battery by managing SOC and making estimates, so it is the core of battery management. At present, SOC estimation mainly includes open circuit voltage method, ampere-hour measurement method, artificial neural network method, Kalman filter method, etc.

　　3.8 Internal resistance

　　Internal resistance (R) refers to the resistance to current flowing through the interior of the battery when the battery is working. Including ohmic internal resistance and polarization internal resistance, among which: ohmic internal resistance includes the resistance of electrode materials, electrolytes, diaphragm resistors and various parts; polarization internal resistance includes electrochemical polarization resistance and concentration polarization resistance.

　　The unit of internal resistance is generally milliohms (mΩ). Batteries with large internal resistance consume large internal power and generate serious heat during charging and discharging, which will cause accelerated aging and lifespan of the battery, and will also limit high-rate charging. discharge applications. Therefore, the smaller the internal resistance is, the better the battery life and rate performance will be. Usually, the measurement methods of battery internal resistance include AC and DC testing methods.

　　3.9 Self-discharge

　　Battery self-discharge refers to the phenomenon of voltage drop during open circuit resting, also known as the battery's charge retention capability.

　　Generally speaking, battery self-discharge is mainly affected by manufacturing processes, materials, and storage conditions. Self-discharge is divided into two types according to whether the capacity loss is reversible: reversible capacity loss, which means that the capacity can be restored after recharging; irreversible capacity loss, which means that the capacity cannot be restored. At present, there are many theories on the causes of battery self-discharge, which can be summarized as physical reasons (storage environment, manufacturing process, materials, etc.) and chemical reasons (instability of electrodes in the electrolyte, internal chemical reactions, and consumption of active materials). etc.), battery self-discharge will directly reduce battery capacity and storage performance.

　　3.10 Lifespan

　　The life of the battery is divided into two parameters: cycle life and calendar life. Cycle life refers to the number of times a battery can be charged and discharged. That is, under ideal temperature and humidity, charge and discharge at the rated charge and discharge current, and calculate the number of cycles experienced when the battery capacity decays to 80%. Calendar life refers to the time span in which a battery reaches end-of-life conditions (capacity decays to 80%) under specific usage conditions under environmental conditions. Calendar life is closely combined with specific usage requirements, and it is usually necessary to stipulate specific usage conditions, environmental conditions, storage intervals, etc. Cycle life is a theoretical parameter, while calendar life has more practical significance. However, the calculation of calendar life is complicated and time-consuming, so generally battery manufacturers only provide cycle life data.

　　4 Development of lithium batteries for drones

　　4.1 Generalization

　　The general-purpose polymer battery 18650 rechargeable has the above-mentioned polymer battery 18650 rechargeable to meet the main performance, etc., and can be applied to drones of different weights. The commonly used battery for drones is T-shaped polymer battery 18650 rechargeable. One end of the power line is connected to a male and female connector (usually XT60) for energy output. The signal line at the other end is usually detected using equipment such as a voltage detector.

　　General-purpose polymer lithium batteries are a low-cost power solution for drones, but they will bring many problems. For example, first, the battery power cannot be monitored in real time, and there is a risk of the aircraft crashing. Second, there is no perfect charging management and discharge management. After charging and discharging are completed, a voltage detector needs to be used to detect the battery. Third, the over-discharge problem cannot be solved. Fourth, due to the frequent use of plug connections, the aging problem of the plug cannot be solved. Fourth, the battery is flammable and explosive, which poses a great safety hazard. Fifth, recycling is inconvenient, and lithium batteries pollute the environment. Fifth, the battery itself has low energy density and cannot meet the urgent need for long-range drones. Sixth, disassembly and assembly are inconvenient, and the battery replacement frequency of the drone is high, which affects the user experience.

　　4.2 Intelligence

　　Intelligent polymer lithium batteries mainly address the pain points of the above-mentioned general polymer lithium batteries, carry out optimized designs, and combine the drone flight control system and optimized battery management system to achieve intelligent management and control of the battery. In terms of battery structure, first of all, ABS+PC fireproof materials are used to improve the protection level of the battery. Secondly, the integrated design of the fast charging port and the addition of a power control switch reflect the convenience of operation. Thirdly, the buckle design on the battery head facilitates quick disassembly. Finally, the appearance of the battery should be taken into consideration to realize the commercialization of smart batteries.

　　In terms of hardware, the battery is equipped with a BMS battery management system (Battery Management System, abbreviated as BMS). BMS is an important link between drone power batteries and electric drones. BMS is used to monitor and indicate battery and capacitance status (voltage, temperature, current, remaining energy), send alarm signals (acoustic and visual) to users under abnormal conditions, and cut off the power transmission link according to the established control strategy in severe cases to protect battery thereby extending battery life.

　　BMS consists of three major parts: terminal module, central processing module and display module. The terminal module is responsible for measuring battery voltage and temperature, balancing battery energy, current sampling and SOC calculation, generating various alarm data, and controlling charge and discharge circuits; the display module is responsible for displaying battery data, giving audible and visual alarms, recording data, etc. When the total number of system batteries is small, the central processing module can be combined with the terminal module to form an integrated BMS system to save costs.

　　Based on the above hardware and structure, real-time monitoring of the status of smart lithium batteries is achieved through software algorithms. The disadvantage of intelligent lithium batteries is that there are many versions on the market and the batteries are incompatible. The standardization of intelligent lithium batteries is an urgent problem that needs to be solved.

　　Smart battery 18650 rechargeable software interface

　　4.3 Solidification

　　The development of solid-state lithium batteries is mainly to solve the safety hazards of general-purpose lithium batteries themselves, green environmental protection, low energy density and other issues. The energy density of existing liquid lithium-ion batteries is generally only 130-160Wh/kg, and the ceiling is around 300Wh/kg. . It also has the disadvantages of long charging time and low safety. The energy density of solid-state lithium-ion batteries will be much higher. The energy density of all-solid-state lithium-ion batteries has a maximum potential of 900Wh/kg, and the structure is safer, so it was once considered an ideal drone power battery.

　　In terms of working principle, there is no difference between solid-state lithium batteries and traditional lithium batteries. The two ends of the battery are the positive and negative poles of the battery, and the middle is the liquid electrolyte. Lithium ions move back and forth through the electrolyte at both ends to complete the charging and discharging process of the battery. The main difference is that solid-state lithium batteries only have a solid electrolyte. The density and structure of solid lithium batteries can allow more charged ions to gather at one end and conduct greater current, thereby increasing battery capacity.

　　Figure 6 Schematic diagram of lithium-ion battery and solid-state battery 18650 rechargeable

　　Solid-state lithium batteries have many advantages and broad development prospects. Among them, the two most obvious advantages are higher energy density and safer operation. After using an all-solid electrolyte, lithium batteries do not need to use lithium-embedded graphite negative electrodes, but directly use metallic lithium as the negative electrode. This can greatly reduce the amount of negative electrode material and significantly improve the energy density of the entire battery. Now in many laboratories, all-solid-state batteries with energy density of 300-400Wh/kg can be trial-produced in small batches. When solid-state batteries operate at high currents, they will not cause short circuits due to lithium dendrites piercing the separator, no side reactions at high temperatures, and no combustion due to the production of gas.

　　Solid-state battery 18650 rechargeable development plan

　　Use high-nickel cathode + quasi-solid electrolyte + silicon carbon anode to achieve 300Wh/Kg before 2020; use lithium-rich cathode + all-solid electrolyte + silicon carbon/lithium metal anode battery to achieve 400Wh/Kg before 2025; fuel/lithium before 2030 Sulfur/air battery achieves 500Wh/Kg.

　　5 Conclusion

　　In the context of energy conservation, emission reduction, new energy utilization and other related policies, battery 18650 rechargeable technology, as the power source of electric power machinery, has the characteristics of green environmental protection, high energy density and high safety factor, and is the best choice for electric drone power system. Choice is also an inevitable trend in the development of drones. However, the road to research and standardization of battery 18650 rechargeable technology and UAV interface technology is still open. There are still many technical barriers that still require continuous innovation and reform.

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