What You Need to Know about Lithium Batteries in UAV
In recent years, in the civilian and military drone markets, the demand for drones has increased day by day. The iteration cycle of UAV products has been significantly shortened, and the avionics systems, navigation systems, data chain systems, and power systems that match the UAV system have also been developed rapidly. Among them, lithium batteries, as the main source of power for electric UAV, affect the range and flight time of UAV, and are the key factors that directly restrict the development and application of UAV.
As an electric chemical power supply, polymer lithium batteries have characteristic parameters such as voltage, internal resistance, capacity, specific energy, and specific power. For two purposes, the parameters of the battery are measured and evaluated. One is to achieve the purpose of active control. For example, the voltage of the battery cell is inconsistent, which reduces the energy storage capacity of the system. If the monomer voltage of the two poles can be actively adjusted, it can have the effect of amplifying the capacity of the system. The other is for safety reasons. Because the parameters of the battery have a fixed range, if the battery parameters can be detected and the monitoring parameters are within a safe range, it can play a role in maintaining the safe state of the battery.
This refers to the open circuit voltage (V) of the battery. That is, without any load or power supply connected to the outside of the battery, we measure the potential difference between the positive and negative electrodes of the battery, which is the open circuit voltage of the battery. The operating voltage corresponds to the open circuit voltage, which refers to the potential difference between the positive and negative electrodes measured when a load or power supply is connected to the battery and a current flows through the battery in the process.
A monolithic battery cell refers to a 1S battery with a rated voltage of 3. 7V. The actual voltage of a single lithium battery cell is 2. 75~4. 2V, and the capacitance of the lithium battery cell is the power obtained by discharging from 4. 2V to 2. 75V. The voltage of each lithium battery is usually 3. 7v to 4. 2v, which means that the air voltage of each lithium battery is 3. 7v and the full voltage is 4. 2V.
It is refer to 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 22000mAh, and when the current is 1A during operation, 2. 2h can theoretically be used.
The unit of energy stored in the battery is Wh (watt-hour)
Energy (Wh) = voltage (V) × battery capacity (Ah).
For example, a 6S battery identified as 3. 7V/22000mAh has an energy of 488. 4WH. By connecting two such batteries in series, it forms a battery pack with a voltage of 44. 4V and a capacity of 22000mAh. Although the battery capacity has not been increased, the total energy has indeed been increased by 2 times.
It is refer to the energy released by a battery per unit volume or per unit mass. If it is a unit volume, it is the volume energy density (Wh/L), which is directly referred to as the energy density in many places; if it is a unit mass, it is the mass energy density (Wh/kg), which can also be called the specific energy. If a lithium battery weighs 300g, has a rated voltage of 3. 7V and a capacity of 10Ah, its specific energy is 123 Wh/kg. The greater the specific energy, the stronger the battery life.
Divide the energy by time to get the power in W or kW. Similarly, power density refers to the power output per unit mass or unit volume of the battery, in W/kg or W/L, which can also be called specific power. The 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? The power battery with high specific energy has good endurance and can work for a long time to ensure the long range of the UAV; the power battery with high specific power has a fast response speed and can provide a high instantaneous current to improve the acceleration performance of the UAV.
The discharge rate (C) refers to the current value required to release its rated capacity (Q) within a specified time, which is numerically equal to a multiple of the rated capacity of the battery. The discharge magnification determines the discharge current (A) of the battery. For example, for a battery with a capacity of 24Ah and a discharge magnification of 5C, its discharge current is 120A. If its discharge magnification is 2C, the discharge is completed in 0. 5 hours; if it is charged with 12A, if its charging magnification is 0. 5C, the charging is completed in 2 hours. ；
The charge and discharge magnification of the battery determines how fast we can store a certain amount of energy in the battery, or how fast we can release the energy in the battery.
State of Charge
SOC, the full name is State of Charge, also known as remaining power, which represents the ratio of the remaining capacity of the battery after discharge to the capacity of its fully charged state. Its value range is 0~1. When SOC=0, it means that the battery is fully discharged, and when SOC=1, it means that the battery is fully charged. The battery management system (BMS) mainly ensures the efficient work of the battery by managing the SOC and making estimates, so it is the core of battery management. At present, SOC estimation mainly includes open circuit voltage method, ampere measurement method, artificial neural network method, Kalman filtering method, etc.
Internal resistance (R) refers to the resistance of current flowing through the inside of the battery when the battery is working. It includes ohmic internal resistance and polarization internal resistance, among which: Ohmic internal resistance includes electrode material, electrolyte, diaphragm resistance and resistance of various parts; polarization internal resistance includes electric chemical polarization resistance and concentration difference polarization resistance.
The unit of internal resistance is generally milliohm (mW). For batteries with large internal resistance, when charging and discharging, the internal power consumption is large and the heat generation is serious, which will cause accelerated battery aging and life attenuation. At the same time, it will also limit the application of large magnification charge and discharge. Therefore, the smaller the internal resistance, the better the battery life and magnification performance. Usually, there are AC and DC test methods for measuring the internal resistance of batteries.
Battery self-discharge refers to the phenomenon of voltage drop during open circuit standing, also known as the battery’s charge retention ability.
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: the capacity loss is reversible, which means that the capacity can be restored after the recharging process; the capacity loss is irreversible, which means that the capacity cannot be restored. At present, there are many research theories on the causes of battery self-discharge. In summary, they are divided into physical reasons (storage environment, manufacturing process, materials, etc. ) and chemical reasons (instability of the electrode in the electrolyte, internal chemical reactions, consumption of active substances, etc. ). Battery self-discharge will directly reduce the capacity and storage performance of the battery.
The life of the battery is divided into two parameters: cycle life and calendar life. Cycle life refers to the number of times the battery can be charged and discharged in cycles. That is, at the ideal temperature and humidity, the rated charge and discharge current is used to charge and discharge, and the number of cycles experienced when the battery capacity is attenuated to 80% is calculated. Battery life refers to the time span during which the battery reaches the end-of-life condition (capacity attenuation to 80%) under the conditions of the operating environment and after specific operating conditions. If the calendar life is closely integrated with the specific use requirements, it is usually necessary to specify specific use conditions, environmental conditions, storage intervals, etc. Cycle life is a theoretical parameter, while calendar life is more practical. However, the calculation of cycle life is complicated and takes a long time, so generally battery manufacturers only give cycle life data.
Above are a simple introduction about some key parameters of lithium batteries in UAV you need know.