Comprehensive Analysis of UAV Battery SOC (State of Charge)

Drone Battery

ENOV High-Energy drone batteries power industrial and commercial drones. Delivering 220–320 Wh/kg energy density, they enable long flight times (30+ mins) and support fast charging (2C). Perfect for aerial photography, surveillance, and delivery drones.

1. Definition and Core Value of SOC

SOC, short for State of Charge, is a percentage indicator that measures the proportion of the current available capacity of a UAV (Unmanned Aerial Vehicle) battery to its total rated capacity. Its specific quantitative standards are clear: when the SOC is 100%, the battery is fully charged, and the available capacity reaches the rated value. For example, a 2200mAh battery will have an available capacity of 2200mAh at this point; when the SOC is 50%, the remaining capacity drops to half of the rated capacity, so the available capacity of the aforementioned 2200mAh battery will be 1100mAh; in theory, when the SOC is 0%, the battery has no available capacity. However, in practical applications, to protect the battery from damage caused by over-discharging, UAVs usually force a landing when the SOC is in the range of 5% – 10%.

SOC is crucial for UAV operation, mainly reflected in three aspects: First, it ensures flight safety. Accurate SOC estimation can effectively prevent the UAV from crashing due to a power failure caused by incorrect power judgment. Second, it assists in planning flight missions. The endurance time can be predicted by combining with SOC. For instance, a 2200mAh battery with an SOC of 80% has a theoretical endurance time of approximately 1.6 hours if the average power consumption of the UAV is 1100mAh/h/h. Third, it extends the battery life. Through SOC management, over-charging (SOC exceeding 100%) and over-discharging (SOC below 5%) of the battery can be prevented, avoiding irreversible damage to the battery caused by these two states.

2. Comparison of Mainstream SOC Estimation Methods

UAV batteries are mostly lithium batteries (such as LiPo lithium-polymer batteries), and their SOC cannot be directly measured, so indirect algorithms need to be used for estimation. Different methods vary significantly in accuracy, advantages, disadvantages, and applicable scenarios:

Estimation Method	Core Principle	Advantages	Disadvantages	Applicable Scenarios
Open Circuit Voltage Method (OCV)	Estimates SOC by using the corresponding relationship between the open-circuit voltage of the battery and SOC when the battery is at rest. For example, for a LiPo battery, an open-circuit voltage of 4.2V corresponds to an SOC of 100%, 3.7V corresponds to 50%, and 3.0V corresponds to 0%.	Low cost, simple and easy-to-understand algorithm, and high estimation accuracy when the battery is at rest, with an error controllable within ±2%.	During the dynamic flight of the UAV, due to large current fluctuations, the estimation accuracy will decrease significantly; moreover, the battery needs to be at rest for more than 30 minutes to obtain an accurate estimation result.	Suitable for SOC calibration of the battery when it leaves the factory and as a reference for SOC when the battery is at rest.
Coulomb Counting Method	A current sensor is connected in series to count the charging and discharging current of the battery in real time. The charged or discharged capacity is obtained by integrating the current, and then compared with the rated capacity of the battery. The SOC is calculated according to the formula “SOC = (Rated Capacity – Discharged Capacity) / Rated Capacity × 100%”.	High estimation accuracy during the dynamic flight of the UAV, with an error of about ±3%, fast response speed, and can reflect the change of power in a timely manner.	There is cumulative error, such as deviations in the current measurement process and unaccounted self-discharge of the battery; the initial SOC needs to be calibrated, otherwise the accuracy of subsequent estimation will be affected.	Widely used in mainstream consumer-grade UAVs, such as products of DJI, Feimi and other brands.
Kalman Filter Method	Integrates the measurement data of the open-circuit voltage method and the Coulomb counting method, and incorporates parameters such as battery temperature and internal resistance. The estimation error is dynamically corrected through a specific algorithm.	Takes into account the estimation accuracy of the battery in both static and dynamic states, with an error controllable within ±2%, and no cumulative error occurs.	The algorithm is complex, requiring high hardware performance, which increases the hardware cost; a large amount of battery characteristic data is needed for calibration to ensure the estimation effect.	Mainly used in industrial-grade UAVs, such as UAVs in professional fields like surveying and mapping, and inspection.
Impedance Spectroscopy Method	Injects a high-frequency small signal into the battery, measures the impedance characteristics of the battery, and estimates the SOC with the help of the correlation model between impedance and SOC.	No cumulative error occurs, and it can monitor the aging state of the battery and understand the health status of the battery.	The hardware structure is complex, requiring a special impedance measurement module, which increases the equipment cost; the response speed is slow, and it cannot keep up with the rapid change of power in a timely manner.	Mainly used in laboratory-level battery testing or special application scenarios in high-end industrial fields.

Consumer-grade UAVs (such as those of DJI, Autel and other brands) often adopt a hybrid algorithm combining Coulomb counting and voltage correction, with a SOC data refresh rate of 30ms. The SOC value seen by the operator during operation is the calculation result of this algorithm.

3. Key Factors Affecting SOC Estimation Accuracy

Even with advanced estimation algorithms, the following factors may still lead to inaccurate SOC estimation results, which need to be focused on in practical use:

(1) Battery Aging (Cycle Times)

The rated capacity of a lithium battery will slightly decrease after each charge-discharge cycle. Generally, after 200 cycles, the battery capacity may drop to 80% of the initial capacity. If the UAV does not update the actual capacity of the aged battery in a timely manner and still calculates the SOC based on the initial rated capacity, the SOC will be falsely high. For example, if the actual capacity of the battery has dropped to 1760mAh, but the calculation is still based on the initial 2200mAh, when the SOC shows 50%, the actual SOC is only 40%.

(2) Temperature Fluctuations

Lithium batteries are sensitive to temperature. When the ambient temperature is lower than -10℃ or higher than 45℃, the voltage and internal resistance of the battery will change drastically, leading to a deviation in the corresponding relationship between the open-circuit voltage and SOC. For example, in a low-temperature environment, the actual available capacity of the battery will decrease, but the displayed SOC value is still within the normal range. This situation is likely to cause the UAV to “lose power quickly”, affecting flight safety.

(3) Charge-Discharge Rate (C-Rate)

The charge-discharge rate has a significant impact on the accuracy of SOC estimation. When the UAV is in a high-rate discharge state, such as rapid acceleration and hovering climb, and the discharge current exceeds 5C, the battery voltage will drop sharply. At this time, the Coulomb counting method tends to overestimate the SOC. In a low-rate discharge state, such as the UAV cruising slowly with a discharge current lower than 1C, the battery voltage is relatively stable, and the SOC estimation result is more accurate.

(4) Self-Discharge

The battery will undergo a slow self-discharge phenomenon when stored idle, usually with a monthly self-discharge capacity of about 5% – 10%. If the SOC estimation system of the UAV does not take the self-discharge factor into account, the displayed SOC value will be higher than the actual remaining capacity of the battery. For example, when the SOC shows 80%, the actual remaining capacity may only be 75%.

(5) Initial SOC Calibration Deviation

The Coulomb counting method needs to take “SOC = 100% when fully charged” as the initial reference for calculation. If the battery is not fully charged, such as the interruption of the fast charging process, or over-charging occurs, such as the battery voltage exceeding 4.2V per cell due to a charger failure, it will directly lead to deviations in subsequent SOC estimation and affect the estimation accuracy.

4. Practical SOC Management Strategies

Reasonable SOC management can not only extend the endurance time of the UAV but also effectively protect the battery. Specific strategies can be adopted in four stages: before flight, during flight, after flight, and regular maintenance.

(1) Before Flight: Calibrate the Initial SOC

Before each flight, the battery should be fully charged until the charger shows a “green light” or “fully charged” state, so that the UAV can confirm that the current SOC is 100%, thereby eliminating the cumulative error that may be caused by the Coulomb counting method. If the battery has been idle for more than 1 month, it should first be charged to an SOC of 50% – 60% (this is the optimal SOC range for long-term storage of lithium batteries), and then fully charged for flight.

(2) During Flight: Pay Attention to the Matching Between "Actual Endurance" and SOC

During the flight, continuous flight when the SOC is lower than 20% should be avoided, because the battery voltage is very likely to experience a “cliff-like drop” at this time, which may cause the UAV to lose control. When flying in a low-temperature environment (temperature lower than 0℃), more redundancy should be reserved for SOC. For example, if the planned flight time is 20 minutes, the UAV should be controlled to return when the SOC drops to 40% to prevent accidents caused by the rapid decrease of battery power due to low temperature.

Different brands of UAVs have different settings and displays for SOC. The 30% SOC displayed by DJI series UAVs is the “endurable time” calculated based on the current power, including voltage compensation. When the battery voltage continues to drop, the SOC will quickly jump to 25% and 20%, and trigger the forced return function. Open-source flight controllers (such as ArduPilot/PX4) trigger automatic return (RTL) by default when the SOC drops to 20%. This threshold can be adjusted in the ground station according to the Depth of Discharge (DoD) requirements. For example, plant protection UAVs can increase the threshold to 30% to better protect the battery and extend its service life.

(3) After Flight: Store at an Appropriate SOC

After the flight, the battery should be set to an appropriate SOC according to the storage time: for short-term storage (within 1 week), keep the SOC at 30% – 70%; for long-term storage (more than 1 month), adjust the SOC to 50%, and place the battery in a dry environment with a temperature of 10℃ – 25℃, avoiding exposure to high temperatures or freezing at low temperatures to prevent damage to the battery performance.

(4) Regularly Calibrate the Battery Capacity

After every 50 – 100 charge-discharge cycles, a “deep charge-discharge calibration” operation should be performed on the battery: first, fully discharge the battery until the UAV is forced to land (the SOC is about 5% at this time), and then fully charge the battery. Through this operation, the UAV can update the actual rated capacity of the battery, correct the SOC estimation algorithm, and ensure the accuracy of subsequent SOC estimation.

5. Common SOC - Related Questions and Answers

(1) Question: The UAV shows an SOC of 30%, but suddenly the power drops to 10% and a forced landing is triggered. What is the reason?

Answer: This situation may be caused by factors such as battery aging (unupdated capacity attenuation), low-temperature environment (reduced actual available capacity of the battery), or high-rate discharge (such as a sudden voltage drop caused by rapid acceleration). You can first calibrate the battery capacity, and then test the battery performance in a normal temperature environment to identify the specific cause.

(2) Question: After the battery is fully charged, the SOC shows 98% and cannot reach 100%. Is this normal?

Answer: It needs to be judged based on the number of battery cycles. If the number of battery cycles is small (less than 50), it may be that the protection mechanism of the charger is working (to avoid over-charging the battery) or there is a deviation in the initial SOC calibration. You can try recharging once to observe whether the SOC can reach 100%. If the number of battery cycles is large (more than 200), it is likely that the battery is aging and the capacity has attenuated, so that the voltage cannot reach 4.2V per cell when fully charged. In this case, it is necessary to consider replacing the battery.

(3) Question: Are the SOC displays of the same – model batteries from different brands consistent?

Answer: Not necessarily. Batteries from different brands have differences in characteristics (such as internal resistance, capacity, etc.). If the UAV is not adapted to the open-circuit voltage – SOC model of the battery of that brand, there may be a large deviation in the SOC display value. For example, when the SOC of a battery from brand A shows 50%, the SOC of a same – model battery from brand B may show 45%. Therefore, it is recommended to use the original battery of the UAV first to ensure the accuracy of the SOC display and flight safety.

6. SOC - Related Associated Indicators

In addition to SOC, the important indicators related to UAV batteries also include SOH (State of Health) and DoD (Depth of Discharge):

(1) SOH (State of Health)

SOH is used to measure the overall health status and performance of the battery over time. It reflects the aging state of the battery by comparing the current capacity of the battery with the original capacity. A battery with a higher SOH has a stronger ability to retain power, better performance in providing the rated capacity, and a longer total service life than a battery with a lower SOH.

(2) DoD (Depth of Discharge)

DoD refers to the ratio of the discharged energy of the battery to its total capacity. For example, a DoD of 50% means that the battery has been discharged to 50% of its total energy capacity.

(3) Cycle Life

Cycle life refers to the total number of complete charge-discharge cycles that a battery can complete before its capacity drops to below 80% of the initial capacity. Under normal use conditions, the cycle life of a LiPo battery is usually 200-300 times.

A practical SOC operating window that can extend the battery life is usually 20%-80%. In the application of solar batteries, to avoid deep loss, the minimum SOC is generally controlled at about 10%-20%. The SOC calibration of the Battery Energy Storage System (BESS) will readjust the measured value over time to ensure the accuracy of the SOC estimation value.

SOC, SOH, and DoD are important indicators for understanding the performance and overall health status of the battery. They are often used in battery management systems to monitor and optimize the use of batteries in various devices such as electric vehicles, renewable energy systems, and UAVs.

7. Summary

As the “core instrument panel” for UAV battery management, a deep understanding of the estimation principle, influencing factors, and management methods of SOC can not only effectively avoid flight accidents and ensure flight safety but also significantly extend the battery life. In practical operation, it is necessary to combine the characteristics of the UAV and the battery, reasonably apply the SOC management strategy, and pay attention to the SOC-related associated indicators to ensure that the UAV battery is always in the best working state and provide a reliable guarantee for the smooth completion of the UAV flight mission. At the same time, the SOC algorithms of different UAV models may have large differences. It is recommended to refer to the product manual of the corresponding manufacturer to understand the specific SOC characteristics and operation requirements.