The voltage characteristics of lithium batteries are the core indicators for measuring their performance and safety, directly affecting the charging and discharging efficiency, cycle life, and application safety of the battery. This article will systematically introduce four key voltage parameters of lithium batteries - open circuit voltage (OCV), operating voltage (WV), discharge cut-off voltage (DCV), and charge limiting voltage (LCV), to help you comprehensively understand the definition, influencing factors, and practical application points of voltage parameters.

This article provides a detailed explanation of the open circuit voltage involved.

1. Open circuit voltage (OCV)

Open circuit voltage (OCV) refers to the voltage value of a circuit when the power supply or component is disconnected from the load. It reflects the electromotive force of the power supply or the inherent voltage characteristics of the components when no current passes through, for example, the open circuit voltage of a battery is usually close to its nominal voltage, while the open circuit voltage of a capacitor is consistent with its charged voltage.

The open circuit voltage is the potential difference between the positive and negative electrodes of a lithium battery in its non working state (with no current flowing), determined by the chemical equilibrium potential difference of the positive and negative electrode materials. OCV is an important basis for evaluating the state of charge (SOC) of a battery, and its value shows a regular curve as the remaining capacity of the battery changes.

The OCV of lithium batteries with different material systems shows significant differences:

Ternary lithium battery: OCV is about 4.2V when fully charged, and drops to 3.6-3.7V when discharged to 50% SOC;

Lithium iron phosphate battery: The OCV at full charge is about 3.65V, and the platform voltage remains stable at around 3.2V;

Lithium manganese oxide battery: The OCV range is similar to that of ternary batteries, but the voltage platform is slightly lower.

In practical applications, OCV needs to be measured after the battery has been idle for more than 2 hours to avoid numerical deviation caused by polarization effects after charging and discharging. Long term storage of lithium batteries is recommended to maintain a 70% SOC (corresponding to an OCV of approximately 3.8V) to minimize capacity degradation.

2. Measurement method and key points of open circuit voltage (OCV) of lithium batteries

Requirements for measuring tools and equipment

① Core Tools

When using a DC voltmeter (such as a multimeter or specialized battery tester) to directly connect the positive and negative terminals of the battery for measurement, it is necessary to ensure that the instrument resolution is ≥ 0.1mV, the accuracy meets the error requirements under the 10V range, and has temperature compensation function.

② Key parameters for selection:

Resolution: High resolution (such as 0.1 μ V level) can identify small voltage differences, facilitating early detection of defective cells;

Accuracy: Calculated based on "reading error+resolution error", it is recommended to use a 10V range for a 4V battery;

Temperature compensation: Every 1 ° C change in ambient temperature may cause OCV to fluctuate by hundreds of μ V, which needs to be converted to the standard temperature value through compensation function.

Measurement steps and timing

3. Precautions

① Polarization effect differentiation

OCV1 (Instantaneous Voltage): The initial value measured immediately after charging and discharging, which is greatly affected by polarization;

OCV2 (Stable Voltage): The equilibrium value that eliminates polarization after settling, which is closer to the true electrochemical equilibrium potential and is the core basis for evaluating SOC and SOH.

② Environmental control

Maintain stable measurement environment temperature or correct readings through device temperature compensation function to avoid measurement errors caused by temperature fluctuations.

③ Safe operation

Confirm that the battery has no physical damage before measurement to avoid short circuits; High voltage battery packs require the use of insulated tools to prevent the risk of electric shock.

4. Application scenarios

Production screening: Remove defective cells with abnormal self discharge through OCV differences to ensure consistency of the battery pack;

Performance evaluation: Analyze battery capacity degradation and aging degree by combining OCV-SOC curve (such as discharge platform voltage);

BMS optimization: Real time monitoring of OCV to dynamically adjust charging and discharging strategies, extending battery cycle life.

5. Special measurement method (patented technology)

Rapid testing method: By short-term charging and discharging (<5s) and multiple cycles of depolarization, the settling time is shortened (the total testing time is reduced by more than 50%), which is suitable for mass production rapid testing scenarios.

Curve fitting method: Draw an OCV curve by combining the midpoint of the charge discharge voltage curve to improve the accuracy of SOC estimation.

Note: In actual operation, the measurement scheme should be selected according to the application scenario (laboratory/production line). For high-precision scenarios, specialized battery testers should be prioritized and environmental variables should be strictly controlled.

6. Summary

In summary, open circuit voltage (OCV), as the core electrochemical characteristic parameter of lithium batteries in non working states, is not only a key basis for evaluating the state of charge (SOC), but its numerical differences also directly reflect the inherent characteristics of different material systems (such as ternary, lithium iron phosphate, lithium manganese oxide). By standardizing the measurement process (static elimination of polarization, high-precision equipment and temperature compensation) and scientific analysis methods (OCV-SOC curve, rapid measurement technology), OCV plays an irreplaceable role in production screening of defective cells, evaluation of battery aging degree, optimization of BMS charging and discharging strategies, and other scenarios. In practical applications, it is necessary to pay attention to the differentiation of polarization effects and environmental control, while following the maintenance principle of long-term storage to maintain 70% SOC (OCV about 3.8V), in order to maximize the performance stability and cycle life of lithium batteries.