What are the nominal voltage, open circuit voltage, termination voltage, and midpoint voltage of a battery.

The characteristics of battery voltage vary significantly depending on the battery chemistry and type of battery.

Nominal voltage- the average voltage at which the battery functions during the better part of the discharge cycle is called the nominal voltage. The value is standardized for the categorization of batteries. The common battery chemistries include;

Lithium-ion battery- the nominal voltage is around 3.7 to 3.8 volts per cell.

Alkaline battery- the nominal voltage is 1.5 volts per cell.

Lead-acid battery- nominal voltage is 2 volts per cell. For instance, a 6-cell battery is 12 volts and a 3-cell lead-acid battery is 6 volts.

Open Circuit Voltage- The voltage of a battery when no load is connected is the open circuit voltage. It shows the maximum voltage when no current is flowing through the battery. Variations can occur depending on the battery temperature and state of charge. The open circuit voltage of a 3.7V lithium-ion battery can be around 4.2 volts.

Termination voltage- the voltage level at which a battery is said to be fully depleted or discharged is the termination voltage. The lower voltage threshold ensures that over-discharging of the battery is avoided as it is harmful. For instance, a 3.7V lithium-ion battery has the termination voltage set at either 2.5V or 3.0V.

Midpoint voltage- the level at which the battery is at approximately 50% of the capacity is the midpoint voltage. This acts as a reference point when gauging the battery's state of charge. For a 3.7V lithium-ion battery, the midpoint voltage can be around 3.6V.

What is the nominal voltage of a battery?

The standardized value used in the categorization of battery voltage is known as the nominal voltage. It describes the average voltage at which the battery functions while undergoing the discharging cycle. The nominal voltage is usually a rounded value as the actual battery voltage can vary depending on the state of charge and battery chemistry among other factors.

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Common nominal voltage for various battery chemistries include;

Alkaline battery- typically, the nominal voltage for a single-cell alkaline battery is 1.5 volts. Alkaline batteries are used in series where a device requires multiple cells to achieve higher voltages.

Lithium-ion battery- for Li-ion batteries, the nominal voltage is around 3.7 to 3.8 volts. There can be variations however depending on the specific manufacturer and battery chemistry. To achieve higher voltages, multiple-cell lithium-ion batteries are used in series.

Lead-acid battery- for a single-cell lead-acid battery, the nominal voltage is 2 volts. Common lead-acid batteries are often 12V. Six 2V cells are connected in series to achieve the required voltage.

Nickel-metal hydride (NiMH) battery- typically, the nominal voltage of NiMH battery is 1.2 volts. Just like alkaline batteries, these too are used in series where higher voltage is required.  

As much as the nominal voltage is used as a reference point, the actual battery voltage can vary during the discharge cycle. The load conditions and temperature can impact the battery’s voltage. Protection circuits should be designed and proper voltage regulation put in place for safe and reliable operation.

How do you determine the nominal voltage of a battery?

The battery chemistry and construction are used in determining the nominal voltage. The characteristics of chemical reactions taking place within battery cells provide a basis on which the manufacturers assign a nominal voltage to different batteries. Ways of determining the battery nominal voltage include;

Identify battery chemistry- nominal voltages vary depending on battery chemistries. The common battery chemistries include;

Alkaline battery with a nominal voltage of 1.5 volts per cell.

Lithium-ion battery with a nominal voltage of 3.7 to 3.8 volts per cell.

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A lead-acid battery with a nominal voltage of 2 volts per cell.

Nickel-Metal Hydride Battery with a nominal voltage of 1.2 volts per cell.

Count the cells- the manufacturer provides voltage for a single cell of the battery. For batteries using multiple cells connected in series, the summation of all nominal voltages of total cells is the nominal voltage.

Read manufacturer's specifications- the label on battery packaging can give specifications regarding the nominal voltage. In most cases, the nominal voltage is indicated clearly.

Using a Multimeter can measure the battery voltage if they have access to a multimeter good enough to measure DC voltage. This can also measure the open circuit voltage which is close to the nominal voltage but can vary slightly based on the temperature and battery state of charge.

Refer to industry standards nominal voltage for different battery types is often specified on industry standards and conventions. The standards ensure consistency in battery specifications and labeling.

Define Battery Open Circuit Voltage. 

The battery voltage measured when no load is connected and no current is flowing across the terminals is called the open circuit voltage. Open circuit voltage is the battery voltage when no device or circuit is connected. The maximum voltage provided by a battery when not connected to any load or is fully charged is represented by OCV. 

This shows the remaining battery energy level or state of charge making it a crucial parameter.

Fully charged state- when the battery is fully charged, it shows the highest OCV. For a 3.7V lithium-ion battery the OCV is around 4.2V.

State of charge- the open circuit voltage serves as a reference point for estimation of the battery state of charge. Comparing the OCV of a battery when fully charged and during the discharge cycle, one can approximate how much energy the battery still holds.

Voltage variations- the OCV can vary based on factors like battery age, chemistry, and temperature. During the interpretation of OCV measurements, the factors should be considered.

Discharge curve- the voltage decreases from the OCV during the discharge cycle to a lower level. The load conditions and battery chemistry determine the voltage decline.

Safety consideration- it is essential to be sure of the battery’s OCV during operation. Extremely low OCV can indicate a depleted battery while extremely high OCV can indicate overcharging.

Measuring OCV- this can be done using a multimeter or voltmeter when there is no load connected or current draw.

Use in battery testing- in the assessment of battery condition, OCV is used in battery testing and diagnostics.

Conclusion

Battery voltage is an important factor to consider when handling batteries. The values of battery voltage will vary depending on the battery chemistries, specific models, and manufacturers. It is important to follow the guidelines and specifications provided by the manufacturers for the battery in use. Safe charging and discharging practices should be followed to ensure safety and battery longevity.