Understanding Direct Current Internal Resistance (DCIR) in Lithium-ion Batteries

 

1. Introduction

Direct Current Internal Resistance (DCIR) is a crucial parameter in evaluating the performance of lithium-ion batteries. It determines the voltage platform during discharge and indicates a battery’s high-power discharge capability. This article aims to explain the concept of DCIR, its components, how it’s measured, and why it matters for battery design and operation.

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2. What is Internal Resistance in a Battery?

Internal resistance refers to the opposition encountered by current as it flows through a battery. It consists of two main parts:

• Ohmic Resistance: Caused by the resistance of electrode materials, electrolytes, separators, and interfacial contact points.

• Polarization Resistance: Resistance arising from electrochemical reactions at the electrodes, including:

  • Electrochemical Polarization: Determined by the inherent nature of the battery system.

  • Concentration Polarization: Caused by changes in ion concentration during electrochemical reactions.

Differences in measurement duration and method can lead to variation in DCIR values, especially due to concentration polarization.

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Figure-1

3. Ohmic Resistance – Instant Voltage Drop (ΔU1)

As shown in Figure 1, when a battery begins to discharge, there is an immediate voltage drop (ΔU1), attributed to ohmic resistance. This drop occurs in milliseconds (typically within 1–2 ms). Therefore, testing systems require both high accuracy and fast response times to capture this behavior.

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4. Polarization Resistance – Gradual Voltage Drop (ΔU3)

Following the initial voltage drop, the battery voltage gradually decreases due to ongoing electrochemical reactions. This gradual decline results from both the battery's polarization and changes in open-circuit voltage (OCV) due to state of charge (SOC) shifts.

When discharging stops, a brief voltage recovery occurs (ΔU2), again due to ohmic resistance. Subsequently, the voltage slowly rises back to a stable level (ΔU3) as the internal reactions settle.

Note: During charging, voltage changes caused by ohmic and polarization resistance are positive in nature.

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5. Why DCIR Matters

Internal resistance is one of the most critical indicators of battery health and performance. It reflects how easily electrons and ions can move within the battery. Initial internal resistance is influenced by factors such as:

• Material composition

• Manufacturing quality

• Cell design and structure

Monitoring changes in DCIR is essential for:

• Predicting battery lifespan

• Ensuring reliable battery operation

• Detecting degradation or failure early

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6. Conclusion

DCIR provides valuable insight into a lithium-ion battery’s internal behavior. By understanding and monitoring both ohmic and polarization resistance, engineers and quality control teams can better manage battery performance, safety, and longevity.