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What Is The Difference Between Internal Resistance And Impedance?

Author: Site Editor     Publish Time: 2024-01-30      Origin: Site


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To grasp the nuances of internal resistance and impedance, it's crucial to recognize that impedance pertains to AC (alternating current), while internal resistance is more associated with DC (direct current). Despite their different contexts, their calculation follows the same formula, R=V/I, where R is internal resistance or impedance, V is voltage, and I is current.

Internal Resistance: The Barrier to Electron Flow

Internal resistance results from the collision of electrons with the conductor's ionic lattice, transforming electrical energy into heat. Consider internal resistance as a type of friction impeding electron movement. In scenarios where alternating current flows through a resistive element, it generates a voltage drop. This drop remains in phase with the current, illustrating a direct relationship between the current flow and the internal resistance encountered.

Impedance: A Broader Concept Encompassing Internal Resistance

Impedance represents a more comprehensive term that encapsulates all forms of opposition to electron flow. This includes not just internal resistance, but also reactance. It's a ubiquitous concept found across all circuits and components.

It's imperative to differentiate between reactance and impedance. Reactance specifically refers to the opposition offered to AC current by inductors and capacitors, elements that vary across different battery types. This variability is evident in the differing diagrams and electrical values characteristic of each battery type.

To demystify impedance, we can turn to the Randles model. This model, depicted in Figure 1, integrates R1, R2, alongside C. Specifically, R1 represents the internal resistance, while R2 corresponds to the charge transfer resistance. Additionally, C denotes a double-layer capacitor. Notably, the Randles model often excludes inductive reactance, as its impact on battery performance, particularly at lower frequencies, is minimal.

Randles model of a lead acid battery

Figure 1: Randles model of a lead acid battery

Comparison of Internal Resistance and Impedance

To clarify, a detailed comparison of internal resistance and impedance is outlined below.

Aspect of Electrical    Property

Internal Resistance (R)

Impedance (Z)

Circuit Application

Utilized   primarily in circuits operating on direct current (DC).

Predominantly   employed in circuits designed for alternating current (AC).

Circuit Presence

Observable   in both alternating current (AC) and direct current (DC) circuits.

Exclusive   to alternating current (AC) circuits, not present in DC.


Originates   from elements that obstruct the flow of electric current.

Arises   from a combination of elements that resist and react to the electric current.

Numerical Expression

Expressed   using definitive real numbers, for example, 5.3 ohms.

Expressed   through both real numbers and imaginary components, exemplified by 'R + ik'.

Frequency Dependence

Its   value remains constant regardless of the frequency of the DC current.

Its   value fluctuates with the changing frequency of the AC current.

Phase Characteristic

Does   not exhibit any phase angle or magnitude attributes.

Characterized   by both a definitive phase angle and magnitude.

Behavior in an   Electromagnetic Field

Solely   exhibits power dissipation when exposed to an electromagnetic field.

Demonstrates   both power dissipation and the capacity to store energy in an electromagnetic   field.

Precision in Battery Internal Resistance Measurement

As a solution provider specializing in monitoring and managing backup batteries, DFUN emphasis on battery internal resistance measurement aligns with established industry practices, drawing inspiration from widely accepted devices like Fluke or Hioki. Leveraging methods akin to these devices, known for their accuracy and widespread customer acceptance, we adhere to standards such as IEE1491-2012 and IEE1188.

High accuracy of Internal resistance measurement

Internal Resistance Test Results Comparison

IEE1491-2012 guides us in understanding internal resistance as a dynamic parameter, necessitating continuous tracking to gauge deviations from the baseline. Meanwhile, the IEE1188 standard sets a threshold for action, advising that if the internal resistance exceeds 20% of the standard line, the battery should be considered for replacement or subjected to a deep cycle and recharge.

Moving from these principles, our method of measuring internal resistance involves subjecting the battery to a fixed frequency and current, followed by voltage sampling. The subsequent processing, including rectification and filtering through an operational amplifier circuit, yields an accurate measurement of internal resistance. Remarkably swift, this method typically concludes within 100 milliseconds, boasting an admirable accuracy range of 1% to 2%.

In conclusion, precision in internal resistance measurement ensures effective monitoring of batteries, contributing to their longevity.  This guide aims to assist those who may find it challenging to differentiate between internal resistance and impedance, facilitating a nuanced understanding of these electrical properties. For more comprehensive information and understanding, you can explore additional resources from DFUN Tech.

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