What Is an Internal Resistance?
Everything You Need to Know
- Quick Answer
- Introduction
- The Physics Behind Internal Resistance
- Mathematical Representation & Circuit Analysis
- Factors Affecting Internal Resistance
- Maximum Power Transfer Theorem
- How to calculate internal resistance of a battery?
- Practical Implications & Applications
- Future Outlook
- Frequently Asked Questions
Quick Answer
Internal resistance is the inherent electrical resistance inside a battery or power source that opposes current flow during charging and discharging.
It causes voltage drop under load and generates heat, directly affecting power capability, efficiency, and thermal behavior.
Lower internal resistance generally indicates better battery health and higher usable performance.
Introduction
Internal resistance is the inherent opposition to current flow that exists inside a power source such as a battery or power supply.
It originates from electrode materials, electrolyte conductivity, and internal interfaces, and causes part of the electrical energy to be dissipated as heat.
Higher internal resistance leads to greater voltage drop under load and reduced usable power.
Why batteries get warm during use?
Internal resistance causes batteries to generate heat when current flows, because part of the electrical energy is lost as resistive (I²R) losses inside the cell.
As load current increases, this heat generation rises sharply, which can reduce efficiency and accelerate cell aging.
Effective thermal management and low internal resistance are therefore critical for safe, high-power battery operation.
The Physics Behind Internal Resistance
Chemical Sources (Batteries)
In batteries, internal resistance originates from electrolyte conductivity and ion mobility, which limit how quickly charge carriers can move between electrodes.
It also depends on electrode surface area and polarization effects, where reaction kinetics and concentration gradients slow charge transfer under load.
Electronic Sources
In battery pack, internal resistance originates from semiconductor junctions (MOSFETs, diodes, regulators) where carrier conduction causes voltage drop and heat.
Additional resistance comes from internal PCB traces, bond wires, and contact interfaces that limit current flow under load.
Together, these effects explain why electronic sources dissipate power and deviate from ideal zero-resistance behavior.
Mathematical Representation & Circuit Analysis
The Equivalent Circuit Model
The equivalent circuit model represents a real battery as an ideal voltage source in series with a small internal resistor.
This series resistance explains load-dependent voltage drop, heat generation, and reduced usable power during high current draw.
It provides a simple, widely used framework for analyzing battery performance in real circuits.
Terminal Voltage Formula (V = E − I*r)
The terminal voltage formula V = E − I*r describes how a power source behaves under load, where E is the electromotive force (open-circuit voltage), I is the current, and r is internal resistance.
As current increases, the I*r voltage drop grows, causing the measured terminal voltage V to be lower than the EMF.
Open Circuit vs Closed Circuit
Open-circuit voltage is measured with no load, so no current flows and internal resistance causes no voltage drop.
When the circuit is closed and a load is applied, current flows and internal resistance creates an I·r drop, reducing the terminal voltage instantly.
This is why batteries show voltage sag under load even when fully charged.
Factors Affecting Internal Resistance
State of Charge (SoC)
State of Charge (SoC) directly affects internal resistance, which typically increases as a battery discharges.
At low SoC, reduced ion concentration and slower electrochemical kinetics raise resistance, causing higher voltage drop and heat under load.
This is why batteries deliver less peak power and show more voltage sag when nearly depleted.
Temperature Sensitivity
Temperature strongly affects internal resistance, which increases as battery temperature drops.
In cold conditions, slower ion mobility and reduced electrolyte conductivity limit current flow, leading to voltage sag and reduced usable capacity.
This is why batteries deliver less power and appear to “drain faster” in winter, even if their rated capacity is unchanged.
Age and Degradation
As batteries age, internal resistance increases due to electrode degradation, electrolyte decomposition, and growth of interfacial layers such as SEI.
Higher internal resistance causes greater voltage drop and heat generation under load, reducing usable power even if nominal capacity remains.
This resistance rise is a key indicator used in State of Health (SoH) assessment for battery aging and end-of-life decisions.
Discharge Rate
High discharge rates increase the apparent internal resistance of a battery due to polarization effects and limited ion transport at the electrodes.
As current draw rises, I²R losses grow rapidly, causing more heat generation and a larger terminal voltage drop.
This efficiency loss reduces available power and accelerates degradation, especially under sustained high-load conditions.
Maximum Power Transfer Theorem
The Matching Principle
The matching principle states that maximum power transfer occurs when the load resistance equals the source’s internal resistance.
Under this condition, voltage division causes half of the source voltage to drop across the internal resistance, maximizing power delivered to the load but not efficiency.
In battery systems, this principle explains why operating near internal resistance limits increases power output while also increasing heat and energy loss.
Efficiency vs Power
Maximum power transfer occurs when load resistance equals internal resistance, but this operating point limits efficiency to about 50% because half the energy is lost as heat inside the source.
High internal losses increase temperature, accelerate aging, and reduce reliability in batteries and power electronics.
For real systems, designers prioritize higher efficiency by keeping load resistance much greater than internal resistance, even if peak power is lower.
How to calculate internal resistance of a battery?
DC Load
The DC load method measures internal resistance by applying two known load currents and recording the corresponding terminal voltage drop.
Internal resistance is calculated from the change in voltage divided by the change in current (ΔV / ΔI), minimizing errors from open-circuit voltage variation.
This method reflects real operating conditions but is sensitive to temperature, state of charge, and load duration.
AC Impedance (EIS)
AC impedance (EIS) measures internal resistance by applying a small AC signal over a range of frequencies and analyzing the battery’s voltage–current response.
It separates ohmic resistance, charge-transfer resistance, and diffusion effects, making it useful for detailed battery health and degradation diagnostics.
EIS requires specialized equipment and controlled conditions, so it is mainly used in labs and advanced BMS development rather than routine field testing.
Practical Implications & Applications
Battery Health Monitoring (SOH)
In battery health monitoring (SoH), internal resistance is a key KPI for defining end of life (EoL) in EVs and smartphones because rising resistance directly limits power delivery and increases heat.
As cells age, resistance growth causes larger voltage sag under load, triggering power throttling or reduced usable capacity even if nominal capacity remains.
For this reason, many BMS define EoL by a resistance increase threshold rather than capacity loss alone.
Safety Concerns
High internal resistance increases I²R heat during charging or discharging, which can raise cell temperature beyond safe limits.
Excessive heat accelerates electrolyte breakdown and gas generation, leading to venting in cylindrical and pouch cells.
If heat is not controlled or dissipated, this feedback loop can trigger thermal runaway, especially under high current or fault conditions.
Design Considerations
High-drain applications require batteries with low internal resistance to minimize voltage sag, heat generation, and efficiency loss under high current loads.
Low-drain devices can tolerate higher internal resistance, prioritizing energy density and capacity over peak power capability.
Correct battery selection balances internal resistance, discharge rate, thermal limits, and the application’s real current profile to ensure safety and performance.
Future Outlook
Solid-state batteries aim to reduce internal resistance by replacing liquid electrolytes with solid ionic conductors that enable faster, more stable ion transport.
Lower internal losses can improve power capability, thermal stability, and cycle life compared with conventional lithium-ion cells.
Frequently Asked Questions
Yes, all batteries have internal resistance because ion transport, electrode reactions, and material interfaces inherently oppose current flow, affecting voltage and efficiency.
A good internal resistance is as low as possible for the battery type—typically a few milliohms for high-current lithium-ion cells—because lower internal resistance means less heat generation and better efficiency under load.
Increasing a battery’s internal resistance causes greater voltage drop, reduced usable power, lower efficiency, and more heat generation during charging and discharging.
A standard multimeter cannot directly measure a battery’s internal resistance, but it can estimate it indirectly by comparing open-circuit voltage and loaded voltage with a known load.
A battery with high internal resistance typically shows rapid voltage drop under load, poor high-current performance, excessive heating, slow charging, and reduced usable capacity.
Internal resistance is reduced by using high-quality electrode materials, improving electrolyte conductivity, increasing electrode surface area, minimizing contact resistance, and maintaining proper temperature and state of health.
Yes, a battery’s internal resistance changes over time—typically increasing with aging, cycling, high temperature exposure, and cell degradation, which reduces performance and efficiency.
Yes, wire length directly affects resistance—longer wires have higher resistance because electrical current travels through more conductive material, causing greater energy loss and voltage drop.
