How does ambient operating temperature influence the internal resistance and polarization behavior of lithium-ion batteries during performance testing? Optimize Testing with Thermal Control

Ambient operating temperature is a major determinant of both internal resistance and polarization in lithium-ion batteries. As temperature falls, electrolyte ionic conductivity, lithium-ion diffusion, and electrochemical reaction rates decrease, increasing ohmic and polarization resistance. The result is a larger voltage drop during high-current discharge and a higher voltage requirement during charging. Higher temperatures generally reduce these dynamic voltage losses, although they accelerate degradation and introduce safety risks.

Temperature changes the battery’s apparent electrical behavior, not merely its available capacity. Accurate performance testing and battery simulation therefore require resistance and polarization parameters measured across the intended temperature range.

Why Temperature Changes Internal Resistance

Low Temperature Raises Ohmic Resistance

At low ambient temperatures, the electrolyte conducts lithium ions less effectively. This increases the cell’s DC ohmic resistance, which represents the immediate voltage loss associated with current flowing through the cell’s resistive components.

The effect is visible as an immediate change in terminal voltage when current is applied:

[ \Delta V_{\text{ohmic}} = I \times R_{\text{ohmic}} ]

For the same test current, a higher resistance produces a larger voltage deviation from the open-circuit voltage.

Electrolyte and Electronic Paths Behave Differently

Temperature does not affect every resistance component in the same way. Electrolyte resistance is strongly influenced by ionic conductivity and generally rises as temperature decreases.

Metallic conductors may show different temperature dependence, but in many lithium-ion cells the low-temperature increase in electrolyte and electrochemical resistance dominates the measured behavior.

Higher Temperature Reduces Immediate Voltage Loss

Elevated temperature improves electrolyte ionic conductivity and accelerates electrochemical reaction kinetics. This generally lowers the cell’s measured polarization resistance and reduces the dynamic voltage loss under load.

The improvement is useful for short-term power performance, but it should not be confused with improved long-term durability.

How Temperature Changes Polarization

Cold Conditions Increase Activation Polarization

At low temperatures, the electrochemical reactions at the electrodes proceed more slowly. The cell therefore requires a greater overpotential to sustain the requested charge or discharge current.

This increased activation polarization appears as a larger separation between the cell’s equilibrium voltage and its operating voltage.

Slow Diffusion Creates Concentration Polarization

Lithium-ion transport through the electrolyte and electrode materials also slows in the cold. During high-rate operation, lithium ions cannot redistribute quickly enough to maintain uniform concentration.

The resulting concentration gradients produce concentration polarization, adding to the voltage loss caused by ohmic and activation resistance.

High-Rate Discharge Magnifies the Effect

Polarization grows with current, so cold-temperature effects become especially pronounced during high-rate testing. A cell discharged at 4 C can exhibit a substantially larger voltage drop at 4 °C than at room temperature; the primary reference reports a drop of approximately 10 V in its stated test setup.

That magnitude should be interpreted in the context of the tested cell or battery configuration, because a single lithium-ion cell cannot normally sustain a 10 V terminal-voltage drop without leaving its normal operating range.

Charging Is More Sensitive Than Discharging

During cold charging, slow lithium-ion transport can cause lithium ions to accumulate near the graphite anode surface. The resulting concentration polarization increases the voltage required to charge the cell.

Under sufficiently severe conditions, metallic lithium can deposit on the anode. This lithium plating reduces usable capacity and can create serious safety risks, including internal short circuits.

What Performance Tests Actually Measure

Terminal Voltage Combines Several Effects

The measured terminal voltage is not determined by open-circuit voltage alone. A simplified representation is:

[ V_{\text{terminal}} = V_{\text{OCV}}

• I R_{\text{ohmic}}
• \eta_{\text{activation}}
• \eta_{\text{concentration}} ]

During charging, the signs reverse in the expected direction because the tester must raise the terminal voltage above the equilibrium voltage.

Temperature changes each of these terms, particularly the ohmic and polarization components.

Pulse Tests Separate Fast and Slow Responses

A current pulse can reveal the immediate ohmic response through the rapid voltage step. The slower voltage change that follows reflects electrochemical polarization, diffusion limitations, and other dynamic processes.

Testing at multiple pulse durations helps distinguish a simple resistance change from a broader temperature-dependent impedance response.

Hysteresis Becomes More Pronounced in the Cold

Low temperatures can increase the difference between the voltage observed during charging and the voltage observed during discharging at comparable states of charge. This voltage hysteresis reflects delayed electrochemical and diffusion responses.

If a model ignores this behavior, it may estimate state of charge, available power, or energy efficiency inaccurately during cold operation.

Why Controlled Temperature Testing Matters

Ambient Temperature Must Be Recorded Precisely

“Room temperature” is not a sufficient test condition when comparing battery results. A cell tested at 20 °C and another tested at 25 °C may show meaningfully different resistance and polarization behavior, especially under high current.

The test record should include cell temperature, ambient temperature, current profile, state of charge, rest duration, and thermal stabilization time.

Thermal Equilibrium Affects Repeatability

The chamber air temperature and the cell’s internal temperature are not necessarily identical. High-current pulses can also create internal heat during the test.

Reliable measurements require sufficient time for the cell to reach the target temperature and consistent thermal conditions throughout the test.

Resistance Should Be Characterized Across the Operating Range

For high-precision simulation, internal resistance should be measured at multiple temperatures rather than represented by a single room-temperature value. The results can be stored in lookup tables or represented through temperature-dependent parameter models.

These parameters should normally also vary with state of charge, current direction, and pulse duration because resistance and polarization are not fixed cell constants.

Understanding the Trade-offs

Higher Temperature Improves Power but Accelerates Aging

Warm cells generally deliver better high-rate power because their resistance and polarization losses are lower. However, elevated temperature accelerates parasitic reactions, capacity fade, and structural degradation.

Temperatures above approximately 40–50 °C therefore require careful interpretation: the immediate performance may improve while cycle life and safety margins deteriorate.

Lower Temperature Preserves Some Processes but Reduces Usable Performance

Cold operation can reduce self-discharge and slow certain degradation reactions. These benefits do not offset the loss of usable power and capacity caused by increased resistance and polarization.

Below 0 °C, charging is particularly concerning because the risk of lithium plating increases as ion transport and intercalation become limited.

Uniformity Matters in Battery Packs

A pack can contain cells at different temperatures even when its average temperature appears acceptable. Keeping the cell-to-cell temperature difference, or ΔT, within approximately 5 °C helps preserve capacity consistency and reduces unequal aging.

Testing should therefore assess not only average ambient temperature but also temperature gradients across the pack.

Capacity Results Are Not Pure Material Measurements

A cold-temperature capacity reduction may result from voltage reaching the test cutoff early because of polarization, rather than from permanent loss of active material. For example, very low temperatures can reduce usable discharge capacity substantially through electrochemical underutilization.

Capacity tests should distinguish reversible temperature-induced limitation from irreversible capacity fade by repeating measurements after the cell returns to a controlled reference temperature.

Making the Right Choice for Your Goal

Temperature-aware test design should reflect the performance question being answered.

• If your primary focus is high-power performance: Measure pulse resistance and dynamic polarization at each relevant temperature, state of charge, and current rate.
• If your primary focus is charging safety: Include cold-temperature charging tests and monitor voltage behavior for signs of excessive overpotential and possible lithium plating.
• If your primary focus is simulation accuracy: Populate temperature-dependent lookup tables or parameter models for ohmic resistance, polarization resistance, and relaxation behavior.
• If your primary focus is cycle life: Test elevated temperatures separately from short-duration power tests because improved immediate conductivity can coexist with accelerated degradation.
• If your primary focus is pack consistency: Control and record cell-to-cell temperature variation, not only the chamber setpoint or average pack temperature.

Accurate lithium-ion battery characterization begins by treating temperature as a core electrical parameter rather than a secondary test condition.

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