When selecting and testing lithium batteries, many people encounter a puzzling scenario: two batteries show nearly identical internal resistance values when measured with an internal resistance tester, yet under actual high-current discharge, they perform very differently. One exhibits minimal voltage drop and low heat generation while maintaining full-power output, whereas the other suffers from a sharp voltage plunge and rapid temperature rise.
I. AC Internal Resistance and DC Internal Resistance: Same Name, Different Nature
Battery internal resistance is not a single value but the sum of ohmic internal resistance, electrochemical polarization resistance, and concentration polarization resistance. 1. AC Internal Resistance (ACR): A “snapshot” taken at 1 kHz. The vast majority of handheld internal resistance testers on the market employ the AC impedance method: a sinusoidal AC signal with a frequency of 1 kHz and low amplitude is injected into the battery, and the internal resistance is calculated based on the phase difference between voltage and current.
This measurement method has two key characteristics:
Primarily measures ohmic internal resistance: Under a high-frequency signal of 1 kHz, slow processes—such as ion diffusion and electrochemical reactions—cannot respond in time; consequently, only “instantaneous” ohmic impedances (associated with electronic and ionic conduction) are captured. It reflects the intrinsic electronic and ionic conductivity of the electrode sheets, current collectors, electrolyte, and separator.
Low value and high stability: Because polarization components are excluded, the AC internal resistance value is typically low and relatively insensitive to State of Charge (SOC) and temperature, making it suitable for the rapid screening of battery consistency. In short: AC internal resistance ≈ ohmic internal resistance; it is suitable only for a basic assessment of battery quality and cannot fully reflect rate capability.
2. Direct Current Internal Resistance (DCR): The “panoramic” DCR under high-current conditions is measured using the pulse discharge method. A high-magnitude DC current pulse (e.g., 1C, 5C, or 10C) is applied to the battery, and the ratio of the instantaneous voltage drop to the current is recorded; the calculation formula is DCR = ΔV / ΔI. This ΔV comprises three components:
Ohmic voltage drop: An instantaneous voltage jump corresponding to ohmic internal resistance, which shares the same origin as AC internal resistance;
Electrochemical polarization voltage drop: A voltage drop caused by electric double-layer charging and charge-transfer resistance, occurring on a timescale of milliseconds to seconds;
Concentration polarization voltage drop: A voltage drop resulting from insufficient diffusion of active lithium ions within the electrode and the formation of concentration gradients, occurring on a timescale of seconds to minutes.
Therefore: DCR = Ohmic internal resistance + Electrochemical polarization resistance + Concentration polarization resistance. This represents the true “total resistance” encountered by the battery during high-current discharge, directly determining the operating voltage, heat generation, and power capability under high-rate discharge conditions.
II. Why batteries with the same internal resistance exhibit different rate performance
Once the distinction above is understood, the answer becomes clear: an internal resistance tester measures AC internal resistance, which reflects only the ohmic component; however, a battery’s rate performance is determined by its DC internal resistance, of which polarization resistance constitutes a significant portion.
Two batteries can have nearly identical ohmic internal resistance—resulting in almost identical readings on a tester—yet if there is a vast difference in polarization resistance, their DC internal resistance will differ drastically, leading to vastly different rate performance.
Consider this intuitive example: Battery A (high-rate type) has an AC internal resistance of 1.0 mΩ, polarization resistance of 0.5 mΩ, and DC internal resistance of 1.5 mΩ; Battery B (high-energy type) has an AC internal resistance of 1.0 mΩ, polarization resistance of 2.0 mΩ, and DC internal resistance of 3.0 mΩ. While they appear identical on the tester, during a 10C discharge, Battery B experiences twice the voltage drop and four times the heat generation (P=I²R) of Battery A, resulting in significantly poorer rate performance.
III. Factors influencing polarization resistance: Why does it vary so much among lithium batteries? The root cause lies in the diametrically opposed design philosophies of high-rate batteries versus high-energy batteries; every structural trade-off redistributes the proportions of ohmic resistance and polarization resistance.
Comparison of Design Dimensions
① Electrode Areal Density: High-energy batteries feature thick coatings and high areal density to maximize the proportion of active material; high-rate batteries use thin coatings and low areal density to shorten ion diffusion paths. High-energy batteries experience significantly higher concentration polarization, resulting in higher DC internal resistance.
② Conductive Agent Content: High-energy batteries use a low proportion (1%–2%) to prioritize active material volume; high-rate batteries use a higher proportion (3%–5%) to establish a robust conductive network. High-rate batteries exhibit lower electrochemical polarization and more complete reactions.
③ Tab Design: High-energy batteries use single or narrow tabs, resulting in longer current paths; high-rate batteries use multiple or full tabs, ensuring uniform current distribution. High-rate batteries benefit from reduced ohmic resistance and lower localized polarization.
④ Electrolyte system: Energy-oriented types feature standard viscosity, balancing cycle life and cost; high-rate types feature low viscosity, high conductivity, and oxidation resistance. High-rate electrolytes facilitate faster ion migration and reduce concentration polarization.
⑤ Separator selection: Energy-oriented types use thin separators with low porosity to maximize energy density; high-rate types use thick separators with high porosity and ceramic coatings. High-rate separators offer lower resistance to ion transport and enhanced safety.
⑥ Cathode and anode materials: Energy-oriented types utilize materials with high tap density, high capacity, and large particle sizes; high-rate types utilize materials with small particle sizes, porous structures, and high specific surface area. High-rate materials provide more reaction sites and exhibit lower electrochemical polarization.
The core logic breaks down as follows: the goal of energy-oriented batteries is to “store more energy,” so manufacturers strive to thicken electrode sheets, densify materials, and minimize inactive components (such as conductive agents, current collectors, and tabs). The trade-off is that the diffusion path for lithium ions—traveling from the electrolyte into the active particles—becomes longer; consequently, under high-current conditions, the deep interior of the electrode suffers from “lithium depletion,” causing a sharp rise in concentration polarization.
In contrast, the goal of high-rate batteries is to “deliver higher current.” To achieve this, they intentionally sacrifice some capacity by using thinner electrode sheets, increasing the content of conductive agents, adding more tabs, and reducing material particle sizes. While these design choices offer limited improvement regarding ohmic internal resistance, they significantly reduce electrochemical polarization and concentration polarization—the very factors that account for the bulk of the increase in DC internal resistance.
This explains the common adage: a battery cannot simultaneously possess both high-rate performance and high energy density.