Last Updated on 12/05/2026 by Bonnen Battery
The Internal Resistance of a Battery: AC Impedance vs DC Resistance Simplified
In short, DC internal resistance (DCIR) is like a battery’s total “friction” under a heavy discharge, while AC impedance (especially at 1 kHz) is mostly the immediate, ohmic part. DCIR requires a big pulse (think 40–80 A for a few seconds) and includes all sources of resistance ⇱ (ohmic, charge transfer, diffusion, etc.). In contrast, AC impedance is measured with a tiny 1 kHz AC signal (~50 mA) and is super quick (≈0.1 s). It mostly “sees” the fast, ohmic resistance because capacitive effects are shorted out at high frequency. Knowing both is key: DCIR tells you about real-world high-power performance, while AC impedance (and full EIS spectra) reveal electrochemical details. Understanding the difference helps you pick the right test and interpret battery health.
What Are They and How Are They Measured
• DC Internal Resistance (DCIR ⇱) – Measured by applying a heavy constant-current pulse (typically tens of amps) for a short time (2–3 seconds) and observing the voltage drop. Using Ohm’s Law (R = ΔV/ΔI), we get the resistance. Specialized testers do this; small cells usually can’t handle such pulses.
• AC Impedance (ACIR) – Measured by injecting a small sinusoidal current (usually 1 kHz, ~50 mA) and sampling the resulting voltage. Because the test current is tiny, it’s quick and causes almost no wear on the battery. This is effectively a snapshot of impedance at that frequency. A full AC impedance spectrum (EIS) ⇱ at many frequencies can give even more detail, but the single-frequency ACIR test is very common for fast checks.
| Feature | DC Internal Resistance | AC Impedance (at 1 kHz) |
| Signal | Heavy DC pulse (40–80 A) | Small AC signal (1 kHz, ~50 mA) |
| Test Time | Longer (2–3 seconds) | Very short (~0.1 second) |
| Battery Size | Only large cells (small ones can’t handle huge pulses) | Works for almost any cell, even small batteries |
| Cell Stress | Can heat and polarize cell interior | Minimal stress (low current) |
| Accuracy | Very high (≈0.1% error) | Good (≈1–2% error) |
| Measured R | Total resistance (ohmic + electrochemical) | Mostly ohmic resistance at 1 kHz |
| Common Use | Power performance, battery health & SoC checks | Fast QC/production testing, quality indicator |
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Key Characteristics
• DCIR Test: You “push” the battery hard and see how the voltage dips. This is great for checking how a battery will handle big loads (important for electric vehicles, power tools, etc.). Because the current is huge, internal components start to polarize quickly. To avoid errors, the pulse must be very short. The result is very accurate (battery internal testers boast ~0.1% precision). On the downside, not all cells can take such a blast – tiny batteries will be damaged or give bad readings.
• AC Impedance Test: Here you “tickle” the battery with a little AC at a fixed frequency (usually 1 kHz). The battery voltage response is measured and converted into an impedance. This is very fast (hundreds of milliseconds) and safe for the cell. Almost no heat is generated, so even small or fully charged cells can be tested without damage. The trade-off is slightly lower precision (about 1–2%) and it mostly captures just the immediate (ohmic) resistance. In practice, ACIR meters ⇱ are widely used on production lines – every battery “passes” this quick test to ensure it meets spec.
• What They Cover: DCIR measures the sum of all internal resistances ⇱ in action. That includes the resistance of the electrodes, electrolyte, separator (ohmic resistance), plus the resistance from charge transfer and ion diffusion happening during that pulse. ACIR at high frequency, however, effectively “ignores” the slower processes. Capacitive and diffusion effects get bypassed at 1 kHz, so the ACIR value is mostly the fast, ohmic part.
• Real-World Numbers: It’s common for a Li-ion cell to show a much lower ACIR than DCIR. For example, a typical 18650 cell might measure ~36 mΩ at 1 kHz but ~110 mΩ under a DC pulse. Both readings are correct; they just highlight different aspects. The AC test “sees” a low resistance because the chemistry doesn’t fully engage at high freq, whereas the DC test forces the chemistry and structure to resist.
What These Tests Tell Us
• DCIR Insights: Since it includes the chemical and ion-movement resistances, DCIR is a good indicator of battery power performance. A higher DCIR means more voltage drop under load, which translates to lower output power. It’s also sensitive to the battery’s state of health (SoH). As a Li-ion battery ages, its chemistry often degrades and internal resistance rises. Monitoring DCIR can thus help predict battery life. In industry, DCIR is also used in battery management systems to estimate state of charge (SoC) or state of power (SOP) by observing how the resistance changes.
• AC Impedance Insights: ACIR (at 1 kHz) is mostly a quick quality check – it tells you about the manufacturing quality and fast-conductivity. Because it’s so fast, it’s ideal for production lines. A cell with poor contacts or coating issues might have a higher ACIR. If you extend to a full impedance spectrum (EIS), you can peel apart processes: mid-frequency “semicircles” correspond to charge-transfer resistance at the electrode/electrolyte interface, and low-frequency tails correspond to ion diffusion. In short, AC impedance (EIS) gives richer electrochemical information, while DCIR gives a “big-picture” performance number.
When and Where They’re Used
• DCIR Testing is common when you need to know how a battery will behave under real load conditions. For example, EV and power tool makers care about it. It’s also used in labs and service centers to evaluate battery health and to verify capacity or warranty claims. Because it’s destructive if done too hard, DCIR tests are more controlled and less frequent in regular QA.
• ACIR Testing is a go-to on the factory floor and in quick troubleshooting. By applying a fixed 1 kHz AC signal, manufacturers can test every cell rapidly. As Tektronix notes, ACIR at 1 kHz is a “good indicator of battery quality” and much faster than a full spectrum EIS. Engineers also use AC impedance in R&D and quality control to catch production defects (like poor electrode coating or welding) without hurting the cells.
Effects of Temperature, SOC, and Charge/Discharge
• Temperature ⇱: As with most electrochemical things, temperature plays a big role. When a cell is cold, all internal processes slow down, so internal resistance goes up. Conversely, a warmer battery generally has lower resistance. (But too hot can hurt battery life in other ways!)
• State of Charge (SoC): The battery’s charge level affects polarization. Typically, internal resistance is lowest in the mid-range of charge (e.g. 20–80% SoC) and rises when almost empty or almost full. At 0% or 100% SoC, ions struggle more to move, so resistance is higher. In practice, this means if you test IR at different SoC levels, you’ll see it vary.
• Charging vs Discharging: While discharging, ions leave one electrode and enter another, which changes internal conditions. During a high-current charge, you might also see a temporary bump in internal resistance due to polarization. The exact effect can depend on how long and how hard you charge/discharge.
Keeping these factors in mind, engineers often compare ACIR/DCIR under controlled conditions (room temp, specified SoC) so tests are consistent.
Putting It All Together
In practice, it’s best to use both methods to fully understand a battery. Think of DCIR as giving you a realistic “stress test” number (how bad is the voltage sag under load ⇱), while ACIR (and especially full EIS) tells you more about where that resistance is coming from. If you see DCIR creeping up over months of use, that signals aging (loss of capacity, increased degradation). If ACIR is unexpectedly high, it might flag a production fault or contact issue.
Ultimately, Bonnen Battery uses rigorous testing (both AC and DC) to ensure our lithium cells meet specs. We know that low internal resistance—both AC and DC—is critical for delivering power efficiently and keeping your devices running cool and safe. Whether you need large-format EV packs or small cylindrical cells, we’ve got the know-how (and test data) to match the right battery to your needs.
Summary
Lithium battery internal resistance has two faces: DC internal resistance and AC impedance. DCIR is measured with a heavy pulse and includes the total of all resistance sources; it’s key for power and SoC assessments. AC impedance (at 1 kHz) is measured with a tiny AC signal; it’s fast and mostly reveals the immediate, ohmic resistance. DC tests take longer and stress the cell (so they need large batteries and short pulses), whereas AC tests are quick, non-destructive, and work on any battery. By choosing the right test (or doing both), battery engineers can get a clear picture of battery health and performance.
For example, a battery might measure ~36 mΩ AC but ~110 mΩ DC – both are true, just different perspectives. Use DCIR when you care about real-world discharge (EVs, big loads), and ACIR when you need fast QC or diagnostic insight. In short: one is a sprint (AC 1 kHz tickle), the other a full blast (DC pulse), and together they reveal the full story.
If you need reliable lithium batteries with guaranteed performance, Bonnen Battery is here to help. Visit us at bonnenbatteries.com or contact our team for more info. Our global exports include every battery-related product you can think of – all backed by rigorous AC and DC testing so you get the best cell for your application.
FAQs
Q: Why do AC and DC tests give such different resistance values?
A: They probe the battery in different ways. An AC test at 1 kHz only “sees” the fast, ohmic resistance (since capacitive and diffusive processes don’t have time to act). A DC test forces the battery chemistry to respond, so it includes additional resistances from ion diffusion and charge-transfer. That’s why a cell can show, say, 36 mΩ at 1 kHz but 110 mΩ under a heavy DC load – both are correct for the conditions of the test.
Q: Which method is more accurate, AC or DC?
A: DC pulses typically yield higher accuracy (~0.1% error) because they give a direct voltage/current reading. ACIR is pretty accurate (1–2%), but can be affected by signal noise or ripple. However, ACIR is fast and safe. It’s not about one being “better,” but about what you need: DCIR is more realistic for high-power loads, while ACIR is ideal for quick checks and doesn’t harm the cell.
Q: Can I use DCIR on small batteries?
A: Usually not. Small cells can’t safely handle the huge currents DCIR needs. For example, DCIR machines often use 40–80 A pulses, which would destroy a coin cell or small pack. That’s why ACIR is great – it uses only milliamps, so even watch batteries or phone cells can be tested without risk.
Q: How do temperature and charge level affect these measurements?
A: Lower temperature always raises internal resistance. Think of a freezing cold car battery vs. a warm one – the cold one will have much higher IR. As for charge level, resistance is usually lowest around mid-charge and increases near empty or full. At extreme SOC, the chemistry is less “active,” so the battery resists more. When testing, try to be consistent with temperature and SoC to compare apples-to-apples.
Q: Which test should I use for my project?
A: If you’re evaluating battery health, performance under load, or need to estimate capacity/SOC, use the DC pulse method. If you need a quick quality check on many cells (like in manufacturing) or a fast diagnostic without draining the battery, the AC impedance test is ideal. And if you want deep electrochemical insight, consider a full AC impedance spectrum (EIS) in a lab setting. Bonnen Battery can help you figure out which test fits your application.
Q: How to calculate internal resistance of a battery?
A: The simplest practical method (DC pulse) uses Ohm’s Law: measure the open-circuit voltage (VOC or OCV), apply a known load/current, measure the loaded voltage (Vload), then:
Make sure the load pulse is short (to avoid heating/polarization) and use Kelvin (4-wire) sensing if possible to avoid lead resistance. For AC testing, measure the AC voltage and current at a fixed frequency and compute magnitude: |Z| = V_ac / I_ac.
Q: Formula for internal resistance of a battery
A: Two common formulas:
DC pulse method: R = (V_ocv − V_load) / I_load (units: ohms, Ω).
AC (single-frequency) method: |Z(f)| = V_ac(f) / I_ac(f) where f is the test frequency (e.g., 1 kHz).
Note: ESR (equivalent series resistance) ⇱ is often used interchangeably with internal resistance in practical specs.
Q: How does internal resistance affect batteries’ performance?
A: Higher internal resistance causes:
Larger voltage sag under load → less usable voltage for your device.
More heat generation (power loss = I²R) → lower efficiency and potential thermal issues.
Reduced max deliverable power (Pmax ≈ V² / (4·R) for a simple load-match idea).
Faster apparent capacity loss under high loads (you see less energy at high current).
So low internal resistance = better power, less heat, and higher efficiency.
Q: What affects the internal resistance of a battery?
A: Main factors:
Temperature (colder → higher R).
State of Charge (SoC) (R varies with SoC; often lowest mid-SoC).
Age / State of Health (SoH) (aging and degradation ↑ R).
Current rate & polarization (higher currents can temporarily increase apparent R).
Manufacturing / materials (electrode coating, electrolyte conductivity, separator thickness).
Contacts & assembly (poor welds, corrosion, or loose connections add extra resistance).
Q: How does internal resistance cause the loss of voltage?
A: Voltage loss is simply the drop across the internal resistor:
So at higher current (I) or higher R, the drop increases and the battery’s terminal voltage falls. That’s why a high-current tool or EV shows voltage sag under heavy acceleration.
Q: How does internal resistance affect current in a battery?
A: Internal resistance forms part of the total circuit resistance. If you have a battery with internal voltage E and internal resistance r, and a load Rload, the current is:
So larger r → smaller current for the same load. Internal resistance also determines how much current the battery can safely deliver before voltage sags or heat becomes a problem.
Q: What is the origin of the internal resistance of a battery?
A: It’s not one single thing — internal resistance is the sum of multiple effects:
Ohmic resistance: metal current collectors, electrode material, electrolyte, separator.
Contact/weld resistance: poor joints or tabs.
Charge-transfer resistance ⇱: resistance associated with electrochemical reactions at the electrode/electrolyte interface.
Diffusion / Warburg impedance: resistance from slow ion transport inside electrodes or electrolyte (shows up at low freq in EIS).
Surface films (SEI) and aging layers that block ion flow.
AC/EIS tests help separate these contributions (high freq → ohmic; mid freq → charge transfer; low freq → diffusion).
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