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Understanding LiPo Battery Internal Resistance

In the specifications of a Lithium Polymer (LiPo) battery, capacity (mAh) and voltage (V) often steal the spotlight. They tell you how much energy is in the “tank” and how much pressure is pushing it. But there is a third, silent metric that arguably matters more for high-performance applications: Internal Resistance (IR).

At Hanery, we often describe IR to our OEM clients as the “friction” inside a battery. It is the invisible force that opposes the flow of current. Whether you are designing a high-torque power tool, a long-endurance drone, or a medical device, understanding IR is the key to predicting voltage sag, heat generation, and the true lifespan of your power source. As a leading Chinese manufacturer specializing in polymer lithium batteries, 18650 packs, and LiFePO4 solutions, we place IR testing at the core of our Quality Control (QC) protocols.

This guide will deconstruct the physics of internal resistance, providing you with the data, testing methods, and actionable insights needed to optimize your battery selection and usage.

Definition and Measurement Methods

Internal Resistance, measured in milliohms (mΩ), is not a physical resistor sitting inside the battery. It is the sum of several resistive factors:

Ionic Resistance: The resistance of ions moving through the electrolyte and separator.
Electronic Resistance: The resistance of the actual materials (anode, cathode, aluminum/copper tabs) and their contact points.
Interfacial Resistance: The resistance of chemical reactions occurring at the electrode surface (Solid Electrolyte Interphase – SEI).

AC vs. DC Measurement: The Industry Standard

If you measure a battery with a standard multimeter, you won’t get an accurate IR reading because the battery is a voltage source, not a static resistor. The industry uses two distinct methods:

AC IR (Impedance): This is the industry standard for factory grading, including at Hanery. We inject a 1kHz AC signal into the battery to measure the impedance.
- Pros: Extremely fast (milliseconds) and does not deplete the battery.
- Use Case: Grading new cells on the production line.
DC IR (Load Test): This measures the voltage drop under a heavy load (e.g., 10 Amps) and calculates resistance using Ohm’s Law (R = Vdrop / I).
- Pros: Represents real-world performance (sag).
- Use Case: Testing how a battery will perform in a specific device (e.g., a drone takeoff).

Note: AC and DC readings are not identical. DC IR is typically 1.5x to 2x higher than AC IR because it accounts for polarization and chemical lag, which the fast 1kHz AC signal ignores.

How IR Affects Capacity and Voltage Sag

A common misconception is that IR reduces the total energy stored in the chemical bonds. In reality, it reduces the usable capacity by creating a premature voltage cutoff.

The Voltage Sag Equation

According to Ohm’s Law (V = I x R), current flowing through resistance creates a voltage drop.

Vterminal = Vopen_circuit – (Current x Internal_Resistance)

Imagine a drone battery at 3.8V (50% charge).

Scenario A (Low IR – 2mΩ): At a 50A load, the drop is 50 x 0.002 = 0.1V. The voltage sags to 3.7V. The drone flies fine.
Scenario B (High IR – 20mΩ): At the same 50A load, the drop is 50 x 0.020 = 1.0V. The voltage instantly sags to 2.8V. The drone’s low-voltage alarm triggers, and it lands, even though the battery still has 50% chemical energy remaining.

High IR turns electrical energy into waste heat (I²R loss) rather than useful work, effectively “shrinking” your usable capacity.

IR in New vs. Used Cells

nternal resistance is the most reliable “odometer” for a battery’s health.

Brand New Cells: A fresh, high-quality high-discharge LiPo cell from Hanery typically measures between 1mΩ and 6mΩ (AC 1kHz) depending on size. The chemicals are active, the electrolyte is fluid, and the contact welds are pristine.
Used/Aged Cells: As a battery cycles, the electrolyte decomposes and the SEI layer on the anode thickens. This thickening creates a barrier that ions must struggle to cross.
End of Life (EOL): When a cell’s IR doubles from its initial value, it is generally considered near the end of its useful life for high-power applications. If it quadruples, it should be retired immediately to prevent overheating.

Temperature Influence

Temperature acts as a modifier for internal resistance. This relationship follows the Arrhenius Equation, where chemical reaction rates decrease as temperature drops.

Cold Weather Spike

In cold temperatures (0°C and below), the electrolyte becomes viscous (thick). This drastically increases Ionic Resistance.

Data Point: A battery that measures 3mΩ at 25°C may measure 15mΩ to 20mΩ at -10°C. This is why electric vehicles lose range in winter and phones shut down unexpectedly; the IR spike causes massive voltage sag.

High Heat Danger

Conversely, heat lowers IR temporarily by making the electrolyte more fluid. However, operating above 60°C permanently degrades the electrode binders, causing the IR to rise permanently once the battery cools down.

Why IR Increases Over Time

Why does a battery that was punchy and powerful a year ago feel sluggish today? The rise in IR is driven by irreversible chemical changes:

Electrolyte Dry-Out: The liquid solvents slowly evaporate or break down into gas (swelling), leaving fewer pathways for ions to travel.
SEI Growth: The Solid Electrolyte Interphase is a protective layer. Over time, it grows too thick, clogging the “pores” of the anode.
Contact Corrosion: Moisture ingress can cause microscopic oxidation on the internal aluminum and copper tabs, increasing Electronic Resistance.

Expected Ranges by Capacity (Reference Chart)

One of the most frequent questions Hanery receives is: “What is a normal IR number?” The answer depends heavily on the Capacity (mAh) and C-Rating of the cell. Larger cells generally have lower resistance because they have a larger total electrode surface area.

Below is a reference chart for New LiPo cells (per cell measurements via AC 1kHz method):

Battery Capacity	Typical IR (High C-Rate)	Typical IR (Standard/Low C)	Application Context
100mAh – 300mAh	80 – 150 mΩ	150 – 250 mΩ	Tiny Bluetooth headsets, Hearing aids
500mAh – 900mAh	30 – 60 mΩ	60 – 100 mΩ	Small IoT sensors, Key fobs
1000mAh – 1800mAh	10 – 20 mΩ	25 – 50 mΩ	Action cameras, Small drones
2200mAh – 3500mAh	5 – 10 mΩ	15 – 30 mΩ	RC Aircraft, Handheld tools
5000mAh – 10000mAh	1 – 4 mΩ	8 – 15 mΩ	Large Drones, EV modules, Laptops
20Ah+ (Prismatic)	< 0.8 mΩ	1 – 3 mΩ	Energy Storage Systems (ESS)

Note: These values are per single cell. A 3S (11.1V) pack would have the sum of three cells plus the resistance of the connector and wire.

IR Testing Equipment

For OEMs and serious enthusiasts, accurate measurement requires the right tools.

Professional Factory Grade (AC Method)

Hioki BT3562 / BT3563: The gold standard in battery manufacturing. These benchtop units use the 4-terminal sensing method (Kelvin connection) to eliminate probe resistance.
YR1030 / YR1035+: Portable, handheld AC 1kHz testers. While not as precise as a Hioki, they are widely used by hobbyists and for quick field checks. They are surprisingly accurate for their cost.

Consumer Grade (DC Method)

Smart Chargers: Many modern chargers (e.g., iCharger, ToolkitRC) have an “IR Test” function. These use the DC load method by pulsing the battery.
- Caution: These readings vary wildly depending on the battery’s state of charge. Only compare readings taken at fully charged (4.20V) states for consistency.

Interpretation for OEMs: The Matching Game

For Original Equipment Manufacturers, IR is a critical Quality Control metric. When Hanery builds a multi-cell battery pack (e.g., a 48V e-bike battery), we don’t just grab cells randomly. We perform Cell Matching.

The Weakest Link Rule

In a series circuit, the current must pass through every cell. If a 10-cell pack has nine cells at 3mΩ and one cell at 10mΩ, that high-IR cell will heat up faster and sag deeper than the others.

Result: The BMS will detect the voltage sag on the “bad” cell and cut power to the whole pack, even if the other nine cells are full.

OEM Advice: When auditing suppliers, request their “IR Deviation” tolerance. At Hanery, our premium packs are matched so that the difference between the highest and lowest IR cell is less than ±1-2 mΩ.

Signs of Defective Cells

High internal resistance is often a symptom of other failures.

Heat During Charging: If a battery gets hot to the touch while charging at a normal rate (1C), it has high internal resistance. The energy that should be stored is being converted to heat.
Immediate Sag: If your device shows “Low Battery” immediately after turning on a motor, but “rebounds” to 80% when the motor stops, the battery has high IR.
Puffing/Swelling: Gas generation separates the electrode layers, drastically increasing IR. A swollen battery always has high resistance.

Optimizing Usage to Reduce IR Rise

While IR will naturally rise with age, you can significantly slow the process.

Storage Voltage: Never store batteries fully charged (4.20V) for long periods. High voltage stresses the electrolyte, causing oxidation that increases resistance. Store at 3.80V.
Avoid Deep Discharges: Draining below 3.0V damages the copper anode collector, leading to permanent resistance increases.
Overspec the C-Rating: If your device draws 20A, don’t use a battery rated for exactly 20A. Use a 30A or 40A battery. Running a battery near its limit generates heat, and heat breeds resistance.

Frequently Asked Questions

Can I lower the internal resistance of an old battery?

No. IR increase is due to permanent chemical degradation (SEI growth, electrolyte loss). You cannot “refresh” or “cycle” a LiPo to lower its resistance permanently. Once it is high, it is permanent.

Why is DC IR always higher than AC IR?

AC IR uses a fast 1kHz signal that only measures “ohmic” resistance (materials). DC IR involves a sustained load, which triggers “polarization” resistance (chemical reaction lag). Both are real, but DC represents the voltage sag you feel during use.

Does capacity affect internal resistance?

Yes. Generally, higher capacity = lower resistance. Think of a battery like a water pipe. A 5000mAh battery is a “wider pipe” than a 1000mAh battery, allowing current to flow more easily.

What is a “good” IR for a racing drone battery?

For top-tier racing, pilots look for cells under 2mΩ to 3mΩ per cell. Anything above 5-6mΩ will feel “soft” or lack punch at full throttle.

At what IR should I retire my battery?

There is no set rule, but a common benchmark is when the IR is double that of a new pack. Or, when the battery gets uncomfortably hot (>50°C) during normal use.

Can I mix cells with different internal resistances?

Never. If you connect a low-IR cell and a high-IR cell in series, the high-IR cell will over-discharge and overheat. If in parallel, the low-IR cell will work harder and wear out faster.

Does the length of the wire affect the reading?

Yes! Wires and connectors have their own resistance. When measuring, always account for the lead resistance. Professional testers use “Kelvin Sensing” (4-wire) to ignore the test lead resistance completely.

Why does my charger show different IR readings every time?

Charger-based measurements are not lab-grade. They are influenced by the contact quality of the plug and the battery’s temperature. Only trust averages taken at the same temperature and charge state.

Is Low IR always better?

Generally, yes. However, for low-drain applications (like a TV remote), high IR is acceptable and often comes with cheaper, high-energy-density chemistries. For high-drain (power tools), Low IR is non-negotiable.

How does Hanery ensure low IR in production?

We use high-purity cathode materials, optimize the “compaction density” of the electrode rollers, and use automated laser welding for tabs. This minimizes the structural resistance of the cell before it is even filled with electrolyte.

Summary & Key Takeaways

Internal Resistance is the “vital sign” of battery health. It dictates how efficiently a battery can deliver its energy and how much heat it will generate in the process.

The Metric of Power: Low IR equals high power and less sag. High IR equals heat and early shutdowns.
Size Matters: Do not compare the IR of a tiny 300mAh cell to a massive 5000mAh cell. Use the reference ranges provided.
Temperature is Key: Cold spikes IR; heat degrades the battery causing long-term IR rise.
QC is Critical: For OEMs, ensuring cell-to-cell IR consistency is the secret to building safe, long-lasting battery packs.

At Hanery, we bridge the gap between theoretical chemistry and real-world application. Our R&D teams are constantly refining electrolyte formulas and assembly techniques to push internal resistance lower, giving your devices more punch and longer life.

Optimize Your Power Solution Today

Are you experiencing voltage sag or unexpected shutdowns in your prototype? It might be an Internal Resistance mismatch. Contact Hanery today for a consultation. Our engineering team can analyze your load requirements and recommend the perfect low-resistance cell structure for your application.

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