Measuring the Internal Resistance of a Battery: An In-Depth Look

Batteries are the powerhouses behind many of the devices we use daily, from our smartphones to electric vehicles. While we typically consider a battery's capacity (mAh or Ah) or its voltage, there's another crucial aspect: the internal resistance (R_int). This property can give us insights into a battery's health, efficiency, and performance. This article delves into the importance of measuring a battery's internal resistance and provides a guide on how to measure it.

What is Internal Resistance?

Every real-world battery has an inherent resistance to the flow of electric current. This resistance originates from the electrochemical processes happening inside and the internal components' resistance. It's termed "internal resistance."

Why is Internal Resistance Important?

  1. Battery Health: A battery's internal resistance can increase as the battery ages. A significant rise in R_int often signals that the battery is nearing the end of its life.

  2. Performance: High internal resistance means more energy from the battery is wasted as heat. This inefficiency can reduce the battery's effective capacity.

  3. Safety: High internal resistance can lead to overheating, making batteries, especially lithium-ion ones, susceptible to failures or even explosions.

How to Measure the Internal Resistance of a Battery

There are several methods to measure a battery's internal resistance. Here, we'll discuss two common techniques: The DC Load method and the AC Impedance method.

1. DC Load Method

This straightforward method involves measuring the battery's open-circuit voltage (no load) and its voltage under a known load.


  1. Measure the open-circuit voltage (V_open) of the battery using a multimeter.
  2. Connect a known resistor (R_load) across the battery's terminals, creating a load.
  3. Measure the loaded voltage (V_load) with the resistor still connected.
  4. Calculate the current (I) flowing through the load using Ohm's law: I = V_load / R_load.
  5. Determine the voltage drop (ΔV) due to internal resistance: ΔV = V_open - V_load.
  6. Calculate the internal resistance (R_int) using Ohm's law: R_int = ΔV / I.

2. AC Impedance Method

This method uses an AC signal, typically at a frequency of 1kHz, and measures the impedance (combination of resistance and reactance) of the battery. Specialized equipment like an impedance meter or LCR meter is required.


  1. Connect the impedance or LCR meter to the battery.
  2. Set the meter to the appropriate frequency (commonly 1kHz).
  3. Initiate the measurement. The device will provide an impedance value.
  4. For most batteries, the reactance component at 1kHz is negligible, so the impedance approximates the internal resistance.

Factors Influencing Internal Resistance

  • Temperature: R_int typically increases with decreasing temperature.

  • State of Charge (SoC): For some batteries, R_int varies depending on whether they're fully charged, half-charged, or nearly empty.

  • Age and Usage: As mentioned, older batteries or those subjected to poor charging habits can exhibit increased R_int.


Understanding and measuring a battery's internal resistance is crucial for assessing its health, efficiency, and safety. Whether you're an engineer, a tech enthusiast, or just someone curious about batteries, appreciating the nuances of R_int can offer deeper insights into the world of portable power. Remember, it's not just about how much energy a battery can hold, but also about how effectively and safely it can deliver that energy.

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