Different Methods for Measuring Batteries’ Internal Resistance

XTAR Light

Apr 26, 2010
The main reason we want to test a battery's internal resistance is to know its health status. A higher resistance means more energy is wasted and turned into heat. A lower resistance means the battery is more efficient. And there are different ways to test the battery's internal resistance as shown below.

*DC load method:
It means to apply a little bit larger current to the battery for a short period of time. And it measures the voltage across the battery before and after the load is applied. The voltage drop is used to calculate the battery's internal resistance. Then the resistance can be calculated using Ohm's law (V=IR). This is one of the reliable test methods.

*AC testing method:
To measure ACIR, an ac signal, typically an ac current (Iac), is passed through the battery and the voltage response (Vac) of the battery measured. The ac current is usually around 100 mA and the frequency is 1000 Hz. The impedance at 1 kHz is calculated as Vac/Iac.

*EIS, or Electrochemical Impedance Spectroscopy:
It's a more advanced form of AC Impedance Spectroscopy, which is used to analyze the electrochemical behavior of batteries. The key difference is that ACIR is completed at 1 kHz. And EIS is completed over a wide range of frequencies by sweeping the ac stimulus current from mHz to 30 kHz—or even higher. The EIS is a high-precision method used in industry and scientific applications.

It's unlikely that any application of the battery will present a 1000Hz sinusoidal current load on the battery. Therefore, this ACIR measurement doesn't reveal much about how the battery will behave in a real-world application. However, ACIR has become a very standard way of assessing the battery's resistance. As for DC load testing, it's a simple and widely used method for measuring battery IR. And most smart chargers adopt the DC load for measurement. This measurement works well to check large stationary batteries. And the ohmic readings of the device are very accurate and repeatable.

So which method do you prefer for testing IR on a smart charger?