EIS Measurement of Small Impedances – Overview
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Chapter 11:
EIS Measurement of Small Impedances
Overview
The Gamry Instruments Reference 3000 is a high-performance measurement instrument used for all types of
electrochemical testing. Unlike many other electrochemical instruments, it offers outstanding performance for
tests with signals consisting of small currents and high impedances, and for tests involving large currents and
very small impedances.
Problems in the measurement of very small currents were discussed in the previous chapter.
This chapter is a discussion of a very different problem, when currents are large, voltages and cell-impedances
are small, and inductance limits measurements. These measurements include those encountered in research
on:
Batteries
Fuel cells
Super-capacitors
Results from fast transient techniques (CV, chronopotentiometry, etc.) or high-frequency EIS measurements on
low-impedance systems are often limited by the cell cable and connections to the cell. An improper cell cable
or a proper cell cable badly connected can cause significant errors in the data obtained.
This chapter should answer these questions:
Why is galvanostatic operation preferred when measuring low impedance?
What are the sources of error in measuring low-impedance cells?
Why use four-terminal measurement techniques
What is mutual inductance?
How do I connect a cable to my cell to minimize errors?
Why Galvanostatic Mode?
Even though potentiostatic EIS is the most commonly used EIS technique, it is poorly suited to impedance
measurements of low-impedance batteries, fuel cells and super-capacitors. This is why:
Current, voltage, and impedance are related through Ohm’s Law. Assume that a high-rate battery has an
impedance of 1 m
. A voltage of 1 mV across a 1 m
impedance leads to 1 A of current. No commercial
potentiostat is specified to control a cell’s potential (typically 0.5 V to 4.5 V) with less than 1 mV of absolute
error. When a potential with a 1 mV (or larger) error is applied to a low-impedance battery or fuel cell, a very
large DC current flows his current (given enough time) can alter a battery’s state-of-charge.
Conversely, a galvanostat can easily control ampere currents to an accuracy of a few milliamperes. The voltage
on a battery or fuel cell is usually unaffected when the galvanostat is connected: the DC current is zero or some
user-defined value.
A modern EIS system with AC-coupling or offset and gain in the voltage measurement can measure microvolts
of AC voltage superimposed on a large DC voltage, as long as that DC voltage is stable.
DC Errors and Four-terminal Measurements
Four-terminal measurements are a common technique used in precision measurement of small impedances. In
a four-terminal measurement, a nominally two-terminal device, such as a resistor or a battery, is connected
using four leads. Two of the leads carry the current that must flow through the device to make the
measurement. The other leads measure the voltage created by that current.
