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Chapter 10 – 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
both tests with small current signals 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 type of 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 mΩ. A voltage of 1 mV across a 1 mΩ impedance leads to 1 A of current.
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 will flow. This 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 milliamps. 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.
Chapter 10 – EIS Measurement of Small Impedances--Overview
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