What is electrical resistance?

The resistance of a component, expressed in Ohms (Ω), measures the opposition to the flow of current in an electrical circuit: the higher the resistance value, the more difficult it is for electricity to flow through the circuit, and conversely, the lower the resistance value, the better the current flow. It is also used to estimate the condition of components or the circuit.

• The resistance can be material: it is an electronic component with a fixed value, integrated in an electric circuit, an electronic card or a thermal component found in domestic and industrial appliances.
• As it can be immaterial: it is called "internal resistance", a term used to describe and measure the resistive behaviour of an assembly such as the human body, a lithium-ion cell... This aspect of resistance only will be developed in the article.

To study the behaviour of internal resistance it is necessary to represent it and use an equivalent electrical circuit model.

How do you measure the internal resistance of a lithium-ion cell?

The internal resistance value of a Li-ion cell is complex to assess as it varies with temperature, SoC, SoH, temperature and other factors. The result depends on the method used, so there is no single value for a cell.

Many equivalent electrical circuits are used to model the internal resistance of a Li-ion cell, ranging from the simple (a single resistor) to the complex (series and parallel combination of electronic components such as resistors and coils). This is the number of methods that exist to estimate the different electrochemical mechanisms from which the internal resistance of the cell is derived. These methods can be divided into two categories:

• the category known as Direct Current (DC), which allows a time-based study
• the category of Alternating Current (AC) allowing a frequency study.

Pulse method: belongs to the "Direct Current (DC)" category and consists of applying a current pulse of several seconds and measuring the voltage response. The total internal resistance of the cell is thus deduced from Ohm's law, taking into account all the resistive electrochemical mechanisms (ohmic resistance, charge transfer polarisation, diffusion etc.). The main interest of this measurement is to provide valuable information on the performance of the cell, particularly in terms of power.

The parameters (intensity and duration of the pulse) applied to this type of test depend above all on the targeted application. There is no standardised procedure, but the procedure described in IEC 61690 is commonly used.

AC method at 1kHz: is a very fast and simple method in the "Alternating current (AC)" category which allows you to estimate the purely ohmic resistance of the cell, which is responsible for the immediate voltage drop under a discharge current and the Joule effect heat release. It uses an alternating current at a fixed frequency, usually 1kHz. Although the result does not allow an effective comparison between different cell references, this method is of interest at the end of the cell production line, as it allows manufacturers to control quality within the same reference.

EIS method: in the same category as the previous one, Electrochemical Impedance Spectroscopy (EIS) is a non-destructive method that involves applying a small alternating current over a wide frequency range from mHz to kHz. The study of the voltage response in the frequency domain allows access to all the processes taking place in the cell, and to isolate them in order to parameterise models for example.

These measurements are useful for :

• determining the maximum power that can be extracted from the cell
• compare cells (if chemistry, format and capacity are close)
• dimensioning a thermal management system, efficiency
• do programming in BMSs using internal resistance
• develop electrical models for simulations
• assess the state of health of the battery
• detect a production fault or defective cell

‍