One essential parameter that greatly impacts battery efficiency and longevity is its "Internal resistance". There are two primary methods to measure cell internal resistance: Alternating Current Internal Resistance (ACIR) & Direct Current Internal Resistance (DCIR).
Alternating Current Internal Resistance (ACIR):
ACIR is a widely-used method that involves applying an alternating current (AC) signal to the cell and measuring the resulting voltage response. The AC signal oscillates at a specific frequency, which is typically 1 kHz for most battery cells. The ratio of the change in voltage to the change in current yields the internal resistance value.
ACIR offers several advantages, such as:
a) High sensitivity: Detects minute changes in resistance, allowing for precise monitoring of battery health.
b) Non-invasive measurement: The low amplitude of the AC signal does not significantly affect the cell's state of charge (SoC), making it a non-destructive technique.
c) Rapid testing: ACIR measurements can be performed quickly, enabling real-time monitoring and diagnosis.
Direct Current Internal Resistance (DCIR):
DCIR is another widely-adopted method in which direct current (DC) is applied to the battery, resulting in a change in voltage. The internal resistance is calculated as the ratio of the voltage change to the current change. DCIR can be conducted using a constant current or a pulsed current method.
While DCIR is generally less sensitive than ACIR, it offers its own set of advantages:
a) Simplicity: Relatively easy to perform, requiring only basic equipment and straightforward calculations.
b) Relevance to real-world conditions: Since most battery applications involve the use of direct current, DCIR provides a more realistic representation of the internal resistance encountered during operation.
c) Compatibility with various battery chemistries: It can be applied to a wide range of battery types, making it a versatile technique.
Other Techniques for Measuring Cell Internal Resistance:
Besides ACIR and DCIR, several other techniques can be employed to assess the internal resistance of battery cells. Some of these include:
a) Electrochemical Impedance Spectroscopy (EIS): An advanced technique that measures the cell's impedance over a range of frequencies. It provides valuable insights into the various processes occurring within the battery, allowing for a detailed analysis.
b) Hybrid Pulse Power Characterization (HPPC): HPPC is a dynamic test that involves applying a series of current pulses to the battery cell. By analyzing the voltage response to these pulses, the internal resistance can be determined, along with other performance parameters.
c) Incremental Capacity Analysis (ICA): ICA is a technique that assesses the internal resistance by analysing the cell's capacity as a function of voltage.