Electrochemical Impedance Spectroscopy

Introduction 

There is expected to be a 3.7-fold rise in the demand for lithium-ion batteries between 2013 and 2020. Keeping batteries reliable when their capacity decreases, spotting problems before they become catastrophic, and anticipating when a battery has reached its useful life all necessitate improvements in diagnostics.

A battery is analogous to a biological entity whose state can only be assessed with varying precision using accessible symptoms. This is meant to mimic a medical exam, complete with many tests and the application of the law of elimination. Rapid battery testing methods have lagged behind other technologies because of their complexity and the lack of certainty surrounding test results for outliers.

Cadex has made significant strides in rapid-test technology because of the significance it places on battery diagnostics. Diagnostic Battery Management (DBM) is a new approach being taken by forward-thinking businesses to battery management and care, and these innovations are its bedrock. Rather than building another new super battery, DBM is necessary to maintain the reliability of current battery systems by monitoring capacity, the primary health indicator, along with other metrics.

Capacity denotes energy storage, internal resistance pertains to the power supply, and self-discharge measures mechanical integrity. A battery cannot be considered suitable unless it possesses all three of these characteristics. In addition to these fixed features, a battery also possesses state-of-charge (SoC)-dependent dynamic features that influence battery performance and make quick testing more difficult.

Technology for testing batteries should be able to identify all battery states and return accurate results regardless of their charge. This is a tall order, considering a completely charged good battery acts similarly to a partially charged faded pack.

Electrochemical Impedance Spectroscopy (EIS) provides a snapshot of the chemical battery in addition to the more conventional voltage reading, pulse or AC impedance method for measuring internal resistance, coulomb counting, and so on. Digital monitoring via coulomb counting is simpler than decoding the chemical battery to estimate capacity. Proprietary algorithms and matrices, which serve as lookup tables like letter or face recognition, are used to delve into the chemical batteries.

Particularly with Li-ion and lead-acid systems, voltage, and internal resistance do not correspond with capacity and fail to adequately anticipate the end of battery life. The chemical battery holds the key to the truth. Because chemical symptoms are not reflected in a computer measurement, it is fallible on its own.

When the battery is fully charged and the circuit is open, the battery’s voltage indicates its level of charge. It is impossible to determine the battery’s SoH based on voltage alone.

When internal resistance is high due to corrosion or mechanical problems, it can be detected with an ohmic test. These irregularities are indicative of the battery’s demise but are not always associated with diminished power. An alternative name for the ohmic test is the impedance test.

The capacity of a chemical battery can be determined by putting it through a full cycle of charging, discharging, and charging again. The smart battery is calibrated to correct tracking faults, and the service takes time and might be stressful, but the results are worth it.

In a hurry? Try time domain testing, where you pulse the battery and see the ions flow in Li-ion, or frequency domain testing, where you scan the battery at various frequencies. Complex software containing battery-specific parameters and matrices functioning as lookup tables is required for advanced rapid-test methods.

BMS: Most Battery Management Systems estimate SoC by monitoring voltage, current and temperature. BMS for Li-ion also counts coulombs.

Counting Coulombs: The number of coulombs relevant to SoH is provided by a smart battery’s Full Charge Capacity (FCC). Instantaneous FCC readout, but data accuracy degrades with time and the battery needs full cycle calibration.

To charge a battery, a charger equipped with RAC technology first determines the battery’s state of charge (SoC) using a proprietary filtering algorithm, and then calculates the necessary number of coulombs. RAC requires a one-time calibration for each battery model; cycling a decent pack offers this parameter that is recorded in the battery adapters. Cadex created the RAC technology.

The State-of-Life-Indicator (SOLI) calculates how much longer a battery will last by keeping track of the number of coulombs it may discharge. A new battery starts at 100%; delivered coulombs diminish the number until the allocation is gone and a battery replacement is near. Manufacturer specifications (V, Ah) are used to get the coulomb count of 1 cycle, which is then multiplied by the specified number of cycles to obtain the entire scale. Cadex’s SOLI can be incorporated into new or existing wheelchairs, medical equipment, traction, and UPS systems. Fleet management is facilitated by wireless communication.

Strong symptoms are required for accurate results. This isn’t always doable, especially with lead acid batteries that haven’t been formatted or storage packs. Readings from a poor battery can be jumbled and irregular, but a healthy battery taken from service often offers firm symptoms with good accuracy. If the battery has turned into a potato, it is impossible to take accurate measurements because the symptoms are either nonexistent or unspecific. Because of this, the battery is now considered an anomaly by the system. Nine out of ten batteries may be predicted with accuracy using well-developed fast test methods. The EIS could eventually become more advanced than competing technologies.

 In typical use, lead acid and Li-ion batteries have a lot in common, including a low resistance. Exceptions to this rule include preemptive battery replacement and heat failure/mechanical issues that increase internal resistance. The depletion of nickel-cadmium, nickel-metal hydride, and even the primary battery may be seen.

Li-ion, with its 99 percent charge efficiency, is ideal for digital battery estimation. This assists in BMS design by enabling capacity estimation using coulomb counting. While the measurements are instant, occasional calibration is essential to address the tracking mistake that develops with unpredictable battery usage. Nickel-based batteries, in contrast, would skew digital tracking due to their poor charge efficiency and excessive self-discharge. Lead acid batteries are only halfway efficient for coulomb counting when used in the appropriate conditions and at a reasonable temperature.

All batteries’ performance and speed of testing degrade in cold weather. However, a battery’s charge acceptance drops below freezing, and charging times must be lengthened by decreasing the current. Li-ion batteries should not be charged at temperatures below freezing, though some chargers will do so automatically.

Summary

According to Mark Twain, “I didn’t have time to write a short letter, so I wrote a long one instead.” Making anything “short” is also relevant to the progress of Diagnostic Battery Management. Adding functionality is simple, but maintaining a reasonable price tag is difficult. By upgrading to newer, more intelligent microcontrollers and streamlining the manufacturing process, manufacturers can add features to their products that were previously unimaginable. However, as Mark Twain alluded to, efficiency does not come overnight.

The end goal is to make batteries that can be relied on, are safe, are inexpensive, and are ecologically friendly. Systems that can accomplish this task automatically and at low cost are necessary. The objective is to maximize the utilization of each battery and make their health status obvious to the driver and fleet manager. This has the potential to eliminate the occurrence of sudden battery failures. Also learn about methods for Electrochemical Corrosion Testing.