The dependability of energy storage devices — particularly batteries — is becoming increasingly important to consumers, industry, and the military. As battery technology becomes more complex and users' expectations become more pronounced, there is a pressing need for highly accurate assessment techniques that can give state-of-health readings in conditions approaching real time.
Until now, embedded monitoring has relied on passive measurements of voltage, current, and temperature, or on impedance methods that can take as long as 10 minutes. Idaho National Laboratory's Energy Storage Group has developed an Impedance Measurement Box (IMB) that can generate the necessary data in 10 to 15 seconds.
The IMB provides more than a single-point measurement, and can generate data during operation that is more useful for assessing battery health (Figure 1). It also can be used on batteries prior to installation, confirming health before entering active service or during regular maintenance.
Impedance is the opposition to the flow of alternating electrical current, and a key performance measurement that correlates with more complex parameters, such as resistance and power capability. Up to this point, standardized impedance measurement techniques required expensive and delicate laboratory equipment, which precluded integrating evaluation of batteries in place. The IMB directly measures the wideband impedance spectrum in seconds during battery operation with no significant impact on service life.
The IMB diagnostic tool uses five key steps to obtain the vital impedance spectrum measurement results (Figure 2). First, an input signal is generated that consists of sinusoids strategically separated by a known frequency spread and summed together. This combined signal is injected into the energy storage device (e.g., battery). The response is then captured by a data acquisition system for data processing and analysis, and display.
In addition to testing plug-in electric vehicle (PEV) batteries, the IMB diagnostic tool can be used to test batteries for many uses including military, telecommunications, and critical infrastructure. The latest development is a third-generation device able to assess a 50-Volt system, making it applicable for testing on battery modules that contain multiple cells. The overarching technological purpose of IMB is the development of smarter and better energy storage and management systems that can more accurately detect pending failures, assist with warranty, and enable smarter management for extended battery life. By keeping careful track of the safety and performance of batteries as they age, manufacturers and their engineering teams can make more informed decisions about the state of health and battery life.