Invention: The Naval Research Laboratory (NRL) has developed a diagnostic technique to identify and monitor the state-of-health (SOH) of lithium-ion batteries (LiBs) to improve safety and reduce the safety hazards associated with lithium-ion battery failures. The technique applies a small AC signal (current or voltage) to a battery over a specific frequency range, termed the SOH frequency, which is unique to a battery’s chemistry, size and form factor. The impedance response to the perturbation at the SOH frequency is invariant of the battery’s state-of-charge and can be applied online to provide real-time SOH monitoring.
The invention relates generally to battery health monitoring. More particularly, the invention relates to a diagnostic method for monitoring the state-of-health of rechargeable batteries and identifying defective batteries to be taken out of service before causing a serious safety hazard. The method first identifies a single-frequency as which a voltage or current perturbation induces an impedance response which is nearly impendent of the batteries state-of-charge. The impedance response to the perturbation at the single-frequency will be largely invariant when the battery is charged and discharged within the voltage and/or state-of-charge ranges dictated by the battery chemistry. When the impedance response deviates outside of a predetermined safety factor, the method diagnoses the battery cell has been exposed to an abusive condition (overcharge). The impedance circuit and diagnostic can be implemented with basic electronics into a battery management system.
Background: Preventing the overcharge of lithium-ion batteries is a significant challenge in battery safety. Lithium-ion batteries have voltage charging limits that are specific to the chemistry of the positive/negative electrode couple and electrolyte materials. The most common cathode material, LiCoO2, has an upper limit of 4.2 V and a lower limit of 2.8 V vs. Li+/Li0. Charging above the voltage limit, or overcharging, destabilizes the cathode and causes thermal runaway and fires. Overcharge of a single cell is significant since failure resulting in explosion or fire can easily cascade to other cells in a battery pack. This method uses electrochemical impedance spectroscopy (EIS) to monitor the resistance increases associated with Li-ion batteries during and after overcharge. EIS is commonly used to measure the resistance within a Li-ion cell and can show changes in resistance behavior originating from chemical and electrochemical reactions occurring within bulk electrodes, electrolyte, and along electrode/electrolyte interfaces. The physical processes in electrochemical cells have different time constants or effective capacitances, which result in different frequency responses. These processes can be differentiated using impedance measurements taken over a broad range of frequencies (i.e.: 100 kHz to 0.01 Hz). We have previously shown the impedance characteristics for overcharged batteries are markedly different from those of the healthy batteries. We observe that the impedance at frequencies ranging from 100 to 500 Hz are responsive to changes in passivation layers, the results are independent of state of charge for healthy batteries, and grossly different for overcharged batteries. The single-point impedance-based tool can be utilized in real-time for monitoring battery health and diagnosing abuse due to overcharge.
- Quick data collection and determination of state-of-health; eliminates collection of full impedance spectrum.
- Technique can be applied on-line for real-time health monitoring, independent of battery state-of-charge.
- Diagnostic works at the battery pack level eliminates the need for individual cell monitoring.
- Single frequency measurement eliminates the need for complex and expensive electrical waveform generators.
- Diagnoses fault after a single severe overcharge or repeated mild overcharges.
Applications and Industry:
- Monitoring Li-ion battery systems for real-time state-of-health determination
- Diagnostic for identifying overcharge abuse in lithium-ion batteries
- Manufacturers of consumer electronics
- Battery manufacturers
- RC Hobbyists
- Electric vehicles
Licensing and Collaboration Opportunities
US Filed Patent Application No. 13/705891 is available for License to companies with commercial interest. Collaborative research and development is available under a Cooperative Research and Development Agreement (CRADA).
Navy Technology Number: 101,468
Technology Status: Naval Research Laboratory and Office of Naval Research are funding the technology for 3 years. NRL estimates this technology’s at approximately Level 2. Prototype is being developed in a laboratory setting for further testing and validation.
Lead Inventor: Corey T. Love
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- Corey T. Love and Karen Swider-Lyons, “Impedance Diagnostic for Overcharged Lithium-ion Batteries,” Electrochemical and Solid State Letters, 15 (4) A53-A56 (2012).
- Corey T. Love et al., “State-of-Health Monitoring of 18650 4S Packs with a Single-Point Impedance Diagnostic,” Journal of Power Sources, accepted (2014).