The high energy density of rechargeable lithium ion (Li-ion) batteries allow for enhanced storage capabilities and longer runtimes, making this technology one of the most popular options for portable electronic devices.
Underground mining workers often utilize Li-ion batteries to power various safety equipment including cap lamps, hand-held gas detectors, tracking devices and communication tools.
Current Use of Li-Ion Batteries in Coal Mines
The potential for the Li-ion battery thermal runway, a situation in which an increase in the temperature of a battery can lead to flame ignition, requires any mining equipment equipped with these batteries to be declared permissible by the Mine Safety and Health Administration (MSHA). The presence of released methane (CH4) gas and coal dust, both of which result from various coal mining processes, that occurs within the coal mine can also increase the severity of an explosion if an Li-powered device ignites.
Researchers from the National Institute for Occupational Safety and Health (NIOSH) are actively investigating how to standardize quality control tests for permissible Li-ion and Li polymer batteries in coal mining equipment.
Testing Li-Ion Battery Safety
Traditional safety tests for Li-ion batteries can include electrical, mechanical and environmental evaluations, with mechanical integrity crust tests being the most common. A standard crush test involves the application of an external force pushing into the battery cell until an internal short circuit, or spark, is achieved.
The nail penetration test was commonly employed for this type of mechanical evaluation, however the ability of the metal nail to exude its own thermal energy onto the Li cell limits its practicality in safety testing.
NIOSH Improves Li-Ion Battery Safety Tests
In an effort to create a more standardized technique for evaluating the safety of Li-ion batteries, a study by NIOSH developed a method that involved a 90° wedge-shaped plastic fixture to compress the Li cell. By utilizing plastic rather than metal, the Researchers eliminated the applied force’s potential electrical or thermal energy to dissipate onto the tested cell
The plastic material also exhibits a high mechanical strength and rigidity, as well as a high resistance to solvents and chemicals. To further improve the accuracy of the cell crush tests to be specific to coal mining purposes, the Researchers also conducted the tests within a 20 L chamber that was filled with CH4-air to stimulate potential hazardous conditions within a coal mine.
The NIOSH study looked at two Li-ion batteries within the aforementioned test conditions: (1) LG Chem ICR18650S2 LiCoO2 and (2) A123 Systems 26650 LiFePO4 cells. Overall, the A123 cells proved to be much safer as compared to the LG Chem cells within each of the applied crush test conditions.
The LG Chem cells readily ignited when placed within the CH4 chamber, whereas the A123 cells did not ignite in any of its crush tests. Under high-applied pressures, the A123 cells did rupture and produce smoke, however ignition was never found in either of the test chambers.
The results of the NIOSH test suggest that both the plastic wedge test and the incorporation of a CH4 test chamber are more appropriate crush tests for Li-ion battery safety. Under the conditions of the study, the A123 LiFePO4 battery demonstrated greater safety potential under fabricated mining conditions as compared to the LG Chem LiCoO2.
The plastic wedge test proved to be more severe as compared to traditional crush tests, as it allowed for a deeper penetration of the wedge into the cell without the need for greater applied force.
Additional experimental research on Li-ion battery ignition within ambient conditions containing methane gas mixtures must still be conducted in order to fully demonstrate the safety of any Li-ion battery equipment.
NIOSH and other research organizations must also continue to investigate safer battery assemblies, cable protection and ignition safe disconnects to fully protect against a potential mining disaster.
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- “Are Lithium Ion Cells Intrinsically Safe?” T. Dubanieqicz, J. DuCarme. IEEE Journals. (2013). DOI: 10.1109/TIA.2013.2263274.