Mining Contract: Methods to De-Energize Batteries

Contract Status & Impact

This contract is complete. To receive a copy of the final report, send a request to mining@cdc.gov.

Batteries present specific hazards during mine emergencies by virtue of the energy they contain and the fact that this energy could be released in a variety of ways to pose a fire or methane ignition risk. A previous study identified such battery ignition hazards from past underground coal mine emergencies. At this time, no technologies exist to de-energize smaller lithium-ion batteries or large lead-acid batteries to ensure they do not serve as an ignition source. The desired technology must be resistant to inadvertent triggering and be capable of both local and remote activation. It should also be free from sparking/arcing risk while being operational until the de-energizing protocol has been fully completed.

Under this contract, TIAX LLC evaluated candidate technologies for de-energizing batteries that take into account factors that drive cost, performance, and ease of implementation. A range of design and circuit options for de-energizing systems were explored and two architectures (single-load, multi-load) appropriate for testing of laboratory prototypes were identified. Single-load architecture uses a latching relay switching element and a load element designed to be integrated with the battery and the equipment it powers to the greatest extent possible. Multi-load architecture uses a two-phase de-energizing sequence in which one circuit discharges the battery across its main terminals, removing the bulk of the resident energy, with cell-level loads subsequently completing the process to ensure that all cells are rendered intrinsically safe by bringing each cell to low voltage and permanently maintaining the voltage there.

This research showed that even when batteries are discharged significantly beyond the specified end-of-discharge (“empty” point), they can retain the ability to deliver large currents, even when the cells are quite small. Testing of both lead-acid and lithium-ion cells showed that to achieve de-energizing of batteries sufficient to render them essentially electrically inert, cells should be discharged to zero voltage, then actively held there. Without an active hold at zero, battery voltage and current capability (and thus safety risk) rebound. As an example, the researchers documented the discharge of a 2.6-amp-hour lithium-ion cell under standard, manufacturer-specified test conditions. Once the discharge completed, the cell was allowed to rest for 24 hours. The cell rebounded significantly in voltage, as would be expected, rising from the 3 V end-of-discharge point to about 3.425 V in approximately five minutes. Subsequently, voltage continued to increase for several hours until it approached an equilibrium value of about 3.45 V. The cell was then found to be capable of a peak short circuit current of 56 amps. Thus, ordinary discharge as might be encountered in normal use of batteries is clearly insufficient to render the batteries electrically inert.

Evaluation of different de-energizing technologies showed that an electronic approach capable of executing the required discharge protocol could be implemented using commonly available components and assembly methods to deliver a system that:

• Fully and effectively de-energized batteries via an autonomous process that could be initiated remotely or locally by simple control signals and also manually as a backup;
• Was robust against false triggering and single-point failures of the technology;
• Did not interfere with or alter normal battery function and performance;
• Removed >99.995% of resident energy as demonstrated in a mining-relevant battery;
• Independently de-energized each individual cell to maximize overall risk reduction;
• Rendered batteries intrinsically unable to deliver current-voltage combinations that would violate MSHA intrinsic safety standards for methane-air mixtures as listed in MSHA ACRI2001.

 TIAX designed two prototypes that share a common set of functional blocks and have the same design considerations. Both the “Single-Load Architecture” (SLA) and the “Multi-Load Architecture” (MLA) are suitable for any battery chemistry. The SLA offers greater simplicity and lower cost, while the MLA offers enhanced performance. As battery voltage increases, the performance advantages offered by the MLA become increasingly important, making this architecture the preferred choice for higher-voltage batteries. Prototypes of both the SLA and the MLA were designed and fully tested.