Detecting Lithium-Ion Cell Internal Faults In Real Time

Mar 1, 2010 12:00 PM
Celina Mikolajczak, John Harmon, Kevin White, Quinn Horn, and Ming Wu Exponent Failure Analysis Asso
Kamal Shah Intel Corporation Portland, OR

Li-ion cell thermal-runaway failures can occur due to poor cell or battery-pack design (electrochemical or mechanical), manufacturing flaws, external abuse, and poor charger or system design. Fortunately, a procedure for detecting future failures shows pr

Internal cell faults continue to lead to thermal runaway failures in Li-ion battery packs used in the field. Though these events are rare, the proliferation of Li-ion-powered consumer electronics has increased the risk for an event occurring on an aircraft, or at a similarly inauspicious location or time.

The average business traveler may be carrying anywhere from two to five Li-ion-battery packs (notebook pack, spare notebook pack, phone battery, camera battery, MP3 player battery, etc.) each time he or she travels. Since battery packs for notebook computers usually contain six to 12 higher-capacity cells (2,000 to 3,000 mAh each), the result of an event in a notebook pack is potentially more severe than an event with a single cell-phone battery pack.

After three incidents in the summer of 2009 involving fires or smoldering of packages containing Li-ion batteries in aircraft cargo holds, the Air Line Pilots Association (ALPA) has called for a ban on bulk shipments of lithium batteries — and products containing those batteries — from passenger and cargo planes.

“The evidence of a clear and present danger is mounting,” states an August 25, 2009 press release distributed by ALPA. “We need an immediate ban on these dangerous goods to protect airline passengers, crews, and cargo…ALPA calls on the agencies charged with protecting the public from hazardous materials to issue an immediate ban on lithium battery shipments to protect airline passengers, crews and cargo until the proper safety regulations are in place and can be enforced.”

The rate of notebook computer recalls due to Li-ion battery faults has apparently decreased since 2007, however they do continue to occur. For example, in May of 2009, a major notebook PC OEM announced a recall of 70,000 notebook computer battery packs.

CURRENT ATTEMPTS TO THWART CELL THERMAL RUNAWAY

Cell thermal runaway failures can occur for a number of reasons, including poor cell design (electrochemical or mechanical), cell manufacturing flaws, external abuse of cells (thermal, mechanical, or electrical), poor battery-pack design or manufacture, poor protection design, and poor charger or system design. The IEEE 1625 and 1725 standards committees have recently focused on conveying the concept that Li-ion battery pack safety is a function of the entirety of the cell, pack, system design, and manufacture. Each of the above aspects has a role to play in ensuring pack safety.

For notebook computers with mature pack and protection designs, Exponent has observed that the most common causes of field failures are internal cell faults that are directly related to cell design flaws, cell manufacturing flaws, or user abuse. For example, we have observed cell thermal runaway failures resulting from internal cell faults caused by contamination (either by materials foreign to the battery or loose pieces of battery material itself), manufacturing-induced electrode damage (scratches, punctures, tears, active material displacement), burrs on electrode tabs, weld spatter from cell lead attachment points, wrinkles or kinks in windings or tabs, electrode misalignment, poorly aging electrodes, post-manufacturing mechanical damage to cells, and cell thermal abuse.

Cell manufacturers continue to work toward designing safer cells, through modifications of cell components (e.g. changes in the cell separator, cell chemistry, etc.), and through improvements in manufacturing practices (e.g. eliminating contaminants, improving the uniformity of processing, etc.). The industry as a whole has also spent considerable effort in developing a collection of standards for improving the safety of Li-ion cells and battery packs (most recently the IEEE and BAJ efforts).

However, even the most respected cell manufacturers' cells have suffered field incidents. In addition, there are a number of new battery manufacturers whose technology and manufacturing approaches are not as advanced or mature as those of traditional suppliers. With the ever-increasing demand for Li-ion cells, these newer manufacturers' cells are increasingly entering the U.S. market in a variety of consumer electronic devices.

Individual cell manufacturers are arguably in the best position to make cell-design and process improvements in their own manufacturing lines to improve cell safety. However, we believe that a system-level approach to detect incipient failures with battery-pack electronics has the potential for reducing the rates of thermal runaway failures and thus could represent a significant contribution to battery safety.

Commercially available notebook battery packs have redundant-protection devices in place to prevent cell overcharging and other potentially damaging or unsafe conditions (charging at high temperatures, charging at high rate when cell voltage is low, etc.). These devices work by monitoring pack voltage, block voltage, current, and temperature in one or more locations within the battery pack.

DETECTING THERMAL RUNAWAY BEFORE IT HAPPENS

At present there is no pack-protection circuitry in commercial use that is designed to continuously monitor the cells for the symptoms of a latent incipient internal cell fault before such a fault causes thermal runaway. Until recently, the ability to detect incipient faults sufficiently early enough to prevent cell thermal runaway has been considered impossible or, at best, impractical. This is most likely a result of the difficulty in simulating faults similar to those that happen in the field, as well as the cell cycling regimes used during simulations.

However, our failure analysis studies, testing associated with our various failure analysis efforts, and testing conducted as a part of the work done for the Mobile PC Extended Battery Life Working Group (EBLWG) — an industry group that facilitates cooperation between cell, notebook, and protection electronics manufacturers — indicates that there are a number of early failure symptoms that might be effectively exploited to identify packs that are likely to suffer thermal runaway failures if left unmitigated. Such symptoms include:

• Excessive block (cell) voltage drop during extended rest periods, indicating high self-discharge rates consistent with micro-shorting (typical cycling programs used for testing to date do not include extended rest periods between charge and discharge steps, although these are common in actual usage of battery packs)
• Long taper-current charging times consistent with dissipation of charge current in a micro-short
• Noisy voltage profiles during charging and discharging (indicating the formation of transient micro-shorts)
• Cell heating during cell charging, particularly near the end of charging
• Charge capacity higher than discharge capacity beyond the typical charge efficiency losses
• Cell-charge/discharge inefficiency change

Exhaustive exploration of detection approaches for a wide variety of internal faults would require significant collaboration with battery manufacturers to produce cells with known faults (e.g. specific contaminants in pre-determined locations). However, in a project sponsored by the Mobile PC Extended Battery Life Working Group, Exponent has accomplished some preliminary rounds of testing that have proved productive.

ANALYZING THE TESTS

In the first round of tests, commercially available Li-ion cells were obtained and faults within the cells were induced through repeated cell overdischarge to induce copper dissolution and subsequent plating. Also, high-rate charging with slight overcharge was used to induce mild lithium plating. As a result, cells either did not develop faults, or failed by triggering designed safety features (charge interrupt devices), before developing an internal flaw that could be monitored over extended cycles.

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