Is Lithium-ion sufficiently safe?
Posted by ecsem1 on Jan 04, 2010
A mild short will only cause an elevated self-discharge. Little heat is generated because the discharging energy is very low down Dell GD761. If, however, enough microscopic metal particles converge on one spot, a major electrical short can develop and a sizable current will flow between the positive and negative plates. This causes the temperature to rise, leading to a thermal runaway, also referred to ‘venting with flame.’
Sony Energy Devices (Sony), the maker of the lithium-ion cells in question, says that on rare occasions microscopic metal particles may come into contact with other parts of the battery cell, leading to a short circuit within the cell. Although battery manufacturers Dell 1721 Battery strive to minimize the presence of metallic particles, complex assembly techniques make the elimination of all metallic dust nearly impossible. Energy dense cells with ultra-thin separators are more susceptible to impurities than older designs with lower Ah ratings.
Lithium-ion cells with cobalt cathodes (same as the recalled laptop batteries as PB992A) should never rise above 130°C (265°F). At 150°C (302°F) the cell becomes thermally unstable, a condition that can lead to a thermal runaway in which flaming gases are vented.
During a thermal runaway, the high heat of the failing cell can propagate to the next cell, causing it to become thermally unstable as well. In some cases, a chain reaction occurs in which each cell disintegrates at its own timetable HSTNN-LB31. A pack can get destroyed within a few short seconds or linger on for several hours as each cell is consumed one-by-one. To increase safety, packs are fitted with dividers to protect the failing cell from spreading to neighboring cells.
The safety precaution is especially critical on larger batteries, such as laptop packs. The hazard is so much greater than on a small cell phone battery if something goes wrong. For this reason, many laptop manufacturers secure their batteries with a secret code that only the matching computer can access. This prevents non-brand-name batteries from flooding the market. The drawback is a higher price for the replacement Dell Inspiron 1501 Battery.
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