Dell's recall of batteries for more than four million laptop computers has brought renewed attention to the temperamental nature of this power source. Rechargeable batteries come in two types. Lithium-ion batteries are generally found in laptops, cellphones and iPods; nickel metal hydride, or NiMH, batteries are used in cordless phones and less-expensive digital cameras, plus a few laptops. Apple has told customers to return 1.8 million lithium-ion batteries used in Mac laptops because they could cause the computers to overheat and ignite.
Lithium-ion batteries are particularly sensitive to heat. To avoid the danger of a fire, they should not be stored in direct sunlight. In addition, the connectors should be kept away from metals that could cause a short circuit.
What are lithium ion batteries
Lithium ion batteries abbreviated Li-Ion are a type of rechargeable battery commonly used in consumer electronics. They are currently one of the popular types of battery, with good energy-to-weight ratios, no memory effect and a slow loss of charge when not in use. They can be dangerous if mistreated, however, and unless care is taken they may have a shorter lifespan compared to other battery types. A more advanced lithium-ion battery design is the lithium polymer cell.
What are the benefits
- High energy density, potential for yet higher capacities.
- Does not need prolonged priming when new. One regular charge is all that's needed.
- Relatively low self-discharge, self-discharge is less than half that of nickel-based batteries.
- Low Maintenance, no periodic discharge is needed; there is no memory.
- Specialty cells can provide high current to applications such as power tools.
- Small and mobile, but more readily stored.
- Li-ion batteries do not suffer from the memory effect.
- Very low profile, batteries resembling the profile of a credit card are feasible.
- Flexible form factor, manufacturers are not bound by standard cell formats. With high volume, any reasonable size can be produced economically.
- Lightweight, gelled electrolytes enable simplified packaging by eliminating the metal shell.
- Improved safety, more resistant to overcharge; less chance for electrolyte leakage.Disadvantages
A unique drawback of the Li-ion battery is that its life span is dependent upon aging from time of manufacturing (shelf life) regardless of whether it was charged, and not just on the number of charge/discharge cycles. This drawback is not widely publicized.
At a 100% charge level, a typical Li-ion laptop battery that is full most of the time at 25 degrees Celsius or 77 degrees Fahrenheit, will irreversibly lose approximately 20% capacity per year. However a battery stored inside a poorly ventilated laptop, may be subject to a prolonged exposure to much higher temperatures than 25 °C, which will significantly shorten its life. The capacity loss begins from the time the battery was manufactured, and occurs even when the battery is unused. Different storage temperatures produce different loss results: 6% loss at 0 °C/32 °F, 20% at 25 °C/77 °F, and 35% at 40 °C/104 °F. When stored at 40% charge level, these figures are reduced to 2%, 4%, 15% at 0, 25 and 40 degrees Celsius respectively.
This makes Li-Ion batteries unsuitable for back-up applications compared to lead-acid batteries, and even to Ni-MH batteries.
Because the maximum power that can be continuously drawn from the battery depends on its capacity, in high-powered (relative to C, battery capacity in A·h) applications, like portable computers and video cameras, rather than showing a gradual shortening of the running time of the equipment, Li-Ion batteries may often just abruptly fail.
Low-powered cyclical applications, like mobile phones, can get a much longer lifetime out of a Li-Ion battery.
A stand-alone Li-Ion cell must never be discharged below a certain voltage to avoid an irreversible damage. Therefore all systems involving Li-Ion batteries are equipped with a circuit that shuts down the system when the battery is discharged below the predefined threshold. It should thus be impossible to "deep discharge" the battery in a properly designed system during normal use. This is also one of the reasons Li-Ion cells are never sold as such to consumers, but only as finished batteries designed to fit a particular system.
When the voltage monitoring circuit is built inside the battery (so called "smart battery") rather than equipment, and continuously draws a small current from the battery even if it is not in use, the battery further must not be stored fully discharged for prolonged periods of time, to avoid damage due to deep discharge.
Li-ion batteries are not as durable as nickel metal hydride or nickel-cadmium designs and can be extremely dangerous if mistreated. They are usually more expensive. They also require a lot of mandatory safety devices built in, before they can be considered safe for use outside of a laboratory. Safety devices take away useful space inside the cells, and add an additional layer of unreliability, also because their triggering point may be biased to the safe side.
- Requires protection circuit to maintain voltage and current within safe limits.
- Subject to aging, even if not in use - storage in a cool place at 40% charge reduces the aging effect.
- Transportation restrictions, shipment of larger quantities may be subject to regulatory control. This restriction does not apply to personal carry-on batteries.
- Expensive to manufacture, about 40 percent higher in cost than nickel-cadmium.
- Not fully mature metals and chemicals are changing on a continuing basis.
Dell and Apple Recalled Lithium-Ion Batteries Manufactured by Sony
Apple has told customers to return 1.8 million lithium-ion batteries used in Mac laptops because they could cause the computers to overheat and ignite. Last week, Dell announced a recall of 4.1 million batteries, the largest recall in the history of the consumer electronics industry. Dell's batteries used cells manufactured by Sony that could potentially short-circuit and cause a fire, even if the laptop is switched off.
The Consumer Product Safety Commission said in a press release on Thursday: "These lithium ion batteries can overheat, posing a fire hazard to consumers."
Apple's 1.8 million recalled batteries represent about 32 per cent of the nearly 5.6 million laptops the company shipped during the quarterly periods covered by the recall, according to IDC. Dell's recall, though larger in number, represents a smaller portion of its sales, given that it sold 22 million laptops during the period covered by its recall, again according to IDC. (The number of batteries recalled does not necessarily mean that 1.8 million laptops were affected, as some users purchase more than one battery for their systems.)
An Apple spokesman said the company does not expect the recall to have a material financial impact on the company, "We discovered that some Sony batteries in previous models of Power PC-based PowerBooks and iBooks do not meet Apple's standards for safety and performance. None of Apple's Intel-based laptops are affected."
Sony will take a financial hit from the combined impact of the Apple and Dell recalls. As with Dell, Sony plans to offer financial support to Apple's recall effort, said the spokesman. The total cost of the Apple and Dell recalls could fall between USD 172 million and USD 258 million, Sony said in a statement.
Safety Concerns
Lithium-ion batteries can easily rupture, ignite, or explode when exposed to high temperatures, or direct sunlight. They should not be stored in a car during hot weather. Short-circuiting a Li-ion battery can also cause it to ignite or explode. Never open a Li-ion battery's casing. Li-ion batteries contain safety devices that protect the cells inside from abuse. If damaged, these can cause the battery to ignite or explode. Internal contaminents inside the cells can defeat these safety devices; the Mid-2006 recall of millions of batteries containing cells by Sony is due to contamination inside the cells with metal particles. These can under some circumstances pierce the seperator, which allows all of the energy in the cell to convert rapidly to heat.
Kuzhikalail M. Abraham, a lithium battery consultant with E-Kem Sciences, says the computer industry's drive to increase battery capacity can test the limits of sensitive components such as the membrane separator, a polyethylene or polypropylene film that is only 20-25 µm thick. He points out that the energy density of lithium-ion batteries has more than doubled since they were introduced in 1991. "When you pack the battery with more and more material, the separator can undergo stress," he says.
The mid-2006 Dell laptop battery recall isn't the first of its kind, but it is the largest. During the past decade there have been numerous recalls of lithium-ion batteries in cellular phones and laptops owing to overheating problems. Last December, Dell pulled about 22,000 batteries from the U.S. market. In 2004, Kyocera Wireless recalled about 1 million batteries used in phones.
The question often asked is what is the typical amount of lithium (in grams) of a lithium-ion rechargeable battery for laptops and other portable devices. The 'equivalent lithium content' of a lithium-ion cell (in grams) is calculated at 0.3 times the rated capacity (in ampere-hours). The lithium-equivalent content of a battery equals the sum of the grams of lithium-equivalent content contained in the component cells of the battery.
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