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Lithium ion battery
Lithium ion batteries (or Li-ion) have become very common and dropped in price recently. They provide one of the best energy-per-weight ratios of rechargeable batteries at present. They have succeeded nickel metal hydride and nickel-cadmium batteries in consumer electronics such as cellular phones and notebook computers.
Specific energy density: ~150-200 Wh/kg
Volumetric energy density: ~250-530 Wh/L
Lithium-ion batteries have a nominal voltage of 3.6V and a typical charging voltage of 4.2V. The charging procedure is one of constant voltage with current limiting. This means charging with constant current until a voltage of 4.2V is reached by the cell and continuing with a constant voltage applied until the current drops close to zero.(Typically the charge is terminated at 7% of the initial charge current.) Lithium-ion batteries cannot be fast-charged and typically need at least four hours to fully charge.
Li-ion batteries are not as durable as NiMH and NiCd designs, and can be extremely dangerous if mistreated. At a typical 100% charge level (notebook battery, full most of the time) at 25 degrees Celsius, Li-ion batteries irreversibly lose approximately 20% capacity per year from the time they are manufactured, even when unused. (6% at 0 degrees, 20% at 25 degrees, 35% at 40 degrees Celsius. When stored at 40% charge level, these figures are reduced to 2%, 4%, 15% at 0, 25 and 40 degrees Celsius respectively.) Every (deep) discharge cycle decreases their capacity. The degradation is sloped such that 100 cycles leave the battery with about 75% to 85% of the original. When used in notebook computers or cellular phones, this rate of deterioration means that after three to five years the battery will have capacities too low to be still usable.
One great advantage of Li-Ion batteries is their low self-discharge rate of only approximately 5% per month, compared with over 30% per month and 20% per month in nickel metal hydride batteries and nickel cadmium batteries respectively.
Lithium ion internal design is as follows. The anode made from carbon, the cathode is a metal oxide, and the electrolyte is a lithium salt in an organic solvent. Since lithium metal which might be produced under irregular charging conditions is very reactive and might cause explosion, Li-ion cells usually have built-in protective electronics and/or fuses to prevent polarity reversal, over voltage and over-heating.
The Li-Ion battery required nearly 20 years of development before it was safe enough to be used on a mass market level. A unique drawback that we can see to the Li-Ion battery is that its life cycle is dependent upon aging from time of manufacturing (shelf life) regardless if it was charged or not and not on the number of charge/discharge cycles. This drawback is not widely publicized.
A more advanced lithium-ion battery design is the lithium polymer cell.
Guidelines to Prolonging Li-Ion Battery Life
- Unlike NiCad batteries or NiMH batteries, lithium-ion batteries should be charged early and often. However, if they are not used for a longer time, they should be brought to a charge level of around 40%. Never use the battery care functions some cellular phones provide for nickel based batteries. (This will deep cycle the batteries.)
- Li-ion batteries should be kept cool. Ideally they are stored in a refrigerator. However, they should not be subjected to freezing temperatures. Aging will take its toll much faster at high temperatures. Keeping them in burning hot cars can kill lithium-ion batteries.
- Buy Li-Ion batteries only when needed. Look at the manufacturing date. That is when the aging process commenced.
- When using a notebook computer running from fixed line power over extended periods, it is advisable to remove the battery and store it in a cool place.
- Removing the battery from a laptop while it is plugged in is against manufacturer's recomendation for many laptops as it can cause permanent damage to the laptop since the power supply is designed to output a voltage assuming a battery is present. It is therefore advisable to ensure that removing the battery from a device with it plugged in is an acceptable mode of operation.
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