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: ~250-300 Wh/L

Lithium ion batteries have a nominal voltage of 3.6V and a typical charging voltage of 4.1V. The charging procedure is "constant current/ constant voltage." This means charging with constant current until the 4.1V are reached by the cell and continuing with constant voltage until the current drops to zero. Lithium ion batteries cannot be fast-charged and need at least 4 hours to fully charge.

Li-ion batteries are not as durable as NiMH and NiCd designs, although they do not suffer from the memory effect. At a typical 100% charge level (notebook battery, full most of the time) at 25 degrees centigrade, 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 centigrade. When stored at 40% charge level, these figures are reduced to 2%, 4%, 15% at 0, 25 and 40 degrees centigrade 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 two to three 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.

A more advanced lithium ion battery design is the lithium polymer cell.

Guidelines to prolonging Li ion battery life

• Unlike nickel based rechargeable batteries (NiCd and NiMH), Li 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 fridge. However, they should not freeze. Aging will take its toll much faster at high temperatures. Burning hot cars can kill lithium ion batteries.

• Buy Li ion batteries only when needed. Look at the manufacturing date. That is when the ageing process commenced.

• A specific note for notebook users: When running from fixed line power over extended periods, it is advisable to remove the battery and store it in a cool place. Remember how hot notebooks can get...