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Lithium-ion Battery Packs

Devices designed to make our lives easier are being developed at an increasingly rapid pace. With the multimedia age dawning, the market is becoming more deversified with such innovations as lightweights, compact video equipment, personal computers, laptops and data processing equipments. Such devices have created a need for high quality, reliable power sources that provide excellent functionality.

Introduction of series of lithium-ion battery packs offering high energy density and capacity than Nickel-Cadmium (Ni-Cd) and Nickel-Meyal-Hydride (Ni-MH) batteries.

Rechargeable lithium-ion batteries are popular in laptop computers. Lithium-ion batteries support longer battery runtimes than NiCd and NiMH technologies and hold a charge well when not in use. Li-ion have an extremely high energy density; that is, they store more energy than other types for a given size and weight. This is primarily because lithium is one of the lightest metals.

Li-ion laptop batteries are normally constructed of several individual cells. The cells are combined in battery packs of normally between 2 to 12 cells, arranged in serial and parallel connections resulting in high voltage ratings. Capacity ratings generally go from 2000mAh to 6000mAh or more. The higher the capacity rating, the longer the battery lasts in a given application.

Lithium is a unstable substance, and when first released, rechargeable lithium-ion batteries had problems. After a number of discharge/recharge cycles, it was found that metallic lithium plating would occur on the lithium electrode, causing a potential runaway condition resulting in a violent reaction. A vent in the battery would prevent an outright explosion; however, extremely hot gases would be released under these conditions, causing damage to the surrounding equipment or even burns to individuals nearby. Several incidents of batteries overheating and venting flames resulted in a number of recalls and redesigns in early 1990s.

Subsequent models were manufactured using built-in protection circuits that limit the peak voltage of each cell during charge, prevent the cell voltage from dropping too low on discharge, limit the maximum charge and discharge currents, and monitor the cell temperatures. With these electronic precautions, the possibility of thermal runaway and violent reactions had been virtually eliminated. But also because of the careful design requirements for Li-ion batteries, raw cells are not sold to the individuals and instead are sold only in assembled battery packs, with the protection circuitry built in.

Working Principle:

The Lithium-ion battery employs a metal oxide material (such as lithium cobalt oxide) or a material with a tunneled structure (such as lithium manganese oxide) as positive electrode - cathode - and a high crystallized special carbon as negative electrode - anode.

Also an organic solvent specialized to be used with the specific carbon works like electrolytic fluid.

The chemical reaction that takes place inside the battery is as follows, during charge and discharge operation:

The main principle behind the chemical reaction is one where lithium in positive electrode material is ionized during charge and moves from layer to layer in the negative electrode (as illustrated to the left image).

During discharge Li ions move to the positive electrode where embodies the original compound.

Features:

High energy density that reaches 400 Wh/L (volumetric energy density) or 160Wh/Kg (mass energy density).
High voltage.
No memory effect. Can be charged any time, but they are not as durable as NiMH and NiCd batteries.
High charge currents (0,5-1A) that lead to small charging times (around 2-4 hours).
Flat discharge voltage allowing the device to stable power throughout the discharge period.
Typical charging Voltage 4,2 ± 0,05V.
Charging method: constant current - constant voltage (CV-CC).
Typical operation voltage 2,8V to 4,2V.
Recommended temperature range 0-40 degree celcius.

Advantages of Lithium-ion Battery Packs:

They have a higher energy density than most other types of rechargeables, resulting in the longest battery runtimes in portable computers. They also operate at higher voltages than other rechargeables, typically about 3.7 volts for lithium-ion vs. 1.2 volts for NiMH or NiCd. This means a single cell can often be used rather than multiple NiMH or NiCd cells.
Lithium-ion batteries also have a lower self-discharge rate. This means that once they are charged they will retain their charge for a longer time than other types of rechargeable batteries. NiMH and NiCd batteries can lose anywhere from 1-5% of their charge per day, (depending on the storage temperature) even if they are not installed in a device. Lithium-ion batteries will retain most of their charge even after months of storage.

So in summary; lithium-ion batteries can be smaller or lighter, have a higher voltage and hold a charge much longer than other types of batteries. Disadvantages of Lithium Ion battery packs:

Lithium-ion batteries are more expensive than similar capacity NiMH or NiCd batteries. This is because they are much more complex to manufacture. Li-ion batteries actually include special circuitry to protect the battery from damage due to overcharging or undercharging. They are also more expensive because they are manufactured in much smaller numbers than NiMH or NiCd batteries. Li-ion batteries are becoming less expensive and over time we should see their price decrease significantly.
Lithium ion batteries are not available in standard cells sizes (AA, C and D) like NiMH and NiCd batteries.
Lithium-ion batteries also require sophisticated chargers that can carefully monitor the charge process. And because of their different shapes and sizes each type of Li-ion battery requires a charger designed to accommodate its particular size. This means lithium ion battery chargers are more expensive and more difficult to find than NiMH and NiCd battery chargers.

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