Battery packaging - a look at old and new systems(B)
Theprismatic cell was developed in the early 1990 to response toconsumer demand for thinner geometry. Prismatic cells are commonlyreserved for the lithium battery family. The polymer version isexclusively prismatic.
The prismatic cell comes in various sizes with capacities from400mAh to 2000mAh and higher. No standard cell size exists; rather,prismatic cells are custom-made for cell phones and other highvolume items.
Thenegative attributes of the prismatic cell are slightly lower energydensities and higher manufacturing costs than the cylindrical cell.In addition, the prismatic cell does not provide the samemechanical stability enjoyed by the cylindrical cell. Prismaticcells have no venting system. To prevent bulging on pressure buildup, heavier gauge metal is used for the container. Some degree ofbulging must be considered in equipment design.
The introduction of the pouchcell in 1995 made a profound advancement in cell design. Ratherthan using expensive metallic enclosures and glass-to-metalelectrical feed-troughs, a heat-sealable foil is used. Theelectrical contacts consist of conductive foil tabs that are weldedto the electrode and sealed to the pouch material.
The pouch cell concept allows tailoring to exact cell dimensions.It makes the most efficient use of available space and achieves apackaging efficiency of 90 to 95 percent, the highest among batterypacks. Because of the absence of a metal can, the pouch pack islight. The main application is cell phones. No standardized pouchcells exist, each manufacturer builds to a special application.
The pouch cell is exclusively used for lithium-based chemistries.Manufacturing cost is still higher than conventional systems andits reliability has not been fully proven. In addition, the energydensity and load current are slightly lower. The cycle life is notwell documented but remains less than that of other packagingsystems.
A critical issue with the pouch cell is the swelling that occurswhen gas is generated during charging or discharging. Allowancemust be made for some expansion, even though battery manufacturersinsist that the cells do not generate gas if correctly charged. Itis best not to stack pouch cells, but lay them side-by-side.
The pouch cell is highly sensitive to twisting. Point pressure mustalso be avoided. The protective housing must be designed to protectthe cell from mechanical stress.
Battery packsfor portable devices
Most manufacturers of cell phones,laptops and cameras develop their own battery packs. A model changeoften results in a redesigned battery. The typical contactarrangements of cell phone and video camera batteries are: batterypositive, negative and temperature sensor. Additional contacts, ifpresent, may serve as control switch or battery type identifier.'Smart' batteries have extra contacts to provide state-of-chargeindication and other information.
There are no norms and standards for thesebatteries. Each manufacturer has its own design.
In the 1990s, the Smart Battery System (SBS) forum made a concertedeffort to standardize on battery norms for laptops, surveyequipment and medical instruments. Beside physical size, thesebatteries ran on a standard SMBus protocol. With miniaturizing andsecuring a lucrative battery replacement market, laptopmanufacturers went their own way. The SMBus batteries (type 2020,1030, 1020, 210, 202, 201, 36, 35, 30, 17 and 15) are still widelyused today for specialty instruments. (See also "The 'smart'battery" in Part One and "How to service laptop batteries" in PartTwo.)