Cell Selection

When choosing a cell for an application, it must match the device's construction and design objectives. Things to consider when choosing include:

• Cell capacity
• Operating temperature and power range
• Cell availability
• Shape
• Cycle life
• Cells in series and/or in parallel
• Cost

Different battery cells have diverse impedance levels and the application current demand may exclude some types of cells. For many years, NiCd batteries were the only suitable battery for portable equipment used on communication and computing devices. In the 1990’s, NiMH and Lithium Ion batteries made their first appearances and began to gain acceptance. Today, Lithium Ion is the fastest growing battery chemistry.

Lithium Ion batteries should be brought to a 40% charge level, and should never be deep cycled like NiCd batteries. Nevertheless, NiCd and NiMH batteries still have usefulness in high-drain applications such as cordless power tools and toys.

Lithium Ion batteries have a nominal open-circuit voltage of 3.6V and typical charging voltage of 4.2V. The charging procedure is one of constant voltage with current limiting. Knowing the minimum and maximum application voltages is critical to the design. In the past, Lithium Ion batteries could not be fast-charged and typically needed two hours or more to fully charge. Today’s generation of cells can be fully charged in 45 minutes or less with some reaching 90% in as little as 10 minutes.