ABC OF RECHARGEABLE BATTERIES （1）

Published:2011/8/9 1:15:00 Author:Phyllis From:SeekIC

By Gert Helles

Basics, pitfalls & recommendations

The use of batteries has never been greater. Batteries are becoming smaller and lighter even as they package more and more energy per unit volume. The main driving force for battery development has been the boom in portable equipment such as mobile phones, laptops, camcorders, and MP3 players.

The following brief overview of charging methods and current battery technologies is intended to lend a better understanding of the batteries used in portable devices. It includes discussions of nickel-cadmium (NiCd), nickel-metal-hydride (NiMH) and lithiumion (Li+) battery chemistries. The article also describes a product for protection of single-cell lithiumion and lithium-polymer batteries.

Definition of a battery

Calling a battery an energy storage system poses a definition that also includes such things as flywheels and clock springs. In the context of modern technology, however, batteries are usually portable, self-contained chemical systems that produce electrical energy.

Disposable batteries (called non-rechargeable or primary cells) create electricity from a chemical reaction that permanently transforms the cell. Discharging a primary cell leads to a permanent and irreversible change in the cell chemicals. By contrast, rechargeable batteries, also called secondary cells, can be recharged by a charger after having been discharged by the application.

The charge or discharge current is usually expressed (in amperes) as a multiple of the rated capacity (called the C-rate). For example, a C/l 0 discharge current for a battery rated at one ampere-hour (1 Ah) is 1 Ah/10 = 100mA. The rated capacity of a cell or battery (in Ah or mAh) is the amount of electricity that it can store (produce) when fully charged under specified conditions. Thus, the total energy of a battery is its capacity multiplied by its voltage, resulting in a measurement of watt-hours.

Defining battery performance

The chemistry and the design of a battery cell together limit the current it can source. Barring the practical factors that limit performance, a battery could produce an infinite current, if only briefly. The main impediments to infinite current are the basic reaction rates of the chemicals, the cell design, and the area over which the reaction takes place. Some cells are inherently able to produce high currents. Shorting a nickel-cadmium cell, for instance, produces currents high enough to melt metals and start fires. Other batteries can produce only weak currents.

The net effect of all chemical and mechanical factors in a battery can be expressed as a single mathematical factor called the equivalent internal resistance. Lowering the internal resistance enables higher currents.

No battery stores energy forever. Unavoidably, the cell chemicals react and slowly degrade, causing degradation in charge stored by the battery. The ratio of battery capacity to weight (or size) is called the battery’s storage density. High storage density enables the storage of more energy in a cell of given size or weight.








Table 1 lists nominal voltage and storage density (expressed in watt-hours per kilogram of weight, or Wh/kg) for the major chemistries used in storage batteries for personal computers and cell phones: Table 2 contains quick comparison data to enable designers to choose the best cell type for a particular application (note that NiCd will be banned soon).

So why not always choose secondary cells, if primary and secondary cells fulfill the same purpose? Secondary cells have drawbacks:
- All practical secondary cells lose their electrical charge relatively quickly, through self discharge.
- Secondary cells must be charged before use.
- Secondary batteries supply less energy at the same volume and weight.

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