A certain battery may be designed for small size and long runtime, but this pack has a limited cycle life. Another battery may be built for durability but is big and bulky. A third pack may have high energy density and long durability but this version is too expensive for the consumer.

Battery manufacturers are aware of customer needs and offer packs that best suit the application. The mobile phone industry is an example of this clever adaptation. Here, small size and high energy density reign in favor of longevity. Short service life is not an issue because a device is often replaced before the battery is worn out.

Let's examine various battery designs, starting with nickel-metal-hydride. The cylindrical nickel-metal-hydride for commercial use offers a mid-range energy density of about 80Wh/kg and delivers roughly 400 cycles. The prismatic nickel-metal-hydride, a battery that is made for slim geometry, compromises on energy density and cycle count. This battery is rated at a moderate 60Wh/kg and offers around 300 cycles. Highly durable nickel-metal-hydride for industrial use are packaged in cylindrical cells, provide a modest 70Wh/kg but last for about 1000 cycles.

Similarly, lithium-ion batteries can be produced with various energy densities. Packing more energy into a cell compromises safety. While commercial lithium-ion batteries are safe, super-high capacity lithium-ion for defense applications are, for safety reasons, not approved for the public at large.

Below is a summary of the strength and limitations of today's popular battery systems. Although energy density is paramount, other important attributes are service life, load characteristics, maintenance requirements, self-discharge and operational costs. Since nickel-cadmium remains a standard against which batteries are compared, we evaluate alternative chemistries against this classic battery type.

-Nickel-cadmium - mature but has moderate energy density. nickel-cadmium is used where long life, high discharge rate and extended temperature range is important. Main applications are two-way radios, biomedical equipment and power tools. nickel-cadmium contains toxic metals.

-Nickel-metal-hydride - has a higher energy density compared to nickel-cadmium at the expense of reduced cycle life. There are no toxic metals. Applications include mobile phones and laptop computers.

-Lead-acid - most economical for larger power applications where weight is of little concern. Lead-acid is the preferred choice for hospital equipment, wheelchairs, emergency lighting and UPS systems.

-Lithium-ion - fastest growing battery system; offers high-energy density and low weight. Protection circuit are needed to limit voltage and current for safety reasons. Applications include notebook computers and cell phones.

-Lithium-ion-polymer - Similar to lithium-ion, this system enables slim geometry and simple packaging at the expense of higher cost per watt/hours. Main applications are cell phones.

-Reusable Alkaline - Its limited cycle life and low load current is compensated by long shelf life, making this battery ideal for portable entertainment devices and flashlights.
Table 1 summarizes the characteristics of the common batteries. The figures are based on average ratings at time of publication. Note that nickel-cadmium has the shortest charge time, delivers the highest load current and offers the lowest overall cost-per-cycle but needs regular maintenance.

Table 1: Characteristics of commonly used rechargeable batteries.

*1) Internal resistance of a battery pack varies with cell rating, type of protection circuit and number of cells. Protection circuit of lithium-ion and lithium-ion-polymer adds about 100mW.
*2) Cycle life is based on battery receiving regular maintenance. Failing to apply periodic full discharge cycles may reduce the cycle life by a factor of three.
*3) Cycle life is based on the depth of discharge. Shallow discharges provide more cycles than deep discharges.
*4) The discharge is highest immediately after charge, and then tapers off. The capacity of nickel-cadmium decreases 10% in the first 24h, then declines to about 10% every 30 days thereafter. Self-discharge increases with higher temperature.
*5) Internal protection circuits typically consume 3% of the stored energy per month.
*6) 1.25V is the open cell voltage. 1.2V is the commonly used as a method of rating.
*7) Capable of high current pulses.
*8) Applies to discharge only; charge temperature range is more confined. 
*9) Maintenance may be in the form of 'equalizing' or 'topping' charge.
*10) Cost of battery for commercially available portable devices.
*11) Derived from the battery price divided by cycle life. Does not include the cost of electricity and chargers.

In subsequent columns I will describe the strength and limitation of each chemistry in more detail. We will examine charging techniques and explore methods to get the most of these batteries. 

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