Power is the primary challenge facing the mobile small computing industry. Consumer expectations are high for mobile small computing devices. They want desktop performance anytime and anywhere they power up. Unfortunately, improvements in power storage technology haven't kept pace with mobile computing technology.
Every mobile small computing device is powered by one or more batteries that have a limited life span after which there is no electricity to operate the device. Engineers reduce power consumption by removing power-hungry components and impose power saving techniques such as deactivating components when a component is not in use.
The length of time a mobile small computing device is operational depends on how well and how long a battery holds its electrical charge. For example, a PDA might operate steadily for 20 hours, while a cellular telephone remains operational for 10 hours. The actual time a battery remains charged depends on a number of factors. These include the type of battery in use, the condition of the battery, and the power consumption of each operation. For example, standby operation for a cellular telephone consumes less power than transmitting a signal.
Power is measured in watt-hours, which means the number of hours the battery can supply one watt of power. A cellular telephone battery is rated at 10 watt-hours, so the battery can supply one watt of power for ten hours. A PDA battery has a rating of 2 watt-hours, supplying one watt of power for two hours.
The amount of watts used by a mobile computing device varies depending on the power-hungry components built into the device. In addition, power consumption depends on the state of the device. Mobile computing devices have three states: active, standby, and off. The active state is when the device is being used. The standby state, also known as the sleep mode, is when all but critical components are powered down, minimizing the drain on the battery. When the device is turned off, the off state still drains power from the battery to maintain information such as a telephone directory in memory.
There are two general classes of batteries: non-rechargeable and rechargeable. Nonrechargeable batteries are further classified as alkaline and zinc-carbon. These are found in PDAs and as an emergency backup power for cellular telephones. Alkaline batteries have a higher capacity to retain an electrical charge than zinc-carbon batteries. Rechargeable batteries fall into four subclasses: lead acid batteries, nickel cadmium (NiCAD) batteries, nickel metal hydride (NiMH) batteries, and lithium ion (LiON) batteries.
Cellular telephone manufacturers originally used lead acid batteries because they can be recharged many times; however, lead acid is a dangerous ingredient, and therefore the manufacturers moved to the safer nickel cadmium battery. NiCAD batteries can be recharged 1,000 times before the battery discharges. However, NiCAD batteries must be fully discharged before being recharged, otherwise the battery may not fully recharge.
Consumers complained about this recharging limitation, and therefore manufacturers dropped NiCAD batteries for either the nickel metal hydride battery or the lithium ion battery.
The NiMH battery does not have to be fully discharged before recharging. It also has a quarter more charging capacity than a NiCAD battery. A lithium ion battery has a longer life and more charging capacity than the NiMH battery. Both NiMH and LiON batteries are considered "smart" batteries because each has a power meter that indicates the electrical charge of the battery.
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