Today's consumers take for granted the ability of the electronics industry to squeeze more functions into smaller, more portable devices. Mobile phones double as cameras and are now adding flash features; still cameras shoot video; laptop computers can do pretty much anything that can be achieved on the desktop, including driving power-hungry peripherals.
This growing array of functions poses a problem, though, for the systems that are used to provide the power: primarily the battery. And, while great strides have taken place in cutting active and standby power consumption of electronic devices, it is the changing nature of the functions that now represents the greatest challenge. Products such as high data-rate GPRS transmitters and cameras with electronic flash and auto-focus drives require significant instantaneous pulses of power that are difficult to deliver from a portable power source.
At the same time as power requirements have become more exacting, product designers have worked to make it possible to use mainstream battery technology in portable devices. Primarily this means moving to what remains the most common, low-cost form of battery power: the alkaline cell, typified by the familiar AA battery.
Alkaline batteries have many advantages:
* They are readily available at reasonable cost.
* Consumers are familiar with the technology and confident in handling them.
* They provide high energy density and long life.
* Finally, they are easy to design-in, long-established and hence well-understood by engineers.
High pulse-power applications take a toll on batteries
Alkaline batteries also have some weaknesses in applications with high pulse power. In particular, their internal resistance (expressed as equivalent series resistance or ESR) causes an internal voltage drop when current is drawn. This means that their ability to deliver short spikes of current is relatively poor. This phenomenon worsens in cold conditions and as the battery discharges -- and in high pulse power applications the system typically shuts down due to the resulting drop in the output voltage during the pulse: worse, in these high pulse power applications the battery typically will have an appreciable amount of energy left at the time of low voltage shutdown that is unused.
In a typical case, a digital still camera driven by two AA batteries might draw a peak power of 3W and current of 1.5A during the auto focus phase of its operation. Such a drive function might last around 0.4s, during which time the battery voltage would drop around 300mV for a new battery but the voltage drop will increase to as much as 600mV when the battery reaches approximately 50% charge level. Results will vary from battery to battery and with manufacturers with some better or worse.