Batteries are easy to use. The only catch is that the battery (or batteries) you choose must supply enough current at the right voltage. With the right choice of battery and a carefully designed system, you can achieve extended operation over very long periods of time. For example, a small PIC- or AVR-based computer can (depending on application and design) operate for up to two years off a single AA battery. A poorly designed system can drain a battery in minutes. A poorly chosen battery unable to supply sufficient current will result in erratic operation or may result in the system being unable to start at all. When choosing a battery, consider not just its average current capability but also its peak current. An embedded computer may need only a constant supply of 20mA but may require as much as 100mA at peak loads. This is especially true of systems using flash memory, which may require high currents during write operations. The battery for such a system must be able to supply not just the continuous load but also the peak load when required.

Power consumption in an embedded system can be reduced in several ways. The use of low-power devices is the most obvious place to start. The power consumption of different devices varies considerably, and many low-power variants of common devices are available. RISC processors often have lower power consumption than comparable CISC processors, so they are often used in preference to CISC in lowpower applications. The PIC and AVR microcontrollers can have current draws of less than 5mA (and as low as 10nA when in sleep mode!). This is considerably less than the 35mA of a 68HC11 microcontroller.

Many memory chips and peripherals will enter a low-power mode when they are not in use. However, the power consumption of some devices can be reduced even further. A useful technique for reducing a system’s power consumption is to turn off devices when not in use. If the processor is executing code from RAM and outputting data to a serial port, then the power to the ROMs and any other I/O devices may be turned off, as they are not in use.

Further, some low-power devices (such as sensors) may need very little current, so little that they can be directly powered from the I/O line of a microcontroller. The I/O signal is the power supply for the device. The devices can be turned on or off under software control by toggling the I/O line. Some processors, such as the PIC and AVR, can sink relatively large currents (20mA) through their I/O pins, and these can be used as ground for some devices (such as LEDs).