Under normal circumstances, the reliability of a portable computer depends on the health of its power source. Portable computers have long relied on batteries for electrical power, and the battery is the source of the computer's electrical current. Because the charge of a battery is proportional to the time that it has been in operation and to the amount of current that is available, there are inherent limitations to how long a portable computer may operate without recharging. However, when the battery is functioning correctly, the computer is capable of performing tasks that are more demanding than simply displaying a screen.
Unfortunately, not all batteries operate efficiently. The power output of the battery is proportional to the voltage of the battery and the current that is provided. Battery voltage has a tendency to drop and battery current declines over time. Typically, the rate of battery voltage decline increases at a faster rate than the decline in battery current. For example, as a battery ages and voltage drops, the battery's charge capacity also declines. Consequently, batteries must be replaced more often.
One approach to improving the reliability of battery-powered portable computers is to make sure that the computer has an adequate reserve of battery power. The reserve of battery power, also referred to as the battery's "capacity", has been estimated by measuring how much battery power the computer will expend at a constant level of current when a particular type of battery is fully charged. With this measurement, the reserve of a battery can be estimated by the battery's capacity rating, which is typically expressed in ampere-hours. For example, if the rated capacity of a battery is 5 ampere-hours, it will be able to provide 5 ampere-hours at a constant level of current for a one-hour period.
Unfortunately, the reserve of a battery is not as easily measured as its capacity. Instead, it has been estimated by continuously measuring the current consumed by the battery when it is fully charged and by continuously measuring the remaining battery charge after the measured current has been expended. By carefully recording the current consumption and battery charge during each hour, the reserve of the battery can be measured accurately.
Since the maximum battery capacity cannot be measured directly, manufacturers must estimate the reserve of the battery based on the battery's capacity rating. When the reserve of a battery is measured with a computer, the computer must rely on the manufacturer's estimate of the reserve of the battery. However, because the current consumption of a battery varies with time, the battery's reserve cannot be accurately estimated unless it is continuously measured. As a result be359ba680
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