Advanced Configuration and Power Interface Specification

ACPI Overview
Table 3-4
Level
Warning
Low
Low Battery Levels
Description
When the total available energy (mWh) or capacity (mAh) in the batteries falls below this level,
the OS will notify the user through the UI. This value should allow for a few minutes of run-time
before the “Low” level is encountered so the user has time to wrap up any important work,
change the battery, or find a power outlet to plug the system in.
This value is an estimation of the amount of energy or battery capacity required by the system to
transition to any supported sleeping state. When the OS detects that the total available battery
capacity is less than this value, it will transition the system to a user defined system state (S1-
S4). In most situations this should be S4 so that system state is not lost if the battery eventually
becomes completely empty. The design of the OS should consider that users of a multiple battery
system may remove one or more of the batteries in an attempt replace or charge it. This might
result in the remaining capacity falling below the “Low” level not leaving sufficient battery capacity
for the OS to safely transition the system into the sleeping state. Therefore, if the batteries are
discharging simultaneously, the action might need to be initiated at the point when both batteries
reach this level.
Critical
The Critical battery state indicates that all available batteries are discharged and do not appear to
be able to supply power to run the system any longer. When this occurs, the OS must attempt to
perform an emergency shutdown as described below.
For a smart battery system, this would typically occur when all batteries reach a capacity of 0, but
an OEM may choose to put a larger value in the Smart Battery Table to provide an extra margin
of safely.
For a Control Method Battery system with multiple batteries, the flag is reported per battery. If any
battery in the system is in a critically low state and is still providing power to the system (in other
words, the battery is discharging), the system is considered to be in a critical energy state. The
_BST control method is required to return the Critical flag on a discharging battery only when all
batteries have reached a critical state; the ACPI system firmware is otherwise required to
switch to a non-critical battery.
3.9.4.1 Emergency Shutdown
Running until all batteries in a system are critical is not a situation that should be encountered
normally, since the system should be put into a sleeping state when the battery becomes low. In the
case that this does occur, the OS should take steps to minimize any damage to system integrity. The
emergency shutdown procedure should be designed to minimize bad effects based on the assumption
that power may be lost at any time. For example, if a hard disk is spun down, the OS should not try
to spin it up to write any data, since spinning up the disk and attempting to write data could
potentially corrupt files if the write were not completed. Even if a disk is spun up, the decision to
attempt to save even system settings data before shutting down would have to be evaluated since
reverting to previous settings might be less harmful than having the potential to corrupt the settings
if power was lost halfway through the write operation.
3.9.5 Battery Calibration
The reported capacity of many batteries generally degrade over time, providing less run time for the
user. However, it is possible with many battery systems to provide more useable runtime on an old
battery if a calibration or conditioning cycle is run occasionally. The user has typically been able to
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