cat /proc/acpi/battery/BAT1/state

Here is a sample output:

present:                    yes
capacity state:             ok
charging state:             discharging
present rate:               0 mA
remaining capacity:         3720 mAh
present voltage:            14800 mV

If the laptop has two running batteries, the 2nd battery can be checked
with the command 'cat /proc/acpi/battery/BAT2/state'.

In the output above, the system indicates that the battery is being used (no AC
power). The field 'remainding capacity' indicates the amount of power is
available in the battery. So how does one know what is the maximum power that a
battery can hold? That information can be obtained with the following command:

     cat /proc/acpi/battery/BAT1/info

Here is a sample output:

present:                     yes
design capacity:             4000 mAh
last full capacity:          4000 mAh
battery technology:          rechargeable
design voltage:              14800 mV
design capacity warning:     420 mAh
design capacity low:         156 mAh
capacity granularity 1:      264 mAh
capacity granularity 2:      3780 mAh
model number:                PA3098U
serial number:               3658Q
battery type:                Li-Ion
OEM info:                    COMPAL

The field 'design capacity' indicates the capacity of the battery. Given the
first output, we can calculate the percentage of the battery level as followimg:

       remainding capacity / design capacity

In the example above, the battery has used up 7% of the total power available.

How does one know if the battery is being charged? Let us say that an AC
power cord has been connected to the laptop. Next we run the command
'cat /proc/acpi/battery/BAT1/state'. Here is the new output:

present:                     yes
capacity state:              ok
charging state:              charging
present rate:                0 mA
remaining capacity:          3720 mAh
present voltage:             14800 mV

Notice the state has changed from 'discharging' to 'charging'. After the
battery has been fully charged, running the command 'cat /proc/acpi/battery/BAT1/state'
would yield the following output:

present:                     yes
capacity state:              ok
charging state:              unknown
present rate:                0 mA
remaining capacity:          4000 mAh
present voltage:             14800 mV

When the charging state is 'unknown', that indicates the battery is fully
charged. Notice the value of the remainding capacity has been updated
accordingly. It now matches the value of design capacity.