By pressing once or pressing and holding the status button on the charge gauge on the battery, you can check:
The battery operating time is largely determined by the number of times it is charged. After hundreds of charge and discharge cycles, batteries lose some charge capacity—or battery health. That is, a battery can show a status of "charged" but maintain a reduced charge capacity (health).
To check the battery health, press and hold the status button on the battery charge gauge for at least 3 seconds. If no lights appear, the battery is in good condition, and more than 80 percent of its original charge capacity remains. Each light represents incremental degradation. If five lights appear, less than 60 percent of the charge capacity remains, and you should consider replacing the battery. See Specifications for more information about the battery operating time.
refer to this DELL document: http://support.dell.com/support/edocs/systems/ins9400/en/om/battery.htm