Dell Latitude E5420 (Early 2011) - Manual (Page 158)

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.
Keyboard Status Lights
The lights located above the keyboard indicate the following:
Turns on when the numeric keypad is enabled.
Turns on when the Caps Lock function is enabled.
Turns on when the Scroll Lock function is enabled.
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