GMC 2010 Sierra 1500 Crew Cab Owner's Manual page 240

For vehicles with reading lamps, they are located on the
overhead console.
To turn on the reading lamps, press the button located
next to each lamp. To turn them off, press the button
again.
The vehicle may also have reading lamps in other
locations. To turn the lamps on or off, press the button
located next to the lamp.
If the vehicle has a DVD Rear Seat Entertainment
(RSE) system, press the lamp lenses to turn the lamps
on or off.
The lamps are fixed and cannot be adjusted.
The cargo lamps come on by turning the instrument
panel brightness control knob to the farthest clockwise
position. This knob is located on the instrument panel
and also turns on the dome lamps.
The cargo lamp can be used if more light is needed in
the cargo area of the vehicle or in the top-box storage
units.
4-16
The vehicle has Electric Power Management (EPM) that
estimates the battery's temperature and state of charge.
It then adjusts the voltage for best performance and
extended life of the battery.
When the battery's state of charge is low, the voltage is
raised slightly to quickly bring the charge back up.
When the state of charge is high, the voltage is lowered
slightly to prevent overcharging. If the vehicle has a
voltmeter gauge or a voltage display on the Driver
Information Center (DIC), you may see the voltage
move up or down. This is normal. If there is a problem,
an alert will be displayed.
The battery can be discharged at idle if the electrical
loads are very high. This is true for all vehicles. This is
because the generator (alternator) may not be spinning
fast enough at idle to produce all the power that is
needed for very high electrical loads.