GMC 2012 Terrain Owner's Manual page 160

6-6 Lighting
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
of the power needed for very high
electrical loads.
A high electrical load occurs when
several of the following are on,
such as: headlamps, high beams,
fog lamps, rear window defogger,
climate control fan at high speed,
heated seats, engine cooling fans,
trailer loads, and loads plugged into
accessory power outlets.
EPM works to prevent excessive
discharge of the battery. It does this
by balancing the generator's output
and the vehicle's electrical needs.
It can increase engine idle speed
to generate more power whenever
needed. It can temporarily reduce
the power demands of some
accessories.
Normally, these actions occur
in steps or levels, without being
noticeable. In rare cases at the
highest levels of corrective action,
this action may be noticeable to the
driver. If so, a DIC message might
be displayed, such as BATTERY
SAVER ACTIVE, BATTERY
VOLTAGE LOW, or LOW BATTERY.
If one of these messages displays,
it is recommended that the driver
reduce the electrical loads as much
as possible. See Driver Information
Center (DIC) on page 5 23.