GMC 2013 Terrain Owner's Manual page 150

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 and it is recommended
that the driver reduce the electrical
loads as much as possible. See
Battery Voltage and Charging
Messages on page 5 24.
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