Cessna SkyHawk SP 172S NAV III 2005 Pilot Operating Handbook page 250

SECTION 7
AIRPLANE AND SYSTEMS DESCRIPTION
ELECTRICAL SYSTEM
(Continued)
CESSNA
MODEL 172S NAV III
The system is equipped with a secondary or "standby" battery
located between the firewall and the instrument panel. The STBY
BATT switch controls power to or from the standby battery. The
standby battery is available to supply power to the Essential Bus in
the event that alternator and main battery power sources have both
failed.
The primary buses are supplied with power whenever the MASTER
switch (BAT) is turned on, and are not affected by starter or external
power usage. Each primary bus is also connected to an avionics
bus through a circuit breaker and the AVIONICS BUS 1 and BUS 2
switches. Each avionics bus is powered when the MASTER switch
(BAT) and the corresponding AVIONICS switch is in the ON
position.
CAUTION
BOTH BUS 1 AND BUS 2 AVIONICS SWITCHES
SHOULD BE TURNED OFF TO PREVENT ANY
HARMFUL TRANSIENT VOLTAGE FROM
DAMAGING THE AVIONICS EQUIPMENT PRIOR
TO TURNING THE MASTER SWITCH ON OR
OFF,STARTING THE ENGINE OR APPLYING AN
EXTERNAL POWER SOURCE
The airplane includes a power distribution module, located on the
left forward side of the firewall, to house all the relays used in the
airplane electrical system. The alternator control unit, main battery
current sensor, and the external power connector are also housed
within the module.
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