Electrical power

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TTP
B1-L3
ATA24
Cessna 172
B2-L3

Electrical energy  is supplied by a 28-volt, direct current, single primary bus, negative ground electrical system. A single 24-volt main battery supplies power to the starting system and gives a reserve source of power if an alternator failure were to occur.  Garmin G1000 system have a second battery known as the Standby Battery.
Standby Battery is controlled and monitored by the Standby Battery Controller and supplies power to the G1000 Essential Bus if there is a failure of the main battery and alternator.
Power junction box, (MCU), is attached to the forward left side of the firewall and includes electrical relays, an alternator control unit (ACU), an ammeter sensor, an external power receptacle, fuses and/or circuit breakers in a single box.
Engine-driven alternator is the normal source of power during flight and maintains a battery charge controlled by the ACU. The external power receptacle is used for ground operation of the electrical equipment and helps the main battery during ground start
Main battery and alternator system is controlled by the MASTER ALT BAT switch. The switch is an interlocking split rocker and is found on the left side of the switch panel. The right half of the rocker controls the main battery and the left half controls the alternator. It is possible in this configuration for the main battery to be online without the alternator. However, operation of the alternator without the main battery is not possible.
BAT MASTER switch, when operated, connects the main battery contactor coil to ground so that the contacts close and supply power to the system from the main battery only.     ALT MASTER switch, when ON, applies positive voltage to the ACU and to the alternator contactor coil at the same time, which then applies field voltage to the alternator field and supplies power to the electrical system from the alternator.
Battery is a 24-Volt, 12.75 Amp-hour or an optional heavy-duty 15.5 Amp-hour flooded lead-acid type battery. Either of these batteries can be installed. The battery is installed on the front-left side of the firewall below the electrical power junction box.
NOTE: The Amp-hour rate is based on a five hour discharge rate.
Garmin 1000 Standby battery is installed behind the Primary Flight Display. If there is no primary power source, the standby battery will give power to the essential bus for a period of time. The standby battery PC board is installed on the back of the switch panel. The standby battery PC board controls and monitors the release of electrical power to and from the standby battery.
Standby Battery, if installed, is controlled by a three-position STDBY BATT switch. Normal flight operation is with the switch in the ARM position that lets the standby battery charge from the G1000 Essential Bus. If there is an alternator failure, the standby battery controller will not let the standby battery discharge to the G1000 Essential Bus until the depletion or failure of the main battery. It is necessary during preflight to do an "energy level" acceptance test.
Main battery ammeter is controlled by a sensor found in the power junction box. In flight, without the use of external power, the meter shows the quantity of current that flows to or from the battery. With a low battery and the engine at cruise speed, the ammeter will show a large positive output and a charge of the main battery.When the main battery is fully charged, the ammeter will show a minimum charge rate.


PTP
Cessna
172
B1
LOC
FOT
SGH
R/I
MEL
TS
B2
LOC
FOT
SGH
R/I
MEL
TS

242001 Alternator, Mounting Bracket, and Electrical Connections - Check condition and                     security. Check alternator belts for condition and proper adjustment. Check belt                     tension.A 05-12-01 120

243001 Main Battery - Examine the general condition and security. Complete the applicable
             main battery servicing procedure.
243002 Main Battery Box and Cables - Clean and remove any corrosion. Examine the                       cables for routing, support, and security of the connections. B 05-12-02 120
243004 External Power Receptacle and Power Cables - Inspect for condition and security.
246001 Switch and Circuit Breaker Panel, Terminal Blocks, and Junction Boxes - Inspect       wiring and terminals for condition and security.
246002 Power Junction Box - Check operation and condition. Check availability and                condition of spare fuse 

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