C172S Electrical System

To what voltage is the system regulated

28V DC

What is the maximum current output of the alternator

60 amps

What is the purpose of the standby battery and where is it located

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. It's located between the firewall and the instrument panel.

What does electrical bus 1 power

Auxiliary fuel pump, beacon, landing and overhead lights, cabin power, flaps, avionics bus 1

What does avionics bus 1 power

PFD, PFD cooling fans, air data computer and attitude heading reference system, nav #1, flight information system (FIS), ADF (if installed), DME

What does the cross feed bus power

Alt master switch, (WARN cct breaker), stall, ap, ELT warnings, main bus voltmeter, hourmeter, starter relay, stdby battery and main bus sense

What does electrical bus 2 power

Avionics switch 2, pitot heat, nav and control wheel map lights, taxi light, wing strobe lights, panel lights

What does avionics bus 2 power

MFD display and fan, transponder, nav 2 and alt avionics cooling fan, VHF communication 2, audio panel, autopilot system

What does essential bus power

PFD, ADC, AHRS, NAV 1 and engine/airframe unit, Comm 1, stdby battery system

What is the master switch and what does it control

A two-pole, rocker type switch. The BAT side of the switch controls the main battery electrical power to the airplane. The ALT side of the switch controls the alternator system. the BAT side of the switch may be selected separately as necessary. The ALT side of the switch can not be set to ON without the BAT side of the switch also being set to ON.

What are the voltage indications

Main bus voltage is indicated by M (measured at the WARN circuit breaker on the cross-feed bus), essential bus voltage is indicated by E (measured at the NAV1 ENG circuit breaker on the essential bus)

Voltage operating

Main or essential bus (24.5-32, average 28V)

Current indications

M indicates main battery (>1.5amps). S indicates standby battery current (positive, which means its charging the battery)

When does the standby battery annunciator come on

When discharge rates are greater then 0.5 amps for more than 10 seconds. This caution annunciation is an indication that the alternator and the main battery are not supplying the power that is required by the essential bus.

When will the standby battery provide power

Main battery supplies power until M BUS VOLTS decreases below 20 volts, then the stdby battery automatically supplies power for at least 30 mins

Low volts annunciator

Main bus is below 24.5 volts. it is an indication that the alternator is not supplying the power that is required by the airplane. ALT Field circuit breaker may pop open during normal start.

What do you do if Low Volts Annunciator comes on or does not go off at a higher RPM

Alt master switch OFF, Alt field cct breaker CHECK IN, Master switch both ON, Check low volts annunciator off, check M bus volts 27.5 (minimum), check M battery amps charging

What do you do if low volts annunciator remains on

Alt master switch off, Reduce electrical load immediately as follows: Avionics bus 1 off, pitot heat off, beacon off, land light off (use as required fr landing), taxi light off, nav light off, strove light off, cabin pwr off

High Voltage annunciation

HIGH VOLTS annunciator will come on when main or essential bus voltage is above 32.0 volts. This warning annunciation is an indication that the alternator is supplying too high of a voltage

What do you do when the high volts annunciator comes on or the M BATT AMPS are more than 40

Alt master switch OFF, reduce electrical load immediately as follows: avionics switch bus 1 off, pitot heat switch off, beacon light switch off, land light switch off, taxi light switch off, nav light switch off, strobe light switch off, cabin pwr 12V switch off, tune comm 1 and nav 1 to active frequency ad=nd select

What is an external power receptacle

A external power receptacle is integral to the power distribution module and allows the use of an external power source for cold weather starting or for lengthy maintenance work on electrical and avionics equipment.

Where is the eternal power receptacle located

The receptacle is located on the left side of the cowl near the firewall. Access to the receptacle is gained by opening the receptacle door.

What type buttons are the cabin heat and air

Double button locking type and permit intermediate settings