Electrical System - Piper Archer III PA-28-181 Handbook

PA-28-181, ARCHER III

7.21 ELECTRICAL SYSTEM

The 28 volt electrical system includes a 24 volt primary battery, a 70
ampere 28 volt alternator, a single external power connector and an isolated 24
volt emergency battery. The electrical system is capable of supplying sufficient
current to all the required equipment for day/night IFR and day/night VFR
operations.
Primary battery
The primary battery provides for electric power to the equipment when the
engine is not running and for engine starting. When energized by the battery
master switch the primary battery supplies electrical power to the starter, as
well as all items on the Essential Bus, Non-Essential Bus and Lighting Bus. If
it becomes necessary to charge the battery by an external source, it should be
removed from the airplane prior to charging. The primary battery is mounted
on a shelf in the aft fuselage area.
Alternator
The alternator is belt driven directly from the engine. Once the engine is
running and the ALTR switch is activated, the alternator becomes the primary
source of electrical power for the aircraft. The primary battery provides stored
electrical power to back up the alternator. During normal operations, the
battery is charged by the alternator.
Voltage regulator
A solid state voltage regulator is located just forward of the instrument
panel on the left side of the aircraft. The voltage regulator is designed to
regulate the electrical system bus voltage to 28 volts and to prevent damage
to the electrical and avionics equipment by removing the alternator from the
circuit if its output exceeds 32 volts. In this situation an ALTR FAIL warning
CAS message will illuminate.
Emergency Battery
The emergency battery provides electrical power to the emergency bus
in the unlikely event of a complete electrical failure. With the EMERG
BATT switch in the ARM position, power is applied to the emergency bus
automatically if electrical power is removed from the primary electrical
system. Functions available via the emergency bus include all standby
instrument functions, PFD functions (nav/com #1 only), and the audio panel.
The emergency battery is sized to provide a minimum duration of 30 minutes
of electrical power to the emergency bus equipment.
ISSUED: December 22, 2017
DESCRIPTION & OPERATION
REPORT: VB-2749
SECTION 7
7-49