Welcome Guest. Sign in or Signup

6 Answers

Alternator or battery power bus?

Asked by: 7102 views Aircraft Systems

Someone made the comment that in a C172 SP with a G1000 the alternator charges the main battery, and then the main battery supplies the power to the bus in normal flight. Rather than the alternator powering the bus directly and charging the battery as needed. Deosn't seem right. I looked through the AFM, but don't have enough electrical background to sort this out. It does say that whenever the ALT part of the Master Switch is on, the BAT part should be on too. Does anyone know for sure? Any place in the AFM that makes this clear?

Ace Any FAA Written Test!
Actual FAA Questions / Free Lifetime Updates
The best explanations in the business
Fast, efficient study.
Pass Your Checkride With Confidence!
FAA Practical Test prep that reflects actual checkrides.
Any checkride: Airplane, Helicopter, Glider, etc.
Written and maintained by actual pilot examiners and master CFIs.
The World's Most Trusted eLogbook
Be Organized, Current, Professional, and Safe.
Highly customizable - for student pilots through pros.
Free Transition Service for users of other eLogs.
Our sincere thanks to pilots such as yourself who support AskACFI while helping themselves by using the awesome PC, Mac, iPhone/iPad, and Android aviation apps of our sponsors.

6 Answers

  1. John D. Collins on Feb 24, 2011

    When the alternator and battery are both in the circuit, the alternator provides all the electrical power for the aircraft plus and any current to charge the battery.  The battery can buffer the load for short duration demand that exceeds the alternator capability, but if this continues for an extended time, the battery will discharge.  Often in a structured buss arrangement, different alternator systems can power only a subset of the electrical requirements. If an essential buss is used, it is usually powered by its own battery and either alternator, whereas the non essential equipment depends on a primary alternator and if it fails, power won’t be shared from the essential buss.  The theory is to keep the important stuff running regardless.

    +2 Votes Thumb up 4 Votes Thumb down 2 Votes

  2. Wesley Beard on Feb 24, 2011

    I have read through the 172S G1000 POH.  It doesn’t specifically say what you are asking but the schematic on page 7-45 clearly shows the alternator powering the circuit breakers that go to the crossfeed bus.  The power is then routed to Electrical Bus #1 and #2.  These buses in turn power the essential bus.  Unforunately, the schematic does not clearly show the alternator charging the battery but am certain that this is how it happens.
    The POH does clearly indicate that it is possible to have the battery switch on without the alternator switch but not the other way around.  The alternator switch cannot be selected on without the battery switch also switching on (7-48).
    My POH is for 172S serial numbers 172S9810 and on.  (Part Number 172SPHAUS-03)

    0 Votes Thumb up 0 Votes Thumb down 0 Votes

  3. Jim Foley on Feb 24, 2011

    I had a similar thought, and contacted Cessna.  Lucky for me, an electrical engineer from Cessna happened to be in my area, and I was put in contact with him.  Your statement is correct.  Think of it as a basic serise circut.  the electrical flow in all C-172 models is Alternator —> Battery —> Main Bus —> ect…  I If you think about it in terms of individual electrons, every single electron comming from the alternator travels through the battery, and then is distributed.  The engineer said it has been proposed to wire in a parallel, where the battery would be charged as needed, but the working electric power would go directly from the Alternator to the Main Bus, then distributed.  Wiring in a serise does decrease the life and efectiveness of the battery, but in small aircraft, it is so small, that the more compicated parallel would be unnecessary.  And no, unfortunately there is nothing in the AFM/POH that depicts this.  I guess I was just lucky to speak to the engineer in person.

    -2 Votes Thumb up 1 Votes Thumb down 3 Votes

  4. John D. Collins on Mar 02, 2011



    I am an electrical engineer and I have to disagree with the explanation of the Cessna engineer.  I have not seen a battery-alternator arrangement that is wired in series.  They are wired in parallel.  Usually the battery is connected to the buss thru a battery relay and an amp meter. The alternator is connected directly to the buss thru a circuit breaker. In most cases the amp meter measures charge or discharge of the battery and when it is being charged, current is flowing from the alternator to the battery and the buss. When the battery is charged, the ammeter reads zero and all the current flows to the buss from the alternator.  If the current load demands temporarily exceed the alternator capability, the battery will make up the difference.


    There are other arrangements than the one I described for the ammeter where it is inserted between the alternator and the buss and acts as a load meter.  This is often used when more than one alternator is used and you can see the load being carried by each alternator. If you can scan a copy of the Cessna circuit diagram, I would be happy to analyze it.  I have the diagrams for the Cessna’s thru 1986, but don’t have a copy of the ones used to power the G1000 equipped aircraft.

    +2 Votes Thumb up 2 Votes Thumb down 0 Votes

  5. Jim Foley on Mar 02, 2011

    My electronics skills/knowledge are pretty rusty.  I just posted what he told me in it’s most simplistic form.  He did tell me that “it is ESSENTIALLY a series.”  I know it wasn’t quite as simple as the basic flow diagram I wrote, but just simplified it for others’ understanding. 
    I really do wish Cessna made those diagrams available to us, then I might be able to understand it better.  Unfortunately, they are not, so I have to rely on word-of-mouth.  There is a possibility I misinterpreted something wrong, but as far as I can recall, that is how he explained it.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes

  6. 0 Votes Thumb up 0 Votes Thumb down 0 Votes

Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.