REFLECTOR: Bus, Battery & Electrical Instrumentation

Laurence Coen lwcoen at hotmail.com
Mon Jun 20 10:32:36 CDT 2005


Keith,

Since Kurt was kind enough to give you voltage numbers I won't.  I actually don't know what the voltage was.  I suspected the alternator because of excessive whine in the headset when I turned on the landing light.  The Cherokee had both an ammeter and a voltmeter, both of which were quite useless IMHO.  The ammeter showed how much current the alternator was supplying and gave no clue to what the load was.  The voltmeter was so inaccurate as to be a waste of panel space.  My caution to you is to understand that if your voltmeter is indicating 13.6 volts your battery may not recover the power you used to start the engine.  Each time you fly your battery gets a little more discharged.  This means that you could experience a total alternator failure at a time when your battery is already toast.  Forewarned is forearmed.

By the way, my ammeter is in the battery lead and shows if the battery is being charged or discharged.  I think your approach is valid so long as you correctly interpret what the voltmeter is telling you.  Since you've gone to the trouble of educating yourself on the subject, you'll be fine.

Larry Coen
N136LC
  ----- Original Message ----- 
  From: Keith Hallsten 
  To: 'Velocity Aircraft Owners and Builders list' 
  Sent: Sunday, June 19, 2005 3:02 PM
  Subject: RE: REFLECTOR: Bus, Battery & Electrical Instrumentation


  Larry,

   

  What was the voltage on the bus when you had the bad alternator diode?  I would expect that it would be lower than usual, and a voltmeter would show that an issue was present.  If the voltage was, say, 14 volts the battery would have charged.  

   

  My point is that a voltmeter will tell you everything you need to know as a pilot.  An ammeter might be handy when troubleshooting, which should be done on the ground.

   

  Keith

   

   


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  From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On Behalf Of Laurence Coen
  Sent: Sunday, June 19, 2005 7:44 AM
  To: Velocity Aircraft Owners and Builders list
  Subject: Re: REFLECTOR: Bus, Battery & Electrical Instrumentation

   

  Keith,

   

  I experienced such a failure mode on my old Cherokee once.  Alternators generally have three separate windings, each with there own diodes.  Metal fatigue broke a lead off a single diode.  The alternator made power, but not quite enough.  As a result it would come real close to floating the load but not enough to recharge the battery.  It took about a month to kill the battery.  The second time the battery died I pulled the alternator and found the problem.

   

  Larry Coen

  N136LC

    ----- Original Message ----- 

    From: Keith Hallsten 

    To: 'Velocity Aircraft Owners and Builders list' 

    Sent: Saturday, June 18, 2005 1:57 PM

    Subject: RE: REFLECTOR: Bus, Battery & Electrical Instrumentation

     

    Al,

    If your alternator stops producing, the voltage on the bus powered by the ammeter will instantly drop to whatever the battery(s) tied to that bus will support.  That cannot be more than 12 volts or so.  Therefore, a low-volts alarm set at 12.5 volts will instantly warn of alternator failure.  That's why I don't think an ammeter is necessary.  I am not aware of any failure mode for an alternator that will result in low amperage without corresponding low voltage.

    Keith 

     


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    From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On Behalf Of Al Gietzen
    Sent: Friday, June 17, 2005 11:35 PM
    To: 'Velocity Aircraft Owners and Builders list'
    Subject: RE: REFLECTOR: Bus, Battery & Back Order

     

      What voltage do you all trigger the low voltage

    warning?  I am guessing that since alternator produce 14V and the

    battery is at 12, simplistically it should be set to 13V or so, taking

    the middle.  Is there a standard value that is the "industry standard"?

     

    My engine monitor will read bus voltage and the current from the alternator.  I can set the alarm limit for low current output (probably something lower than what it takes to just run the engine) to give early detection of low alternator output.  Possibly one reason for an ammeter. 

     

    Al


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