REFLECTOR: Bus, Battery & Electrical Instrumentation

Mon Jun 20 17:54:11 CDT 2005

 Oh boy,
I am going to try and wade deeper into this one.  Larry, if your
in-flite load, as measured by the loadmeter/ammeter is for instance 30
amps and you have a 35 amp alternator which we will assume is putting
out rated power thru a good voltage regulator set at 13.8 VDC you will
only have 5 amps at best going to your batt.  As long as Batt volts are
12 and alt volts are 13.8 you will be charging your battery.  As I
understand it, it cannot be otherwise.  Pressure is pressure.  

Once the batt is recharged, say after 30 to 45 mins of flite, your
ammeter reads your total system draw, because by that time the batt will
be charged and accept no more of the spare amps  Nor, in this state,
will the alternator indicate or provide more electron flow (amps) than
the sum of what units demand.  I think of alternators/voltage regulators
as supplying volts as a function of amps demanded.  This in no way
negates your caution about what we all have, and that is no affective to
predict rate of battery recharge after start or batt endurance in
emergency .  Help me out here.  What am I missing?  

Terry

-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Laurence Coen
Sent: Monday, June 20, 2005 11:33 AM
To: Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: Bus, Battery & Electrical Instrumentation

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 <mailto:KeithHallsten at quiknet.com>  
To: 'Velocity Aircraft Owners and Builders  <mailto:reflector at tvbf.org>
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

  _____  

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 <mailto:KeithHallsten at quiknet.com>  

To: 'Velocity  <mailto:reflector at tvbf.org> 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 

  _____  

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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