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Voltage drop under load

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bbradburry(at)bellsouth.n
Guest

Posted: Thu Jan 24, 2008 2:56 pm Post subject: Voltage drop under load

I noticed today that my voltage would drop from about 14.5 volts down to

13.3 volts when I turned all my lights on.  That is two 100W landing, two

100W taxi, position, and strobe lights.

I made sure that I had plenty of rpm to be certain that the alternator was

putting out its max.  The alternator is rated at 55A.



Is it possible that I am pulling more amps than the alternator can handle?

Is the drop of voltage an indicator of this?



What should be the size of the B lead wire for a 55A alternator?



Thanks,

Bill B

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klehman(at)albedo.net
Guest

Posted: Thu Jan 24, 2008 3:56 pm Post subject: Voltage drop under load

Yes voltage starts to sag as the alternator output starts to lag demand.

Quick math suggests that your alternator is working properly. (100 watts 

divided by 13.3 volts is about 7.5 amps per 100 watt load) My alternator 

can handle all loads but the voltage will drop a bit if I pull the 

throttle to idle on approach. No big deal if the battery helps out for a 

few dozen seconds on final approach. In cruise my taxi lights wig wag 

which halves the current draw.

Ken



Bill Bradburry wrote:

 	  Quote: 	
		   

 

  I noticed today that my voltage would drop from about 14.5 volts down to

 13.3 volts when I turned all my lights on.  That is two 100W landing, two

 100W taxi, position, and strobe lights.

 I made sure that I had plenty of rpm to be certain that the alternator was

 putting out its max.  The alternator is rated at 55A.

 

 Is it possible that I am pulling more amps than the alternator can handle?

 Is the drop of voltage an indicator of this?

 

 What should be the size of the B lead wire for a 55A alternator?

 

 Thanks,

 Bill B

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nuckolls.bob(at)cox.net
Guest

Posted: Sun Jan 27, 2008 9:03 am Post subject: Voltage drop under load

At 06:52 PM 1/24/2008 -0500, you wrote:



 	  Quote: 	
		  



Yes voltage starts to sag as the alternator output starts to lag demand.

Quick math suggests that your alternator is working properly. (100 watts 

divided by 13.3 volts is about 7.5 amps per 100 watt load) My alternator 

can handle all loads but the voltage will drop a bit if I pull the 

throttle to idle on approach. No big deal if the battery helps out for a 

few dozen seconds on final approach. In cruise my taxi lights wig wag 

which halves the current draw.

Ken



Bill Bradburry wrote:

>

><bbradburry(at)bellsouth.net>

>  I noticed today that my voltage would drop from about 14.5 volts down to

>13.3 volts when I turned all my lights on.  That is two 100W landing, two

>100W taxi, position, and strobe lights.

>I made sure that I had plenty of rpm to be certain that the alternator was

>putting out its max.  The alternator is rated at 55A.

>Is it possible that I am pulling more amps than the alternator can handle?

>Is the drop of voltage an indicator of this?

>What should be the size of the B lead wire for a 55A alternator?

>Thanks,

>Bill B


	


    13.3 is TOO low.  You need to put a temporary lead wire

    on the alternator's b-lead terminal and bring it into

    the cockpit for attaching a voltmeter. You MAY find that

    the alternator voltage at the b-lead is holding up just

    fine and that your wiring is too small . . . i.e. accounts

    for the drops under load that you've observed.



    I recommend 4AWG minimum for all the "fat" wires in

    the airplane. There's so little of it that weight deltas

    are insignificant . . . but 4AWG or larger makes for

    very solid voltage regulation at all loads.



    Bob . . .

        ----------------------------------------)

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

        ----------------------------------------

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