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peterc(at)pipcom.com
Guest

Posted: Fri Mar 02, 2007 7:32 am Post subject: batteries

Bob would you mind commenting on this?

  

  First battery selection. I can understand that AH  capacity is a good measurement for people with "electric" airplanes that need  emergency capacity but suppose your prime concern is cranking ability  (amps/speed) as may be the case for those of us flying Rotax "magneto" engines.  What specs should we be using to compare batteries or is it even possible  considering the variation in how the numbers are derived between  manufacturers?  Don't I want something like a max cold cranking amps number  (which also seems to be calculated differently between  manufacturers)?

  

 Next, your Rotax diagram shows #4 wire. Would I get  any noticeable benefit going to #2 if my runs are less than 3 feet?

  

 Finally, since cranking is my priority I plan to  take a ground run directly from battery to starter then back to your style  firewall ground. Worthwhile?

  

 My battery is on the floor between the rudder  pedals. The starter solenoid has its battery post bolted right to the +  battery post; a manual Hella switch is mounted with a short plate right on  the neg battery post so there are only two cables + and -  to starter with  a third from starter to ground bolt on firewall.

  

 Like it?

 Thanks

 Peter



   [quote][b]

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N1DeltaWhiskey(at)comcast
Guest

Posted: Fri Mar 02, 2007 10:11 am Post subject: batteries

Peter, I have some  comments for you to consider.

  

 Battery and wire size selection:  Look for a  battery, regardless of size, that has low internal resistance (such as the  Odyssey brand) RG battery - that will likely have a bigger impact on starting  than anything else in your circuit.  What is the voltage drop you expect in  #4 vs. #2 wire for a 6' run (out and back) while flowing about 250 amps?   If you determine that, then you can answer your own question on wire  size.

  

 Ground routing: Makes sense.  The return wire  from the starter ground to the firewall then could be much smaller, sized to  handle the other loads that you might have and same as battery to fuse  block/breaker bus for the positive side.

  

 Solenoid mounting:  I think mounting anything  to the battery posts other than a connector wire is asking for trouble.   Vibration will tend to cause the solenoid weight to eventually work harden and  break the battery post.  Would recommend a much different approach  here.

  

 Doug Windhorn

 [quote]   ---

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peterc(at)pipcom.com
Guest

Posted: Fri Mar 02, 2007 11:36 am Post subject: batteries

Doug, thanks for your comments.

  

 OK, so I did the calculation and the difference in  voltage drop is .34 volts between using 2 vs 4. I'd guess that this isn't  significant but is it in cold temps?

  

 With regard to bolting to the battery lugs, I've  also modified the battery case to provide additional support to the switch and  solonoid.

 Peter

   [quote][b]

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nuckollsr(at)cox.net
Guest

Posted: Fri Mar 02, 2007 2:49 pm Post subject: batteries

At 10:30 AM 3/2/2007 -0500, you wrote:



 	  Quote: 	
		  Bob would you mind commenting on this?



First battery selection. I can understand that AH capacity is a good 

measurement for people with "electric" airplanes that need emergency 

capacity but suppose your prime concern is cranking ability (amps/speed) 

as may be the case for those of us flying Rotax "magneto" engines. What 

specs should we be using to compare batteries or is it even possible 

considering the variation in how the numbers are derived between 

manufacturers?  Don't I want something like a max cold cranking amps 

number (which also seems to be calculated differently between manufacturers)?



Next, your Rotax diagram shows #4 wire. Would I get any noticeable benefit 

going to #2 if my runs are less than 3 feet?


	


   No



 	  Quote: 	
		  

Finally, since cranking is my priority I plan to take a ground run 

directly from battery to starter then back to your style firewall ground. 

Worthwhile?


	


   Probably not.

 	  Quote: 	
		  

My battery is on the floor between the rudder pedals. The starter solenoid 

has its battery post bolted right to the + battery post; a manual Hella 

switch is mounted with a short plate right on the neg battery post so 

there are only two cables + and -  to starter with a third from starter to 

ground bolt on firewall.


	


    Please don't mount hardware directly to any battery post.

    4AWG SOFT jumpers fabricated from welding cable are recommended.

    The only two in-flight failures of batteries I've encountered

    were broken off connection posts precipitated by too much

    mass mounted too rigidly to the posts.



    For the distances you're talking about, 4AWG will be fine.



    Bob . . .

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

        ( IF one aspires to be "world class",  )

        ( what ever you do must be exercised   )

        ( EVERY day . . .                      )

        (                  R. L. Nuckolls III  )

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

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europa flugzeug fabrik

Joined: 13 Feb 2006
Posts: 65
Location: North Coast, USA

Posted: Fri Mar 02, 2007 5:28 pm Post subject: Re: batteries

 	  N1DeltaWhiskey(at)comcast wrote: 	
		  What is the voltage drop you expect in  #4 vs. #2 wire for a 6' run (out and back) while flowing about 250 amps? 	
This is apparently about a Rotax 91x engine, though?  The starter relay supplied with my kitplane for a 912/914 is 70A.  More consistent with the kW spec of the starter motor.  Maybe 50A average?



Fred F.

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nuckollsr(at)cox.net
Guest

Posted: Sat Mar 03, 2007 12:33 pm Post subject: batteries

At 05:28 PM 3/2/2007 -0800, you wrote:



 	  Quote: 	
		  

<n3eu(at)oh.rr.com>

N1DeltaWhiskey(at)comcast wrote:

 > What is the voltage drop you expect in  #4 vs. #2 wire for a 6' run 

 (out and back) while flowing about 250 amps?

This is apparently about a Rotax 91x engine, though?  The starter relay 

supplied with my kitplane for a 912/914 is 70A.  More consistent with the 

kW spec of the starter motor.  Maybe 50A average?


	


   "Ratings" on switching contacts are to be considered with

   a grain of salt . . . or sand.  The probability of satisfactory

   service life is only loosely related to "ratings".



   The ratings are based on numbers of times a contact can

   SWITCH certain kinds of loads . . . interestingly enough,

   starter motors on engines are almost NEVER included in

   the suite of standard loads for the establishment of

   ratings.



   For example, the el-cheeso battery contactors used on

   tens of thousands of GA aircraft are RATED at 70A, yet

   we've routinely cranked engines through these devices

   with draws of 200-300 amps for decades. Given the enduring

   presence of these critters in the marketplace, one must

   deduce that users are seeing a satisfactory service life.

   I.e., good value for the investment.



   The thing that's hard on devices that switch starters is

   not the average running current. The switch-killer

   is the locked-rotor or inrush current that flows in the

   milliseconds that the motor is accelerating up from

   zero velocity while at the same time, contacts on the

   switch are making MULTIPLE closures. See:



http://www.aeroelectric.com/Pictures/Curves/switch_transition.jpg



   These are switches and not contactors but the effects

   are similar for all such devices. In a millisecond to

   a millisecond and a half, these switches close, open

   and re-close many times.



   This is why we SWITCH starter loads with STARTER CONTACTORS.

   These have exceedingly thin and light contacts compared

   to continuous duty contactors . . . these contacts are

   driven together with much greater force so as to minimize

   the bouncing and contact resistance. See:



http://www.aeroelectric.com/Pictures/Contactors/S702-1b.jpg



   Another consideration for service life is total operations

   per flight hour extrapolated out to total operations per

   year. In the average GA aircraft, any switch or contactor

   is hard pressed to see more than a few hundred operations

   per year . . . a device that is RATED in the tens of thousands

   of operations at RATED loads that may not be representative

   of what the switch really sees in the airplane.



   It's easy to see that many components might deliver satisfactory

   service life in terms of chronological age in the airplane

   in spite of being "overloaded" with respect to catalog ratings.



   Keep in mind too that connections, and contacts for switching

   devices ADD TO THE TOTAL loop resistance and may DOUBLE the

   total loop resistance when compared wire resistance alone.



   These are some of the reasons why it's not terribly useful

   to agonize over the resistance of 2AWG vs 4AWG or the ratings

   of any particular switch or contactor over another. The

   ultimate test of your cranking system's ability to get the

   engine moving is only measurable with some pretty sophisticated

   test equipment. I have a DAS that will gather 2000 samples per

   second so that instantaneous energy losses in the various

   components and wire lengths can be quantified and decisions

   made for optimization.



   I'm not trying to discourage anyone from a quest to understand

   the finer details of their ship's electrical system . . . but

   know too that decisions made on rudimentary, single item

   calculations and/or catalog data for components may not

   stand on solid physics.



   Bob . . .

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