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Battery Cranking Power

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dreel(at)cox.net
Guest

Posted: Sat Mar 25, 2006 11:41 am Post subject: Battery Cranking Power

I took some measurements today because my Panasonic LCRD1217P battery cranking power seems marginal on cold, 45 degree, days with my O360A1A and SkyTek permanent magnet starter.  I saw 5.5 to 6.5v from the starter side of the starter relay to a stud on the vaccum pad of the engine accessory case.  I also saw 5.5 to 6.5 v from the battery + terminal to airframe ground.  The battery was fully charged and measured 12.8v and was new.  Assuming the battery - terminal to airframe ground connection is good, it's new, and that about 200a is being drawn by the starter, this gives a 25 milliohm internal battery resistance to explain the 6 volt drop on starting.  The specs claim the battery has 12 milliohm resistance at 77 degrees.  Would the 30 degree temperature drop be enough to reasonably explain this big increase in internal resistance?  Could the starter be taking 400a?  When the starter would stall, the battery voltage dropped to about 4.5v.



Dave Reel - RV8A

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brian

Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA

Posted: Sat Mar 25, 2006 2:29 pm Post subject: Battery Cranking Power

DAVID REEL wrote:

 	  Quote: 	
		   

 

 I took some measurements today because my Panasonic LCRD1217P battery cranking power seems marginal


	


If you want to see if your battery is holding up under cranking current, 

you need to measure the voltage on the battery terminals. If you are 

looking for losses, you need to measure the voltage on the starter 

itself. You need BOTH measurements to figure voltage drop.



There are inexpensive meters that you just hold against the hot lead 

from battery to starter that will show starter current draw. You should 

be able to get one at a well-stocked auto parts store.



With those three measurements in hand you can get a really good feel for 

now your battery and starting system are performing.



-- 

Brian Lloyd                         361 Catterline Way

brian-yak at lloyd dot com          Folsom, CA 95630

+1.916.367.2131 (voice)             +1.270.912.0788 (fax)



I fly because it releases my mind from the tyranny of petty things . . .

- Antoine de Saint-Exupery

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_________________
Brian Lloyd
brian-yak at lloyd dot com
+1.916.367.2131 (voice) +1.270.912.0788 (fax)

I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery

markn(at)fuse.net
Guest

Posted: Sun Mar 26, 2006 6:57 am Post subject: Battery Cranking Power

Brian:



You have hit the problem on the head. A serious drawback to the SkyTec

starters is their very high current draw on each compression stroke. I

believe it is entirely possible that your starter is indeed drawing 400 amps

twice per engine revolution. It was for this reason, and using dual

Lightspeed III ignition systems, that I swapped my SkyTec over to a field

wound starter. They weigh more, but their cranking current is around half of

the series wound models. High current draw like this will pull the battery

voltage way down as you measured



As a footnote, I discussed this phenomenon with a representative from SkyTec

at Oshkosh in 2004. He could not have been more evasive and curt.  I believe

his response to my query was something like "Cranking current is not our

concern, that is the airframe manufacturer's problem"



Mark Neubauer

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nuckollsr(at)cox.net
Guest

Posted: Mon Mar 27, 2006 11:54 am Post subject: Battery Cranking Power

At 09:51 AM 3/26/2006 -0500, you wrote:



 	  Quote: 	
		  



Brian:



You have hit the problem on the head. A serious drawback to the SkyTec

starters is their very high current draw on each compression stroke. I

believe it is entirely possible that your starter is indeed drawing 400 amps

twice per engine revolution. It was for this reason, and using dual

Lightspeed III ignition systems, that I swapped my SkyTec over to a field

wound starter. They weigh more, but their cranking current is around half of

the series wound models. High current draw like this will pull the battery

voltage way down as you measured



As a footnote, I discussed this phenomenon with a representative from SkyTec

at Oshkosh in 2004. He could not have been more evasive and curt.  I believe

his response to my query was something like "Cranking current is not our

concern, that is the airframe manufacturer's problem"



Mark Neubauer


	


     This is precisely why B&C stayed with wound-field

     starters after considerable evaluation of the simpler

     technology many years ago. The PM starters may indeed put

     out more torque at a given terminal voltage but at higher

     current draws too (you don't get something for nothing).

     You can take advantage of this feature only if your battery's

     internal resistance combined with the system's series resitance

     is sufficiently low to deliver the power demand.



     Yes, it is the manufacturer's problem to deal with the

     hardware they select for integration into their products . . .

     unfortunately, few folks in the airframe systems business

     choose to be very well informed.



     Bob . . .

      < What is so wonderful about scientific truth...is that >

      < the authority which determines whether there can be   >

      < debate or not does not reside in some fraternity of   >

      < scientists; nor is it divine. The authority rests     >

      < with experiment.                                      >

      <                            --Lawrence M. Krauss       >

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william.p.dube(at)noaa.go
Guest

Posted: Mon Mar 27, 2006 12:42 pm Post subject: Battery Cranking Power

The PM starters will try to run at a specific RPM. They will draw the 

current needed to do that, or they will sag the battery voltage trying 

to do it. If the engine turns hard, they will draw a very large current. 

If the engine turns easy, they will draw much less current. (The PM 

starter RPM will be proportional to the supply voltage.)



The behavior of a series-wound starter is more complicated, but they 

tend to put out a constant HP (rather than a constant speed.) They run 

faster if the engine turns easier, and turn slower if the engine turns 

harder. The current draw from the battery tends to be much more constant 

than with a PM starter.



Bill Dube'

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