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Engine start on Z14 with different size batteries

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bobbarrow

Joined: 04 Sep 2011
Posts: 9

Posted: Thu Jun 28, 2012 1:40 am Post subject: Engine start on Z14 with different size batteries

I have the Z14 architecture (dual independent buses with crossfeed contactor) on my RV7. But the architecture is not symmetrical as per Bob Knuckolls drawings.



In my set-up I have an Odyssey 680 battery (17 AH) plus a B&C 40 amp alternator on Bus 1. On Bus 2, I have an Odyssey 310 (7.5 AH) plus a B&C 20 amp alternator.



Yes, I know as a system it's overkill but let's not go there because it's already installed. 



In my architecture Bus 1 with the big battery is the sole starter cranking bus for normal operations. Accordingly all "starter current path" wires on Bus 1 are 2 AWG to reduce resistance during engine start.



By comparison I calculate that the largest wire on Bus 2 could be 8 AWG for normal operations on that bus including Xfeed operations to bus 2 (for non cranking operations).



However it occurs to me that if my Bus 1 battery (Odyssey 680) was struggling to crank the engine in a remore location I would be tempted to switch on the Xfeed and get whatever current boost I could from the smaller Odyssey 310 on bus 2.



My question is this:   What size wire do I need to incorporate in the "starter current" path of bus 2 to facilitate assisted engine cranking on the odd occasion from the smaller battery. Presumably bigger would always be better....but what's reasonable.



Regards Bob Barrow

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wtmills

Joined: 05 Feb 2007
Posts: 32

Posted: Thu Jun 28, 2012 11:35 am Post subject: Engine start on Z14 with different size batteries

Check out the two page pdf from Bob's web site:



Battery Replacement - a plan for throwing in the towel . . . a  capacity test fixture.  



Regards -

Bill



On Thu, Jun 28, 2012 at 11:14 AM, Dick Wildman <dick(at)minetfiber.com (dick(at)minetfiber.com)> wrote:

 [quote]--> AeroElectric-List message posted by: "Dick Wildman" <dick(at)minetfiber.com (dick(at)minetfiber.com)>

  

 Question;

 

 See below. How is the test performed to see if the battery has at least 70% capacity?

 

 Dick Wildman

 7S5

 

 

  	  Quote: 	
		     If you plan to frequent remote locations, then keeping a

   sharp eye on the condition of the larger battery is

   important. You need to replace it when it drops to 70%

   or less of capacity . . . this isn't an endurance issue,

   it's a cranking issue for ventures into higher risk airports.

 

   Being able to 'stack' the 7.5 a.h. battery on top of a soggy

   17 a.h. battery is poor insurance irrespective of wire

   sizes.

 

 

   Bob . . .

  

 

 

 

 	
 

 

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dick(at)minetfiber.com
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Posted: Thu Jun 28, 2012 12:58 pm Post subject: Engine start on Z14 with different size batteries

Thanks.

  

 Dick

 [quote]   ---

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