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Engine start on Z14 with different size batteries

 
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bobbarrow



Joined: 04 Sep 2011
Posts: 9

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

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 Smile Smile Smile


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wtmills



Joined: 05 Feb 2007
Posts: 32

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

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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PostPosted: Thu Jun 28, 2012 12:58 pm    Post subject: Engine start on Z14 with different size batteries Reply with quote

Thanks.

Dick
[quote] ---


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