Chris,

The back battery (behind the aft bulkhead) does have a trickle drain via the panel's clock.  This is why the voltage trickles down over time (days, weeks) relative to the isolated battery on the firewall.  My observation is independent of this variable.

But when both batteries are fully charged, either with dual alternator outputs or plug-in charger, the delta is 1.2V to 1.3V as seen at nearly the same point behind the panel.

I had originally planned for copper cables, and measured the drop based on 6ft, but found the weight to high.  Aluminum is used, and the drop is significantly greater than copper (as we know), and the result of roughly 6ft of 2 gauge is 1.2-1.3V demonstrated.

If this were the cranking battery, it has an impact on amps available tot he starter, which is another 3-4 ft further forward.  When I have experimented with cranking off the aft battery, it definitely is weaker, a problem if cold start.

Jeff





On Oct 24, 2013, at 6:00 AM, Lancair Mailing List wrote:

From: Chris Zavatson <chris_zavatson@yahoo.com>
Subject: Re: [LML] Re: Odyssey PC680 Batteries on Firewall -- Cool Enough?
Date: October 23, 2013 7:45:51 AM EDT
To: lml@lancaironline.net


Jeff,
Something sounds amiss with your batteries.  Longer cables will indeed incur a voltage drop under load, but I assume the voltages you listed were open circuit or lightly loaded.  You mentioned the difference is larger after sitting longer.  Is there perhaps a low current drain on one battery?
A 6 ft section of 4 gauge wire will see a voltage drop of 1.3 V when carrying about 900 A.
Any open circuit voltage south of 12.5 V is worth investigating.
 
Chris Zavatson
N91CZ
360std
http://www.n91cz.net/