I have my personal doubts, Bob, if paralleling at the 6 volt level provides
any advantage, and I can think of cases where it could be a disadvantage.
Let's face it.  ALL 12 volt batteries are made up of pairs of 6 volt
batteries in series, ( or 6, 2 volt in series) and they are frequently
paralleled without even access to the 6 volt point.
If the 6 volt batteries were all perfect, NO current would flow anywhere in
the 6 volt level circuit.
So current is only going to flow in that circuit when one battery starts to
get weak and others have to come to its support.  Is that good?  Do you want
to deplete all of one 6 volt bank trying to boost up one bad member, or
would it be better without the extra circuit and let the one battery die
peacefully on its own.
Let's take a worst case scenario.
Assume one of the batteries in the lower 6 volt bank develops a bad cell.
All the others in that bank are now in parallel and capable of dumping large
amounts of power directly into the overheating bad cell with only 2 other
cells in series with it to absorb power.  Without the extra circuit, there
will be 5 cells in series with the bad one which can all gass and dissipate
excess energy, perhaps preventing a fire or melt down.
It DOES NOT provide a lower source impedance.  It may limit some batteries
from reaching their natural gassing voltage when equalizing.  It
significantly increases the hardware involved and in my opinion gives you no
advantages.
Colin Foster,
tech@yandina.com

Message: 33
Date: Wed, 22 Jan 2003 18:13:08 -0500
From: Bob McLeran rmcleran@ix.netcom.com
Subject: TWL: Getting 12 volts by connecting 6 volt batteries in
series/parallel
To: trawler-world-list@lists.samurai.com

I'm looking into replacing my house bank (2 4D batteries) with whatever
number of golf cart batteries I can fit into the same space (I think the
box will hold six but I don't have the exact measurements yet). In doing
some research (read every TWL email posted in 2002 regarding golf cart
batteries and looked around the internet at specific products) I came
across this . . .

Quote:

If you wish to go even further to insure equal voltages, you can
"CrossConnect" the bank, that is - connect together all the points of equal
voltage with jumpers. Imagine a drawing of a battery bank of three pair of
6 volt batteries connected series/parallel to form a 12 volt house battery
with the positive end of the bank at the top of the page and the negative
end of the bank at the bottom of the page and the 6 volt batteries in two
horizontal rows with all positive terminals up and all negative terminals
down. In addition to all the normal connection wires, make up 12 more
jumpers. Use them to connect all the positive terminals of all the upper
row of batteries together, then all the negative terminals of those
batteries. Then all the positive terminals of the lower row of batteries
together and finally all the negative terminals of the lower row of
batteries. This way you can insure that equal voltage points are as equal
as you can get them.

Endquote.

This quotation is from
http://www.geocities.com/bill_dietrich/BoatElectrical.html, from a post in
1999.

Does it make sense? Does it work better than not making all the "jumper"
connections?
<><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
Bob McLeran      Email: mailto:rmcleran@ix.netcom.com
M/V Sanderling  At new home in Manatee Cove Marina, Patrick Air Force Base
Hampton 35 Trawler      Melbourne, Florida

Bob quoted from an electrical web site:

If you wish to go even further to insure equal voltages, you can
"Cross Connect" the bank, that is - connect together all the points of
equal voltage with jumpers.

While reading the installation manual for my current inverter
installation project I came across a mention of 'cross connecting'
batteries.  What they meant was on a bank of say 3 8D batteries wired in
parallel to give a 12 volt bank of 600 amp hours use the positive
terminal of the first battery and the negative terminal of the last
battery.  This ensures that all three batteries get more or less the
same use.

Gregg McKay
Tolly 37
Victoria, BC