On Fri, Jun 24, 2011 at 05:29:23PM -0000, Eric wrote:
> Ben,
>
> If you are running six 12V batteries in series then each battery is "seeing" the full amperage load. So the cabling to each battery needs to be full size. Likewise, if you use twelve 6V batteries, each battery still gets hit with the full amp load.
What I was talking about - specifically for the purpose of decreasing
the loss down the chain - is splitting the batteries (however many I end
up with) into three sets, with one set being twice the size of the
others (this splits the weight in half.) The batteries in all three sets
would be series-parallel connected to produce 36V - e.g., 3 x 12 in each
of the "small" sets and 2 parallel chains of 3 each in the large set -
and the large set would be mounted in the motor compartment while the
small ones would be mounted forward, where the diesel tanks live
currently. This would, as far as I recall, halve the current across the
cables, and thus decrease the losses. If I'm reasoning incorrectly,
please let me know (it's been a mort of years since I've done this kind
of calculation - been living in the digital world for a long time now.)
> So, using my boat as an example, I've got sixteen 3.2V cells connected in series. The cells are clamped in groups of 4 cells with very short "bar" connections between the cells (actually, the bar connections are tinned flat braid) so each group of 4 (about 50 lbs)can be handled like a 160Ah 12V battery. I've got two of these 4-cell blocks on each side of my cockpit. All of my connections between the "batteries" is 2/0 cable, about 10 inches long for the "batteries" next to each other and an eight foot cable running under the cockpit to connect to the other two "batteries". Then more 2/0 cable goes from the battery bank(s) to the controller which is mounted near the motor. I chose the cabling to support loads of more than 200A (my main fuse is 225A) even though my system does not draw more than 110A from the batteries at full power. The "extra" cable capacity helps keep my voltage drop to a minimum, though the drops using 2AWG cable would be acceptable to many people he!
> re.
There's also the simple fact that connections do age (oxidize) despite
the best we can do, and lose some current-carrying capacity. In my
(quite long) experience with marine electronics, 99% of all wiring
problems happen at the connections.
E.g.: I helped a fellow cruiser a while back who was running his engine
for about 6 hours a day (huge alternator, very nice, large, new battery
bank.) Turned out he never came up above 12.7 or so. I chased that all
the way back... he was dropping all the voltage across the battery clamp
on the negative post. New, clean post; new, clean clamp. Even somewhat
tight - took a good bit of effort to twist it off by hand. After I
tightened it up, though, his charge current shot up from nearly zero to
30 amps. A couple of hours later, he had a fully charged set of
batteries for the first time since the initial launch, and it only took
him about 80 minutes of running time to make up his daily usage from
that point forward.
> BTW, when you go to source cabling, the best place that I have found is www.genuinedealz.com Check them out and compare to West Marine. I learned about them from another group member here. I believe that marine cabling is a better alternative to non-tinned welding cable for boats, it should last longer in a marine environment. The ABYC specs tinned wire for all connections ....
No doubt about it in my mind. I've been looking at boat systems for well
over 20 years now, and tinned cabling, especially when finished with
high-quality fittings and silicone-filled shrink tubing, shows very
little sign of deterioration even in the worst conditions.
--
Ben Okopnik
-=-=-=-=-=-
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