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.
However if you put together twelve 12V batteries in two banks of six batteries that are then connected the two banks in parallel, each bank sees half the total amp load. To make this work, each bank needs to be at full voltage, 72V in your case. But you have to be aware that if you take one bank off line, the remaining bank might have to support all of the load. You can manage that by reducing throttle, but it is something to think about.
Using twelve batteries, if you wire each pair of batteries in parallel then wire the pairs in series, then each pair is essentially a larger 12V battery. All of the series connections would then need to be able to handle the full load.
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 here.
If I was going to add another 16 cells to double my capacity and range, then I would probably "re-wire" each 4-cell block into a 6V battery and continue to link them in series. My personal opinion is that a single large bank is better than 2 small ones, but that is a "coke or pepsi" kind of choice. Both strategies work fine.
I hope that this helps clarify your requirements.
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 ....
Eric
--- In electricboats@yahoogroups.com, Ben Okopnik <ben@...> wrote:
>
> Hi, Tom -
>
> On Fri, Jun 24, 2011 at 03:04:06PM -0000, Tom wrote:
> > Hi Ben,
> > If you're still talking about using 12 L-16s in series, then you'll need to size the 72' cables much larger than 2 AWG to keep the voltage drop at 180 amps within reason.
> > No room in a 45 foot boat to run bigger cable? Sounds like you 'll need a bigger boat!
>
> I was obviously not very clear in what I wrote; sorry about that. The
> batteries would obviously be connected in series/parallel; if I have a
> third of them split between port and starboard and connected in parallel
> to the other two-thirds, the current being drawn from that remote bank
> is going to be about 1/3 of the total - i.e., about 60A of the total
> 180. In addition, the total length of the cable involved is 72'; that's
> four pieces (two negative and two positive runs to the split "remote"
> bank) of 18' each, so #2 should work very well; it's about 70% bigger
> than the nominal size for that current and length.
>
> Oh, and I didn't say I don't have the room - I do indeed, lots of it;
> it's just that bending #2 cable around corners is not the easiest job in
> the world. I might just go with welding cable, which is nice and
> flexible.
>
>
> --
> Ben Okopnik
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