Hi, Eric -
(if you don't mind, I'm going to combine your emails into one.)
On Fri, Jun 24, 2011 at 09:12:18AM -0000, Eric wrote:
> So it's been recommended that you should have a 400Ah x 72V pack? That's 28.8kWh. Very few people think in usable capacity when talking about batteries, so I would guess that they're talking about rated capacity.
Right - I just found that out last night after calling them. Just based
on the early projections, the 13-14k for the batteries plus the ~9k for
the motor plus another (say) 5-7k for all the little extra bits that'll
be needed, i.e., ~$30k, is well outside my range. FLAs at $3K bring that
closer to $20K, which is a lot higher than I originally thought but
possible (especially if I manage to recover anything from selling the
diesel.)
> That is one of the advantages to an electric conversion. You can always upgrade later. You might find that the FLA "test" pack is sufficient for your purposes and the upgrade can be delayed indefinately. You could then put the extra $10k towards new sails, winches, solar panels, etc.
Actually, almost everything else on the boat is in excellent shape - and
the biggest problem with solar panels isn't the cost, it's the space for
them. If I can manage to mount 5 of the 135W Kyoceras over the cockpit
(I've been away from the boat all week, and can't measure it exactly),
that'll be a big win - and I'm also trying to figure out how to possibly
attach four more to the boom while I'm not sailing, and store them along
the lifelines when I am (quick-detach high-amp terminals and big
splashproof bags of some kind, I suppose...) In any case, even if my
dreams of being able to fit all of those come true, that's less than
$3.5K.
> You're definately on the right track. You should see if you can put at
> least some of the batteries where the fuel tanks used to be. That puts
> the weight right back wher it started. Many people split their packs,
> my 48V pack is built with 24V on either side of my cockpit, it's just a
> little more cable and if the cabling is sized right, virtually no
> performance drop.
Yep, I realized that as soon as I sent off the previous email. The
problem with that is two-fold: first, the tanks are rather high up and I
was trying to keep the weight down low, and second... I realized that I
was reacting negatively to the idea because I had subconsciosly decided
(interesting, what it takes to fish out those subconscious decisions!)
to hold off the tank removal until everything else was installed, since
it looks like a really big job. I guess I'll have to change that
decision; it just makes sense to move at least 600 lbs of batteries
forward. The only thing that bothers me is the amount of cabling that
I'd have to put in between the two banks: it's about 6' laterally from
the centerline of the boat, where the engine is located, to the center
of the tank; there's also at least a 4' difference in height, and about
a 3' difference in fore-and-aft position. All that, plus snaking the
cables through the bilge, around corners, etc. sounds like a minimum 13'
trip (probably more like 17' or 18' by the time all the bends and
connections are done.) That's 72' of series wiring; if I've got
(roughly) a third of the batteries, that's going to have to carry a
third of the current - call it 180A (13kW/72) divided by 3, plus a 50%
safety factor (and a small bump because it's a bit more than a third,
anyway) - then I'm looking at #2 wire. Yikes, python wrestling. :)
(By the way, I love the wire calculator that I just found at
http://circuitcalculator.com/wordpress/ - useful gadget.)
> I'm going to take a few liberties with the data provided and say that your predicted performance might be:
>
> 4kts = 35A or 2600W
> 5kts = 65A or 4800W
> 6kts = 120A or 9000W
You have my thanks for taking that liberty. :)
> To my eye, the progression isn't quite right, I've seen with 30' 5 ton boats that each knot doubles the
> electric demand and a little more as you get toward the top end. This probably has to do with the propeller
> losing efficiency as the speed goes up. But these numbers are probably not too far off and close enough for
> this exercise.
Excellent - again, this kind of rules of thumb are what I'm looking for
right now (I want to have a "gut feel" for sanity checking when I get
down to doing actual number crunching.)
> I crunched some numbers for your 395Ah batteries using the predicted performance numbers listed above. I
> assume a depth of discharge of 60% and a Peukert's Exponent of 1.25. Here's the results:
>
> 4tks = 5.9hrs = 23.5nm
> 5kts = 2.7hrs = 13.5nm
> 6tks = 75min = 7.5nm
>
> Honestly, this is not too bad. My target for my boat was 20 nautical miles at 4knots, and my boat exceeded my
> target by being able to do it at 4.2kts. Your boat looks like it is right there.
Terrific - this gives me exactly what I need, beginning to end (a "bank
size to speed/distance" equivalence.) If I make it out to California any
time in the near future, please consider me indebted for at least a few
good quality beers, or whatever beverage of your choice!
> You may also want to reach out to a couple of other vendors to see what your drive choices are in the 10kW
> range. I think that it pays to shop around.
Yes indeed. I've been looking at the ASMO motors, which look very
impressive; I'm also going to research a few others. There seems to be
quite a range out there.
--
Ben Okopnik
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
"I know by now that the glitter of romance as seen from afar often turns
out to be pretty shoddy at close quarters, and what appears to be a
romantic life is invariably an uncomfortable one; but I know, too, that
the values in such living are usually sound. They have to be, or you
don't survive. And occasionally you are rewarded by an insight into
living so splendid, so wholly magnificent, you can be satisfied with
nothing less ever after, so that you go on hoping and searching for
another glimpse for the rest of your life."
-- Ann Davison (first woman to cross the Atlantic single-handed),
"My Ship is So Small"
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