Hi, Eric -
On Thu, Jun 23, 2011 at 04:37:51PM -0000, Eric wrote:
> Hi Ben,
>
> Predicting battery performance under various loads is easily quantifiable. The trick will be to accurately predict the electric loads that your drive will generate for your boat, i.e. the watts to knots conversion. Typically I look for observed performance figures for similar boats that have already been converted to estimate these numbers.
I've been looking around for something like that, but haven't found
anything: there seems to be a big "hole" in the 35-50' range. Lots of
folks at 35' and under (and mostly quite happy about the conversion), a
few at 40' - and then, there's the "way upscale" set, with 70' and up
boats and unstated (but ungodly by implication) amounts of money spent
on very complex, best-of-everything systems.
My 45-footer is quite light for her length - 21000 lbs - which is more
like a typical 36-38' cruising boat. She's reputed to be somewhat wet
when sailed hard (I can attest to a bit of that), but the tradeoff is
supposed to be a better capability for sliding through the waves. In
addition, the Perkins 4-108 aboard her is well known for delivering way
under its rated power output: something like 34 or 36HP from what is
nominally a 50HP motor. Given all of that, it sounds like quite a
favorable setup for making way with a reasonable-sized electric motor.
> Surprisingly, most of the available drives appear to deliver similar efficiencies (+/- 10%), but choosing one that is too small will limit your top speed and available power. The boat will still be operable, but things may get dicey in adverse conditions.
That's exactly the limiting factor here - that's what I want to avoid
(and thus my concern about what it will do at WOT.)
> The choice of drive systems is all about having the desired speed capabilities and appropriate power for tight spots. Going too big generally costs more money without providing much more performance. One rule of thumb that has worked well for many conversions in this group is 1kW of continuous rating for each ton of displacement.
Excellent - that's exactly the kind of info I'm looking for. I had a
pretty good feeling that the 5kW units I'd been seeing were too small;
sounds like 10kW is the right range for me to look at. Thanks, Eric!
> On a completely seperate front, your decisions in the size and type of the battery pack will define your range. Once you've got some estimated "watts to knots" figures for your boat, I can quickly give you expected ranges for battery packs of different sizes and technologies.
Well, the estimate that I've got from EY is here:
http://okopnik.com/misc/ibl360.pdf
I suspect that it's way too optimistic (maybe a third to a quarter over
realistic figures), but it's a start. Here's a series taken off their 72v
chart in that PDF:
25A 3.4kt
50A 4.6kt
100A 5.8kt
150A 6.6kt
200A 7.3kt
250A 7.8kt
300A 8.3kt
350A 8.7kt
(One of the things that makes me a bit skeptical of those charts is that
my boat, fast as it is, surfs at about 7.5kt (36'9"LWL). Unless, of
course, they're supplying free wings with the motor... :)
> Flooded Lead Acid (FLA) cells are the cheapest, LiFePO4 (one type of lithium-ion) are more expensive, but more store more power for the size and weight, and AGM fall somewhere in between for cost and performance. Here's some cost examples for the three types of batteries based on rated capacities:
>
> FLA (Trojan T-105) = $0.11/Wh (Wh = Ah x V, i.e. 100Ah 12V battery = 1200Wh)
> AGM (Odyssey PC1800) = $0.25/Wh
> Lithium (Thundersky LFP160AH + Simple BMS) = $0.45/Wh
>
> It looks like FLA are 1/4 the cost of Lithium for the same capacity.
>
> But as a boater, you know that you can't run consistantly batteries to fully discharged without adversely affecting their lifespan. Many people believe that FLA should be kept above 60% depth of dicharge (DoD), AGM should be kept above 70% DoD and LiFePO4 are rated for 3000 cylces to 80% DoD. So if we apply those guidelines to calculate cost for usable capacity we get something like this:
>
> FLA = $0.11/Wh rated divided by 60% usable = $0.183/Wh usable
> AGM = $0.25/Wh rated divided by 70% usable = $0.357/Wh usable
> LiFePO4 = $0.45/Wh rated divided by 80% usable = $0.562/Wh usable
So for 400AH, which is what they're suggesting, I'm looking at 80 T-105s
(x 4 for the 400AH, x 12 for the string voltage, and / 0.6 for the above
figures)? That seems like a rather large battery farm. Time to start
looking at forklift batteries, maybe?
> Of course, you may be able to find cheaper batteries and I know you can find more expensive ones, but these examples seem to be available to people in most parts of the US. But you can see the math that I did and can work up your own comparison after your shop around.
Eric, I really appreciate the overview; as I said, this is exactly the
kind of info I was looking for, since it allows me to start boxing in
the possibilities. Thanks again!
Ben
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