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.
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. 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.
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. 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 looking at cost per usable Wh, FLA should deliver about the same range as Lithium for 1/3 the price. Lithium is only less than 60% more than AGM for the same range. The Lithium batteries weigh about 60% less than FLA or AGM for the same range.
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.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Ben Okopnik <ben@...> wrote:
>
>
> This is the area where I'm trying to figure out what's reasonable. E.g.,
> the proposed system from ElectricYacht was their ibl360 motor and a
> 400AH@72v bank. For a motor that draws 360A at max output, that seems
> woefully inadequate; given the Peukert effect, heat issues, etc., that
> sounds like about 20 minutes at WOT.
>
> I realize that looking at max draw is not really reasonable - the thing
> that matters the most is where the breakover point in the curve is
> (i.e., where you get a good product of speed vs. time, a.k.a. reasonably
> efficient usage.) The big question is, what is the best approach for
> getting the greatest effect within, say, 1800 lbs. of batteries + motor?
> Or is this still an inexact science, and all you can do is take a good
> guess?
>
> I'm not rich by any means, but given the price of a new diesel plus the
> hassle (or the cost) of installing it, I'd be quite happy to spend at
> least that amount for a good electrical drive system. Cheaper would be
> even better, of course. :)
>
>
> Ben
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