Bob: I chose the makers 1.5 kW spec to keep it a simple caparison to fuel. your batts also seem to be about 1kw so there is no fundamental difference in the distance that the example boat could run. Throwing around various discharge rates and acceptance rates doesn't refute the point that batteries are not a reasonable replacement for fuel in boats.
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The other thing I got to wondering about is the fire danger of big parallel battery banks. Are there lots of breakers between batteries?
One failed in short battery would cause all the rest to dump current at a very fast rate. We see this happen with just a few in parallel causing some excitement but 12 or 24 or more on parallel could get very exciting with several hundred amps from many batteries available
One failed in short battery would cause all the rest to dump current at a very fast rate. We see this happen with just a few in parallel causing some excitement but 12 or 24 or more on parallel could get very exciting with several hundred amps from many batteries available
deckofficer
Senior Member
That is why I'm using LiFePO4 chemistry. It doesn't have the energy density of other lithium cells but they are safer than all the others including lead acid.
I never parallel for capacity (even when I was putting together off grid LA banks), but chose cell capacity for the job in a single series string.
LiFePO4 cells are 3.25 volts and come in 20, 40, 60, 90, 100, 160, 180, 200, 300, 400, 700, and 1000 ahr sizes.
I never parallel for capacity (even when I was putting together off grid LA banks), but chose cell capacity for the job in a single series string.
LiFePO4 cells are 3.25 volts and come in 20, 40, 60, 90, 100, 160, 180, 200, 300, 400, 700, and 1000 ahr sizes.
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deckofficer
Senior Member
I'm not throwing rates for the fun of it. Usable energy storage is what is being discussed for propulsion and the question was asked is how a group 27 stacks up to a LiFePO4.
On a 100 amp load (typical in electric propulsion)
The LA group 27 has 192 usable Whr so range on a single 12 volt battery assuming 300 Whr per mile, is 0.64 miles
The LiFePO4 has 1040 usable Whr at 1/2 the weight, and range on a single 12 volt pack is 3.5 miles.
A rather large difference don't you think?
Read up on Peukert Effect.
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I am familiar with reduction depending on rate it is shown in the Trojan data. As I said I was trying to demonstrate the inadequate energy density of batteries for propulsion of boats. Using worst case Lead discharge rates for your comparison merely says that your batteries are better. So what, they are still not going to power a troller.
How many GP 27 Li equivalents would be needed to produce 25 hp for 1 hour?
How many GP 27 Li equivalents would be needed to produce 25 hp for 1 hour?
deckofficer
Senior Member
You just need to pick the right boat and you won't need 25 hp. The designers of the Motorcat 30 have done some tests with electric propulsion and achieved 5 kt on just 1500 watts. That is 300 Whr per mile.
Do the math, 750 lbs of LiFePO4 cells is 36,400 Whr X .8 (80% DOD) = 29,120 usable Whr / 300 = 97 mile range.
ranger58sb
Guru
Is there some way to know LiFePO4 Ah at a comparable 20-hour rate? Would it be 2000 Ah (from the math)?
What are costs like? Initial acquisition, arm and a leg and your firstborn? Several boat bucks? Or...?
Switching one bank at a time would mean needing a separate charger for the different chemistry, yes?
-Chris
5 kts on 1500watts is5 knots on 2 hp. Does anyone believe that? Maybe with a canoe but who cares about canoes.
Americans aren't familiar with kW ratings for engines so the discussion becomes foggy where as we all know hp.
Just remember 746 watts or .746 kW equals one hp. (Thanks Ski)
Americans aren't familiar with kW ratings for engines so the discussion becomes foggy where as we all know hp.
Just remember 746 watts or .746 kW equals one hp. (Thanks Ski)
deckofficer
Senior Member
It has got to the point that your not seeking information but rather arguing every point.
I've known 746 watts equals 1 hp for about 50 years.
I also know in a water medium it takes close to a 8 fold increase in power to double your speed.
Since boat builders do not give drag coefficients for their hulls, you have to compute from the vessel's top speed and hp required down to hp needed for the slower cruise speeds.
The Motorcat 30 runs 25 kt on 120 hp. Do the math!!!!
So it comes as no surprise the Motorcat designer was able to achieve 5 kt on a 1500 watt Minn-kota.
E-MC 30 - Powercat - MOTORCAT
I believe I am sticking to the point of battery propulsion of the type of boats discussed herenot constantly throwing out diversions and saying do the math. Be nice if you would show the math you promote so we can all learn something.
deckofficer
Senior Member
Peukert effect is very low, so at a 20 hr rate a 100 ahr cell would give around 120 ahr.
Costs run about $1 per ahr at 3.25 volt cell level, so figure $4 per ahr at 12 volts (13 volts).
There is a very large thread on CF for all of us that have made the switch. These cells are not quite plug and play. For instance their internal resistance which allows very little voltage sag under heavy loads, when charging can really tax you charging system. Because a LA battery when discharged and charged takes on a surface voltage during charging that is higher than resting voltage. Charge rate (electron flow) is determined by voltage differential between the charge source and battery. On alternator charging sources for LA, even with a flat battery, only a short period will be at the alternator's max output because of the false surface voltage that will appear. So a 150 amp alternator into a LA bank will charge a 150 amps for a short period and taper. On LiFePO4 cells it will stay at 150 amps for better than 90% of the charge cycle, thus causing the alternator to overheat. On programmable regulators they have a feature called "Belt saver" that allows to to adjust the alternator's output.
So much more to learn here....
LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks - Cruisers & Sailing Forums
but be prepared there is 3800+ posts to that thread but all your questions will be answered.
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deckofficer
Senior Member
I did show the math.
Now that you know there are hulls out there that are efficient enough to cover 1 nm on 300 whr, if you have 30,000 usable whr you have a 100 mile range.
Actually I don't know there are hulls that are efficient. The motorcat only weighs 4400 # dry so it is not especially slippery just light weight for less wetted surface. Have you cruised the motorcat?
That is a very very light boat compared to the trawlers discussed here. A reasonable potable water supply and provisions for cruising plus several people would be a significant percentage of the dry weight and change the numbers a lot. Do the math.
That is a very very light boat compared to the trawlers discussed here. A reasonable potable water supply and provisions for cruising plus several people would be a significant percentage of the dry weight and change the numbers a lot. Do the math.
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Ski in NC
Technical Guru
The motorcat might have hit 5kts with 1.5kW of propulsion, but that is not going to be practical in the real world. Get a good headwind and some seas on the nose and MinnKota is not going to do the job. Add normal cruising weight, it will be even slower, if it moves at all.
There needs to be more available power and more available stored energy to make a successful cruiser. The cost of going full electric, the weight of stored energy and the limited range make it impractical.
Add a diesel genset, that may do the trick.
There needs to be more available power and more available stored energy to make a successful cruiser. The cost of going full electric, the weight of stored energy and the limited range make it impractical.
Add a diesel genset, that may do the trick.
CPseudonym
Moderator Emeritus
How many people do you usually "long term" cruise with?
I ask because that is what the electric powered cat seems best suited for reading both Bob and rjtrane's previous threads on the matter both here and at Cruiser Forum. Two or less seems ideal for that means of propulsion in the 40' and under class and then only if short distance(less than 80 mile) island hopping is your thing. If more than 2 long term cruising or longer distance would pretty much mandate diesel propulsion IMO.
brian eiland
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Energy Independance
I posted this over on the 'Pilgrim' discussion, but had to spend some time trying to relocate that posting.
Interesting material...
I posted this over on the 'Pilgrim' discussion, but had to spend some time trying to relocate that posting.
Interesting material...
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deckofficer
Senior Member
You neglected both cycle life and usable ahr.
Group 27 400 cycles to 50% DOD.
LiFePO4 2000 cycles to 80% DOD.
Usable ahr over cycle life for a group 27 is 20,000 for $100
Usable ahr over cycle life for LiFePO4 is 160,000 for $400
Per ahr lead $0.005
Per ahr LiFePO4 $0.0025
So over the battery lifetime, LiFePO4 is 1/2 the cost.
deckofficer
Senior Member
Define "reasonable"? I would say crossing an ocean would require a reasonable boat.
No fuel or sails, crossed Atlantic on batteries (heavy lead acid) and solar. transatlantic21: The world's first crossing of the Atlantic on a solar boat
ranger58sb
Guru
Thanks. Sort of.
Have to admit, I start reading that thread over there, but it mostly made my head hurt. Simple factoids seemed to have quickly gotten lost in the sales pitches... so I gave up.
Mostly I do short-hand. For example, how much does it cost to run our whole boat for a day at anchor with up to 2x generator sessions (while we're cooking) -- with FLAs, AGMs, gels, LiFePO4s, whatever -- for the whole of all our battery banks' lifetime? (Not propulsion, just nav and house loads.) Given useful info on costs like that, I might not really care about chemistry. I think that means your conclusion it's about half the cost with LiFePO4s is useful info.
Then again there's start-up (change-over) costs, another short-hand for me.
So you're saying a 100 Ah LiFePO4 (@ 20 hour rate) would really be a 120 Ah cell, and it'd cost either $400 -- or maybe $480 -- and that would be a number to compare against flooded lead-acids, AGMs, gels
And I do see your point about boatloads of more cycles to 80% DoD...
And the part about chargers, etc. (and associated costs).
-Chris
deckofficer
Senior Member
Chris,
As with all batteries, LiFePO4 cells have a given shelf life. They are new enough that they have only been around about 10 years. Those early ones are still going, but 10 years would be a good starting point for shelf life.
Unless you are cruising full time and cycling the batteries 200 times a year, lead acid might be more economical for your usage. If you were electric propulsion then the energy density, depth of cycle, and cycle life would make the LiFePO4 a no brainer. If used as a house bank on a powerboat with partial year usage, go with lead.
I used to do off grid home power and back then lead was all there was. I had two rules for battery banks back in those days and that was a single series string and the weight of each battery must be light enough for one person to handle. The smallest system was just (2) golf cart batteries in series and a 12 volt inverter, but most systems I built were 48 volt and a 48 volt inverter. As the size of the system grew, the amount of cells per battery were less. The most common bank size was (24) 700 ahr 2 volt cells in series. A few were pretty large banks, (24) 1150 ahr L16 2 volt cells in series.
As with all batteries, LiFePO4 cells have a given shelf life. They are new enough that they have only been around about 10 years. Those early ones are still going, but 10 years would be a good starting point for shelf life.
Unless you are cruising full time and cycling the batteries 200 times a year, lead acid might be more economical for your usage. If you were electric propulsion then the energy density, depth of cycle, and cycle life would make the LiFePO4 a no brainer. If used as a house bank on a powerboat with partial year usage, go with lead.
I used to do off grid home power and back then lead was all there was. I had two rules for battery banks back in those days and that was a single series string and the weight of each battery must be light enough for one person to handle. The smallest system was just (2) golf cart batteries in series and a 12 volt inverter, but most systems I built were 48 volt and a 48 volt inverter. As the size of the system grew, the amount of cells per battery were less. The most common bank size was (24) 700 ahr 2 volt cells in series. A few were pretty large banks, (24) 1150 ahr L16 2 volt cells in series.
