The battery thread...some facts please

Discussion in 'Hybrid' started by Mick@itc, Feb 24, 2012.

  1. kerosene
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    kerosene Senior Member

    gonna go to zero is likely. If it doesn't keep the voltage after charge its probably toast.

    The curve of voltage is usually such that it stays pretty level (above 12) and once it starts to drop you have only a fraction left.

    This varies depending on the chemistry but as said its probably gone.
  2. Submarine Tom

    Submarine Tom Previous Member

    It's toast.

    Recycle it.

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    FAST FRED Senior Member

    "If battery tech hasn't improved, how do my camera, my phone, and my laptop run 5x longer on a charge than in the year 2000? And charge faster to boot?"

    Not much faster than ancient Ni Cads.

    And a battery for a camera will not power most boats very far.

    The question is are batterys practical as energy sources for boat propulsion?
    sadly the answer is not yet , IF EVER!

  4. mydauphin
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    mydauphin Senior Member

    Actually computers are using a lot less juice now.
  5. johneck
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    johneck Senior Member

    One of the main changes in your computer, phone and camera is less power to perform the same functions. The drill motors have also improved. We need incremental improvements in all parts of the equation, storage, usage and control to get to actaully workable, useful solutions.
  6. Stumble
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    Stumble Senior Member

    Fundamentally it just comes down to energy density. Wether it is a battery, diesel fuel, flywheels, whatever, the technology just doesn't matter. What matters is how much power is usable from a given volume and weight. So let's look at some numbers...

    Diesel 45.4mj/kilo 38.6mj/liter
    Li ion batter .75mj/kilo. 2.33mj/liter
    Nickel-metal .288mj/kilo. 0.504-1.08mj/liter
    hydride battery

    Fundamentally it takes around 50 times the weight in the best lithium ion batteries in the world to equal the energy availability of diesel fuel, and twenty times the volume. While Li batteries are much better than the alternatives, they are no where close to the available power of diesel fuel. So while the Li are 2-3 times better than alternatives, that just isn't good enough for propulsion use.
  7. Mick@itc
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    Mick@itc Junior Member

    Good stuff everyone...

    There is some good stuff here, thanks to all for your input.

    Two questions for discussion purposes.

    We speak of energy density which seems a useful number and tool for discussion. what energy density would batteries have to get to before they would be classed as good/adequate for sailboat propulsion. lets use a 10 tonne disp sail boat as a model for discussion. That would need say a 30¬40hp diesel conventional motor to be classed as suitably powered. (we can change the numbers if necessary. So what battery energy density would be sufficient to see this capable of moving to an electric propulsion system?

    The second question is being a devils advocate. From an overall propulsion systems viewpoint is energy density as important as other factors or is it the most important factor? Just to give an example, I read a blog of a totally electric propulsion boat and he highlights some other factors that he believes are more relevant in the overall system.

    Thanks for the good civilised conversation so far...lets maintain this example of how battery threads can work ;)

  8. CatBuilder

    CatBuilder Previous Member

    I also have a battery question pertaining to this thread. I've been using wet cell forever and I am curious about something:

    If I wanted, say, a battery bank with 30KW of energy (2500AH @ 12VDC), what would be the best way to store this energy? LiFePO4 batteries? What brand would be best to set up a 2500AH bank?

    I'm noticing that a gallon of gasoline has 33KW of energy in it. Interestingly, my 5KW gasoline generator uses 1 gallon per hour at full load. When I run my 5KW generator, producing 5KW in an hour, I lose 28KW of the energy in the gasoline, by the time it's converted to electricity to use. I am left with just the 5KW of actual, usable energy.

    This means it would take me 6 gallons of gasoline every day to make 30KW of energy, assuming no losses. That's $22.50 per day.

    That means I need 42 lbs of gasoline, plus a 140lb generator to make my 30KW each day. 182lbs of weight to make this energy for one day. If I want to go more than one day, I need closer to 300 or 400lbs to go for a longer period.


    How much does a 30KW (2500AH@12VDC) LiFePO4 battery bank weigh? Which batteries could I use to get the most energy density?
  9. Stumble
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    Stumble Senior Member


    Energy density really isn't the important factor, range under power really is. But the higher the energy density the easier it is to store the necessary amount of energy. So let's assume a standard usage pattern that I just did the fuel calculations for. I am about to move a Beneteau 381 from Panama City to New Orleans. The distance covered is around 220 miles, but let's say 200 to make the math easy. The engine burns .5 gallons at 5 knots cruise, so I will use the engine for about 40 hours, so I need 20 gallons of diesel on board.

    To make the same trip using batteries I would need to carry 155 times the weight of diesel in batteries to get the same energy availability. Now diesel weighs about 6 pounds per gallon, so I will be carrying roughly 120lbs of fuel, if I used batteries that would go up to 18,600lbs of batteries to go the same distance.

    Now if I use Li batteries the numbers are better... I only need 7,200lbs of batteries instead of 120lbs of diesel. But the cost goes through the roof.

    On the other hand if all I wanted to do was travel 1 mile, then I would need about 3/5 a pound of diesel, or 100lbs of batteries. Which might be reasonable for this boat in and out of the harbor, but isn't really enough for anything else. If you have a boat with very low power requirements, batteries might be an option but not for normal usage.


    There is no one best battery. The higher capacity longer life batteries, with deeper discharge capability cost more. It really depends on how price sensitive you are, and how much you are willing to spend for the capabilities. Personally I love the Rolls-surret batteries, specifically the traction batteries that are purchased in individual cells. They are much easier to move around, if a cell goes bad you only replace the one bad cell, and traction batteries can be discharged down 80% without damage. However they are more expensive than other options.
  10. CatBuilder

    CatBuilder Previous Member


    I've been doing some reading and it appears that have the same batteries in LiFePO4. They claim 2000 cycles at 80% discharge. They also claim C/3 charging rates and a much lowered weight and volume compared to regular Rolls batteries or AGMs or any other lead batteries. Lastly, they also come in small, 3v units you tie together.

    Seems they could very easily give the Rolls a run for its money.

    In my possible application, I was exploring the idea of having a massive battery bank. 2500AH@12VDC. Since I'm building a new boat, the canvas is pretty much wide open and budget is of no concern, assuming the large battery bank makes something else cheaper and lighter in the fit out.

    My thinking is that the amount of gasoline I need to carry to run a generator to get 30KW for a day is very heavy (and very expensive). I am exploring to see if a large battery bank and solar array could better suit my purposes.

    This is why I'm looking for specific LiFePO4 battery brands and systems. To see how big of a bank I'd need, how much it would cost, how much it would weigh and how much volume it would take up.
  11. WestVanHan
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    WestVanHan Not a Senior Member

  12. Stumble
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    Stumble Senior Member

    To generate 30kw a day in power you would need something like 40 x 230 watt solar panels. Figure around $300 a panel, you are in it for $12,000 and, at a weight of 43lbs per panel, 1,700lbs in panels.

    To replace something like .5 gallons or 3 pounds, or $2 a day in diesel fuel. Some specific applications might justify it, but a 30kw generator is about a 40hp engine, which will burn .4-.5 gallons/hr. In fact the engine on the Beneteau 381 is a 42hp westerbeak rated at .42 gallons/hr at 2500rpm.
  13. CatBuilder

    CatBuilder Previous Member

    No, the 30kw is the daily power budget, not the number of kilowatts needed per hour...

    Will post from computer later, but something seems off about that beneteau engine stuff. My 5kw generator takes a gallon of gasoline an hour. So does a 30hp yanmar take about a gallon of diesel an hour if I remember correctly.
  14. Stumble
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    Stumble Senior Member


    The panels I specced would generate around 30kw a day. Assuming I did the math correctly, though I am not an expert in pv panels. Assuming reasonable production, and 6 hours a day of sunlight.

    If you need 30kw a day instead of continuously that would be about a 1.25kw generator. Which is much smaller than I assumed. A 30kw generator will burn around 2 gallons an hour (I made a mistake in my earlier assumption, this is four times the consumption I assumed). I couldn't find a 1.25kw generator, but a westerbeak 4kw burns .48gallons/hr, why these engines burn the same, when the output is so different I have no idea.

    Either way, the 4kw would need to run for ~7 hours a day to generate your 30kw, so that would be 3.5 gallons a day in fuel burn.

  15. WestVanHan
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    WestVanHan Not a Senior Member

    You guys need to get things on the same level.

    Engines..peak or continuous?
    Gas vs diesel
    Air cooled vs. water cooled.Gas powered air cooled genset is poor.
    Gensets..peak or continuous and what efficiency?

    Diesel is about 20 hp/ US gal/hr. Turbo etc up to 24.
    OHC/OHV water cooled gas engines about 14 hp/gal/hr.Varies by valve count
    Air cooled gas with typical flathead valves about 10hp/gal/hr

    So take a gas genset @ 5 kw continuous burning a gal an hour,which is 7.5 hp plus generator loss and it's at 9-10hp output.
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