Lead carbon battery use

Discussion in 'Electric Propulsion' started by gschuld, Aug 30, 2022.

  1. gschuld
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    gschuld Junior Member

  2. philSweet
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    philSweet Senior Member

    Nope. With any lead chemistry battery, you basically have a built-in four-hour endurance minimum if you want to see any of the advertised duty cycle and life cycle cost numbers. And there is no way you are going to build an eboat that will stay on plane on batteries for four hours. You need a lithium battery that can operate at rated capacity with a much higher rate of discharge and recharge - like 1C, not C/4.

    An advanced lead battery for a 10kW motor will be something like 24 L16 6V batts in a 3 X 8 configuration. The cells will cost more than 15k US. The rest of the battery management and wiring will cost another 10k US. You need an 8' x 3' x 2' box for the battery, and a cabinet the size of a household fridge for the chargers and fuse blocks and switches. You need to be able to charge at a minimum of 5kW beginning immediately after the discharge ends. The battery box and the charger cabinet will need forced air cooling that is isolated from all occupied spaces. Battery system weight is more than 3000 pounds.

    https://www.cdnsolar.ca/wp-content/uploads/2022/04/L16-Lead-Carbon-batteries-SWE6-420-2021.pdf

    The Victron item you linked to is not suitable because you can't parallel enough of them. You need much bigger cells. You don't want more than 4 parallel strings in the battery. If you are planning to operate in a hot climate, the L16 form factor isn't the best for cooling. Use a thinner, slabbier battery that cools better such as the NorthStar Blue. Those don't quite push 10kW for 4 hours, but they are pretty close and keeping them cool is important.

    <edit> I just wanted to add that the nope was with regards to a planing boat. If you have a hybrid with perhaps 1 kW electric per ton of boat, then advanced lead might be worth a look. But you need a large volume to put a heavy battery where it helps with stability. That tends to be where you want to put people. So you need something like a raised salon GA or a tourist schooner where everyone stays on deck for a three hour excursion.
     
    Last edited: Sep 1, 2022
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  3. rnlock
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    rnlock Senior Member

    I was looking at the specs and thought that they seemed comparable in weight to regular lead-acid batteries.
     
  4. philSweet
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    philSweet Senior Member

    Yes, the difference is in life cycle cost when operating at partial state of charge. They are tougher under harsh conditions. You get more charge/discharge cycles out of them and more total energy throughput. But you do still have to recharge immediately and not let them sit in a discharged state. They are an improvement over wet lead, but not really different than them.
     
  5. seandepagnier
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    seandepagnier Senior Member

    Do you think it is possible to build a hydrofoil boat that can be powered from lead acid and stay on foil for 4 hours? I think its possible, but it wont be 20 knots like the lithium powered ones, so a larger foil to support weight and slower speeds like 10 knots.

    A boat at 10 knots on hydrofoils could use 25% of the power of a boat planing to do the same 10 knots, and have a smoother ride, especially in waves.

    It probably makes more sense to use LiFePO4 for this.
     
  6. gonzo
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    gonzo Senior Member

    Even the most advanced batteries only have a small fraction of the power density of gas or diesel fuels. Lead/acid batteries can be effective at low speeds where the weight is not as critical. LiFePO4 is a family of chemistries that include very different characteristics. For example, the operational temperature range.
     
  7. seandepagnier
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    seandepagnier Senior Member

    That is all true but doesn't change anything.

    Consider hydrofoil using pedal power is possible, it is obvious that lead acid batteries have enough energy to lift themselves in this way and power a boat at speeds above 10 knots. They may not have nearly as good specifications such as range or efficiency as other energy storage because of their weight and efficiency loss at high load, but they are still way more than sufficient to achieve high speeds.
     
  8. philSweet
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    philSweet Senior Member

    Because I can't resist a grok-able comparison ...

    The Max Maintainable Power for four hours of an elite athlete is about 300 W. At a world-class VO2 value of 75, they would weigh a skosh more than one L16 6 volt battery. If you amortize chargers and wiring, I'd call it a tie.

    From the chart in my first post, a four hour discharge entails a 0.2 C rate, so 84 A in this case, or 504W. So advanced lead has less than twice the specific power of advanced people at four hours duration, but about four times the specific power of regular people at four hours.

    Hydrofoil performance ...

    Lift to drag ratio's of 16 in small boats seem to be about the upper limit. Most of these are trying for speeds greater than ten knots, though, and that can be tricky for small foils due to cavitation. So to lift a 135 pound battery system that can make 504 watts, you will have about 8.5 pounds of foil drag, or 37.5 Newtons. At 5.144m/s, that's 193 W. Propulsive efficiency for a small boat at ten knots might just make 70%, but that's a totally custom job. So 353W propulsion. So in the most optimistic case, just the battery and wiring will be 60% of the maximum total weight of the craft for four hours endurance on plane. The maximum HPV on a plane seems to be 2.4 hours on a foiler paddleboard at 9.3 knots average speed. But they can surf waves, I haven't seen a flatwater duration record.

    But no one says you can't cheat a bit at the expense of some battery life. At 3.5 hours endurance, things look a bit better, as in it might actually work.
     

  9. DogCavalry
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    DogCavalry Senior Member

    Um. Wrong? Changes everything.
     
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