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Thread: How many MAH packs for "p" mono

  1. #1
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    Default How many MAH packs for "p" mono

    New to fast electric, but not boat racing. What's capacity does a P mono need for a full IMPBA race? Will 5000 do it?

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    It wont. I carry 8000. My son usually runs 10,000mah. He doesn't dump all of that but it makes the boat stay on the water a little better.
    Noisy person

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    glad I asked. thanks

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    In fairness, you could run it on 5000 if your C rating was high enough. You would just have to go slow enough to not run out of mah before you finished. In other words.....you'd get killed speed wise in a race.
    Noisy person

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    Fred
    Here is something to remember about Batteries and racing.
    This is also the reason that I run 4S2P - 5000 packs (10,000mAh total) in my 33" Delta Force for "P" mono

    The higher the mAh total, and the higher the "C" rating = higher Voltage under load
    Seeing that the RPM our motors are rated by KV (rpm per volt)
    A higher voltage under load = more RPM under load.

    Larry
    Past NAMBA- P Mono -1 Mile Race Record holder
    Past NAMBA- P Sport -1 Mile Race Record holder
    Bump & Grind Racing Props -We Like Em Smooth & Wet

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    Quote Originally Posted by TRUCKPULL View Post
    Fred
    Here is something to remember about Batteries and racing.
    This is also the reason that I run 4S2P - 5000 packs (10,000mAh total) in my 33" Delta Force for "P" mono

    The higher the mAh total, and the higher the "C" rating = higher Voltage under load
    Seeing that the RPM our motors are rated by KV (rpm per volt)
    A higher voltage under load = more RPM under load.

    Larry
    Great info! Thank you.

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    Excellent tip on battery capacity. Another thing to keep in mind, the battery’s resistance. There are many low cost, high capacity batteries that may have higher resistance under race conditions. Chargers won’t read the “true” resistance, that while delivering high current to the load and elevated temps. We, as a group, need to keep each other informed on the best deals. A battery with high capacity and high resistance is not much better than a low capacity battery...

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    Quote Originally Posted by CraigP View Post
    Excellent tip on battery capacity. Another thing to keep in mind, the battery’s resistance. There are many low cost, high capacity batteries that may have higher resistance under race conditions. Chargers won’t read the “true” resistance, that while delivering high current to the load and elevated temps. We, as a group, need to keep each other informed on the best deals. A battery with high capacity and high resistance is not much better than a low capacity battery...
    Craig

    Your statement above is why these are my go-to packs for racing.

    https://www.mojoracingproducts.com/c...70c-140c-lipos

    Larry
    Past NAMBA- P Mono -1 Mile Race Record holder
    Past NAMBA- P Sport -1 Mile Race Record holder
    Bump & Grind Racing Props -We Like Em Smooth & Wet

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    Nice lead... Thanks!

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    Giant Power are all I own these days. I am totally biased and totally admit it. Bob from Mojo helps Ty and I out a lot.

    Another interesting brain up (pushups with yer brain).......haha Amperage. A cell deliver a different voltage at different amps. The more amps the lower the voltage. The C rating tells us at what amperage the cell is supposed to be able to deliver that 3.7 volts per cell. Static voltage is higher. Max static off the charger is 4.2v. Usually 4.189 or so by the time you get them plugged in. Our motors are volts times rpm per volt (kv). Our inclination is to throw more prop at a boat, more prop, more prop. Doesn't always translate to more speed though. My boat might turn a x447 prop but be no faster than the x445 because the rpm turned out a tic lower with the 447 than the 445 due to reduced voltage available at the higher amp draw. You can do the same mental exercise with pitch. 1.4 vs 1.5.

    Carrying more mah than you actually need like Larry and I do ensures that you maintain higher voltage even when you get into stupid amp draws. We never get anywhere near the actual C rating this way. Higher voltage...more rpm. Cells hold up longer too. In theory.

    Then each motor has it's happy place where it is most efficient. It might be capable of 200 amps but is more efficient at 160 amps for example. This is where testing is so fun. You have the boats which are beautiful, throw in some physics, some educated guessing, and a sprinkle of crazy............mwahhhhh. Glorious!
    Noisy person

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    I didn’t know that about C-Rate Terry, thanks, a valuable piece of info! I don’t run the amperage even close to the C-Rate, I like to leave a little in the Bank. No battery likes being used at its stated specs. Those are the max-max specs, but they won’t live there long if you use them that way. But you guys are 100% right maintaining a higher voltage thru the race is a distinct advantage!

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    I posted a thread a short while ago asking about efficiency differences between larger and smaller props. I was hoping that this kind of info would creep into the conversation. But it didn’t... The thread was left with the impression that the larger prop is more efficient, which it is from strictly a prop’s perspective. But there is the electrical perspective and that was explained very well here! Thanks to Larry and Terry for that insight.

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    There is some guess work and a balancing act that happens. Sometimes you have the right prop for a boat and the wrong motor. Sometimes you have the right motor but can't quite settle on a happy prop. A larger prop will typically have more blade in the water and accelerate better. If you don't give rip about acceleration go with the tiny prop and high rpm. For ovals and especially offshore you're forever throttling to turn and then accelerating again.

    Back in the dark ages when we ran Nimh some of us would use a gear box. Ed Hughey was the absolute king of the gear drive. Later the Turbo trans from Fine Design. Sometimes we let a motor scream and gear down so we could turn a big fat prop. Fat prop comes out of a corner better. It was also better for the Lemans starts we were doing at the time too. That drag race to the first turn sucked if you were running high kv and a tiny prop. Eventually you would get to the same speeds. Watts are watts and all but it was too late. You were running in slop left behind the guys that had a better hole shot. It's like big fat tires on a F1 car or skinny tires on a land speed record car. You don't need big fat tires on a speeder.
    Noisy person

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    Less amps=more volts=more rpms?

    Higher mah and C ratings allow more volts to potentially be drawn?

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    Quote Originally Posted by CHowarth View Post
    Less amps=more volts=more rpms?

    Higher mah and C ratings allow more volts to potentially be drawn?
    You don’t “draw” volts, you draw amps... you’re looking at it backwards. Volts is like line pressure in a water line. The water flow rate is like amps. MAH is an expression of the amp draw capacity. C Rate is the highest amp draw you should make and still maintain 3.7V out. The discussion is saying get batts with higher mah to keep off the lower voltage part of the discharge curve. So you have to understand you don’t “use” voltage, you use amps. Amps being delivered at a higher voltage creates more power, V X A = Watts.

  16. #16
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    High flow in piping makes more pressure loss due to friction. Same thing with amperage. The higher the amps the more loss of voltage.

    It is a difficult thing to get the brain around.
    Noisy person

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    You got that right! I know plenty of people with BSEE’s that don’t have a clue how a battery really works...

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    Thank you both. It's all new to me and most likely way beyond where I need to be right now, but I like understanding what's happening. I'll quietly try to follow along!

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    We don’t want to squelch your questions! One of the base goals of this site is to support new interest. It’s a difficult thing, and we wouldn’t know if you don’t chime in. I welcome your comments 100%!

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    I will beg to differ on the C rate being maximum current at which point battery has the ability to maintain 3.7V. / Cell under load.

    It is the maximum current that the battery can maintain for a full discharge cycle without heating up past 62c where the battery is likely to puff. It can be argued whether a full discharge is to 0%, to the rated capacity, or to 20%, and whether the puffing point is 60, 62, or 65degrees C, but that is essentially it.

    While the C rate in the current crop of good cells at the moment may well coincide with a 3.7V/cell output somewhere during its discharge curve, that is a temporary thing. When LiPos first came out they were 4C and you could only expect an average of about 2.8V/cell during the discharge at that 4C and it would only be over 3.7v/cell for a second or so. As the cells have improved that voltage had risen and each generation of high C cells is capable of holding a voltage under load closer to the resting voltage than the previous generation. Extrapolating consistent past improvements into the future the next generation of cells should average a higher voltage per cell.at maximum constant C rate/cell than current ones. Although there has already been huge improvements, and the biggest increases are generally when a technology is new, 3.7v/cell is about the resting voltage of a flat LiPo, so there is still plenty of room for improvement.
    Paul "tug Killer" Upton-Taylor, Cat lover.
    FastElectricBoats.co.uk

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    I like it Paul. That makes more sense to me than the 3.7v nominal. Discharge on a curve and the curves shape looks different at a higher C rating.

    Thinking back, I remember someone telling me he felt that the cells would eventually lose zero voltage over the entire discharge. This would correspond to you explanation.
    Noisy person

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    That is not what I am saying Terry. While I can't rule out the possibility that some future battery technology wont lose voltage over the discharge, I can say that LiPos always will, and as all previous battery technologies have done, and I find it hard to imagine that future ones won't.

    The best I could hope for with technology based on our current understanding of science is a superconducting cell with zero IR, in which the voltage under any load would match the resting voltage. With lithium based chemistry it would still start off at 4.2v/cell and come off at 3.7v/cell, it would just maintain those voltages at any ampage you wanted to pull. There would still be a discharge curve, it would just look the same at 1mA as it would at 1000A.
    Last edited by NativePaul; 03-30-2018 at 05:01 AM.
    Paul "tug Killer" Upton-Taylor, Cat lover.
    FastElectricBoats.co.uk

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    Ok. That's a bunch of info. Next question is "P" limited? Will 5000 mah do in that class? Thanks to everybody for helping out the new guy.

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    No I follow Paul.

    Might seem a strange analogy but a similar course has transpired with fire pumps. Available pressure drops on a curve based on gpm. As manufacturing improved so did the curves. Now you can get a pump that has very nearly a flat curve. Still a curve though. Eventually when you exceed the design points the curve gets steeper.
    Noisy person

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    Fred, limited depends on the class. 5k can get it done. Again, propped accordingly. Not great in offshore. It still could work for offshore but you have to prop accordingly. I like to use a pair of 2s6500's wired in series just because they fit in the boats well. Puts the weight low in the boat. Give me a little cushion too.

    With the right setup you can get away with even less in limited hydro. Less will work in sport too but it has to be light on it's feet if you know what I'm saying. We've used a pair of 4s2200's in sport. Usually you need more weight though.
    Noisy person

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    Those Giant Power packs look interesting. I just may spring for a couple to run in my 21" Oval Master. I had planned on using a 2S system but have decided to go 3S and 2950KV Leo 3650 motor. Using more volts is more efficient.
    I also fly electric planes and discovered several years ago that using more cells and propping down is more efficient...also using a larger motor works wel but that;s for model planes not necessarily for boats.
    I used one of those Fine design gearboxes in a 26" FD deep vee mono several years ago. Thought it worked quite well.
    Up where I live in Northern Michigan, there are no RC boat clubs that I know of so I just burn around the lake.

  27. #27
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    Hi all!!!

    I am new to fast electrics, but not new to racing. Really loving my new Promarine Skater X2 This is a very interesting thread for me as i was wondering how the motors/esc's work as far as power ratings go(don't know if i'm saying this correctly) So for eg i know that with my gas boats once i have a high revving motor, a smaller higher pitched prop will get me in and out of the corners quickly, and gives me enough speed down the straights. This battery discussion is interesting as i am wondering the same thing as far as power/battery life/ for a six lap race!!

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    I was going to ask a seemingly stupid question as to which is better 1 4s or 2 2s. I must have had too many beers. Please forgive me, it's my birthday. More packs = more amps. It's that simple.

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    Quote Originally Posted by Mad Hatter View Post
    I was going to ask a seemingly stupid question as to which is better 1 4s or 2 2s. I must have had too many beers. Please forgive me, it's my birthday. More packs = more amps. It's that simple.
    I’m glad not to be the only one who drinks then gets on forums

    I believe it’s the same difference 2x2s=4s 1p and the C rating stays the same as a single 4s pack of same spec cells.
    "Any intelligent fool can make things bigger and more complex... It takes a touch of genius - and a lot of courage to move in the opposite direction."
    --Albert Einstein

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    Quote Originally Posted by rearwheelin View Post
    I’m glad not to be the only one who drinks then gets on forums

    I believe it’s the same difference 2x2s=4s 1p and the C rating stays the same as a single 4s pack of same spec cells.
    One thing to consider is battery placement. In some cases two 2S packs work better than one single 4S pack.........
    Futaba Team Driver
    - IMPBA Hydro Technical Director -

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