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  1. #1
    RC Racer
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    Sorry, another battery question

    Ok, I was trying to figure out what the difference between the c rating on lipos. The 3s lipo I use has a 20c rating, 6400 mah. I have a 4s lipo with a 40c rating, 5000 mah going in soon. I tried to look up the difference and it all just confused me. Some said it didn't matter, some said it does. So that's why I am asking. I am running a 2400kv castle motor with a mmp esc, 14 pinon. Thanks for the help.

  2. #2
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    The C rating is essentially the amount of current the battery can supply.. the higher C rating means Higher current output from the battery.. the people who say it doesnt matter are flat out wrong.. for a mmp 2400kv it most definitely matters... Lower C rated batterys have significantly higher voltage drop under load.. causing problems for many ESC's and their low voltage protection... quite possibly unable to supply the full amount of power your truck could use........ essentially under powering your rig....

  3. #3
    RC Turnbuckle Jr. Jimmie Neutron's Avatar
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    Here is a clip from my "LiPo 101"-

    To calculate the constant discharge rate of your pack
    mAh x minimum discharge C rate / 1,000 = available amps
    For example:
    The same 2200mah 20-40C pack.
    2,200 x 20 / 1000 = 44
    This battery pack can consistently provide up to 44 amps.

    To calculate the peak/burst discharge rate of your pack
    mAh x maximum discharge C rate / 1,000 = available amps
    For example:
    Again, the same 2200mah 20-40C pack.
    2,200 x 40 / 1000 = 88
    This battery pack can provide a peak discharge rate of up to 88 amps.
    Peak rates are limited in time... unfortunately, this amount of time does not have an industry standard. This is how a lot of inflated discharge ratings can be claimed. In my experience, honest retailers/manufactures of packs will list the constant discharge rating along with the burst discharge rating and not only the burst rating.

    Ratings
    When setting up your power system for your RC, make sure that the lower discharge C rate matches or beats the continuous amperage rating of the ESC. I try to beat it by at least 10%. This practice will provide you the most out of your system and it will also keep your batteries from working too hard. A LiPo should NEVER be hot... during use or charging. If it is, you are abusing the pack. Also, the less your pack has to work the longer it will last. However, LiPo's perform at their best once they are between 100F - 120F. (yet another reason to have a temperature gun)
    Whatever it is I just said... I could be wrong.

  4. #4
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    This info is most helpful Jimmy.. just to clarify something... the VXL-3S is rated at 200A continuous current.. 320A Peak??... so a stock traxxas powercell 2s 7.4 lipo can only put out 125A??
    5000mah X 25C 1000 = 125Amps Wouldn't that be underpowering the Esc?

    So would it then be advised to look for atleast a 40C 5000mah 7.4v 2S rated battery???
    5000mah X 40C 1000 = 200A?????

  5. #5
    RC Turnbuckle Jr. Jimmie Neutron's Avatar
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    Correct, but that is assuming everyone is using the same method for C ratings.

    The C rating of a LiPo does not have an industry standard... meaning Traxxas' 25C rating could be 100C for some other company or 10C for yet another. From what I understand, Traxxas really under-rates their packs... they under promise and over deliver.

    In short, you should be fine with Traxxas packs in your Traxxas RC running Traxxas power equipment. --And they stand behind this too.
    Whatever it is I just said... I could be wrong.

  6. #6
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    Thanks, you guys have been a lot of help.

  7. #7
    RC Turnbuckle Jr. Petertje60's Avatar
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    Don't be fooled by the VXL-3s specs. It will never survive 200A continuous. I think about 100A is the max that it can handle.
    Nobody is born with experience.

  8. #8
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    You can't underpower an ESC. The motor draws current from the battery by way of the ESC. Both the battery and ESC are only going to supply what they can supply. If the motor asks for less current than the battery and ESC are rated for, then everyone is happy. When the motor starts asking for more current than the battery and ESC can handle, then you have heat issues everywhere.

    Technically, you would want to know the average/max amperage your motor can draw, then find an ESC and battery that can supply more than the motor can ask for. Getting real world amperage draw on a motor would require a logger since everyone has different setups, gearing, terrain and driving style. Castle provides their own average, but I don't believe they provide a max. Regardless of pack size (mAh), a 200A discharge rate should be your minimum due to the variances discussed above. My advice is to buy the highest C rating you can comfortably afford.



    Quote Originally Posted by T4KT1KZ View Post
    This info is most helpful Jimmy.. just to clarify something... the VXL-3S is rated at 200A continuous current.. 320A Peak??... so a stock traxxas powercell 2s 7.4 lipo can only put out 125A??
    5000mah X 25C 1000 = 125Amps Wouldn't that be underpowering the Esc?

    So would it then be advised to look for atleast a 40C 5000mah 7.4v 2S rated battery???
    5000mah X 40C 1000 = 200A?????

  9. #9
    RC Turnbuckle Jr. Petertje60's Avatar
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    Quote Originally Posted by Slaughter33 View Post
    You can't underpower an ESC. The motor draws current from the battery by way of the ESC. Both the battery and ESC are only going to supply what they can supply. If the motor asks for less current than the battery and ESC are rated for, then everyone is happy. When the motor starts asking for more current than the battery and ESC can handle, then you have heat issues everywhere.

    Technically, you would want to know the average/max amperage your motor can draw, then find an ESC and battery that can supply more than the motor can ask for. Getting real world amperage draw on a motor would require a logger since everyone has different setups, gearing, terrain and driving style. Castle provides their own average, but I don't believe they provide a max. Regardless of pack size (mAh), a 200A discharge rate should be your minimum due to the variances discussed above. My advice is to buy the highest C rating you can comfortably afford.
    A motor does not draw, the battery pushes. As hard as it can, where the C rating is specification how hard it can safely push without too much voltage drop.
    The ESC controls the amount of current that comes from the battery. If you go full throttle, the ESC passes as much current as it can to the motor, where the ESC spec tells you which current (or power) is the safe limit.

    I did quite some logging with different batteries, ESCs and motors and IMHO a battery that can supply 100A is enough to push the VXL-3s to its maximum. A higher C-rating will never harm. It will prevent voltage drops and give more punch.
    Nobody is born with experience.

  10. #10
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    Quote Originally Posted by Jimmie Neutron View Post
    Here is a clip from my "LiPo 101"-

    To calculate the constant discharge rate of your pack
    mAh x minimum discharge C rate / 1,000 = available amps
    For example:
    The same 2200mah 20-40C pack.
    2,200 x 20 / 1000 = 44
    This battery pack can consistently provide up to 44 amps.

    To calculate the peak/burst discharge rate of your pack
    mAh x maximum discharge C rate / 1,000 = available amps
    For example:
    Again, the same 2200mah 20-40C pack.
    2,200 x 40 / 1000 = 88
    This battery pack can provide a peak discharge rate of up to 88 amps.
    Peak rates are limited in time... unfortunately, this amount of time does not have an industry standard. This is how a lot of inflated discharge ratings can be claimed. In my experience, honest retailers/manufactures of packs will list the constant discharge rating along with the burst discharge rating and not only the burst rating.

    Ratings
    When setting up your power system for your RC, make sure that the lower discharge C rate matches or beats the continuous amperage rating of the ESC. I try to beat it by at least 10%. This practice will provide you the most out of your system and it will also keep your batteries from working too hard. A LiPo should NEVER be hot... during use or charging. If it is, you are abusing the pack. Also, the less your pack has to work the longer it will last. However, LiPo's perform at their best once they are between 100F - 120F. (yet another reason to have a temperature gun)

    I think I have a man crush..... Thanks for this breakdown. Really awesome!

  11. #11
    RC Turnbuckle Jr. Jimmie Neutron's Avatar
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    Whatever it is I just said... I could be wrong.

  12. #12
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    I guess they push in the sense that the electrons want to get out of the battery. But they only push with enough force to meet the demands of the circuit, which in the case of an RC is motor amp draw. If a battery was to push with full force, everything you attached to it directly would have to be able to handle the max amount of amps the battery could supply, but I am 99% sure circuits don't work that way.

    Say for instance you have a battery capable of 500A of discharge. If you hook it up directly to an LED, the battery is only going to "push" out say 30mA because that is all the circuit is drawing. I guess you could say they are equally relational to each other. I know we are probably getting at the same thing, but just to be clear for those reading this thread.



    Quote Originally Posted by Petertje60 View Post
    A motor does not draw, the battery pushes. As hard as it can, where the C rating is specification how hard it can safely push without too much voltage drop.
    The ESC controls the amount of current that comes from the battery. If you go full throttle, the ESC passes as much current as it can to the motor, where the ESC spec tells you which current (or power) is the safe limit.

    I did quite some logging with different batteries, ESCs and motors and IMHO a battery that can supply 100A is enough to push the VXL-3s to its maximum. A higher C-rating will never harm. It will prevent voltage drops and give more punch.

  13. #13
    Traxxas Employee TireSlinger's Avatar
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    +1 to Slaughter.

    The motor pulls current based on load. (Gearing, driving style and a host of other factors.) If the battery can comfortably deliver the required current you should have a great time. If the battery cannot you'll get a voltage drop as it tries to provide the required amperage and it will likely get hot quickly.

    If the battery forced power into the system then it would always discharge at the same rate, or close to it. This simply isn't the case. A Stampede 4x4 with a 17t pinion for high speed pulls a lot more current than with the factory 11t pinion and discharges the battery much faster.

  14. #14
    RC Turnbuckle Jr.
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    The battery is the source of all potential power within an electric RC vehicle and as such the battery voltage "pushes" the current out of whichever ESC mosfets are opened at any given moment. This and the total downstream impedance determines the Amp load and related voltage drops across each component.

    Petertje60 was correct!

  15. #15
    Marshal ksb51rl's Avatar
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    Quote Originally Posted by rcapilli View Post
    I think I have a man crush..... Thanks for this breakdown. Really awesome!
    Quote Originally Posted by Jimmie Neutron View Post


    The marshals have a vast collection of PMs and visitor messages that were addressed to "Jimmie Neutron" but were flagged and diverted due to the phrase "Be My Valentine" in the title or body of the message. IDK - I can't explain it.


    Push or pull, so long as you can get out of your own way, it's all good.
    Last edited by ksb51rl; 07-31-2013 at 01:29 PM.
    Alt-248 on the number pad =

  16. #16
    RC Turnbuckle Jr. Petertje60's Avatar
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    I think it's a matter of the used terms in the world of electricity. Those are a bit confusing.

    "Drawing current" sounds to me like an active role. A LED is bound by its resistance which simply limits the current, whatever the power source can deliver, as long its enough to feed it.
    But what if there is a power source which can only deliver 10mA? Will the LED still "draw" 30mA?

    Back to the OP. The MMP/2400 combo can consume quite some power.

    A 6400mAh 3S 20C is able to deliver 128A on 11.1V (1420.8W). With a 14T pinion and the stock 54T spur you probably won't consume 1400W, except for rare peaks.
    A 5000mAh 4S 40C can deliver 200A on 14.8V(2960W), which is more than twice the power. Even in extreme overgeared situations, you will probably not get that far.
    Nobody is born with experience.

  17. #17
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    See what I mean't by no one could quite agree on the other articles and forums I read, it's happening here as well, lol. But im learning a lot either way.

    Thanks Peter, that explained it well.
    Last edited by mattstang2007; 07-31-2013 at 04:43 PM.

  18. #18
    RC Turnbuckle Jr.
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    Quote Originally Posted by mattstang2007 View Post
    See what I mean't by no one could quite agree on the other articles and forums I read, it's happening here as well, lol. But im learning a lot either way.

    Thanks Peter, that explained it well.
    I don't know, I agree with Peter and he agrees with me and KSB agrees but he is trying to be polite..

  19. #19
    RC Turnbuckle Jr. Jimmie Neutron's Avatar
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    Quote Originally Posted by ksb51rl View Post
    The marshals have a vast collection of PMs and visitor messages that were addressed to "Jimmie Neutron" but were flagged and diverted due to the phrase "Be My Valentine" in the title or body of the message. IDK - I can't explain it.
    Thanks for the chuckle! First time in a while I actually lol'd.
    Whatever it is I just said... I could be wrong.

  20. #20
    RC Turnbuckle Jr. Petertje60's Avatar
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    I think we all agree that this is a topic with the right amount of fun.
    Nobody is born with experience.

  21. #21
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    Motors operate using electromagnetic induction, which interacts with conductors(winding), currents and magnetic fields. Every time an electrical current passes through a conductor(winding) it causes a magnetic field around the conductor. But anytime a magnetic field moves through a conductor, it induces electrical current in that conductor.
    Is this correct. I know voltage pushes current, But i also thought motors pulled/sucked current because of the explanation above.
    Problems.The manual's good starting point. Simple

  22. #22
    RC Turnbuckle Jr.
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    Think of the electrical RC system as similar to the water system in your home. The water is under pressure (analogous to voltage) as it enters the home, but unless a faucet or valve (analogous to ESC) is opened somewhere in the home, no water flows at all. Once a faucet is opened, the water flows towards the lower pressure outlet at a rate dictated primarily by the differential pressures (inlet and outlet), the diameter and length of the piping and the internal roughness of the piping system. There are other parameters at play controlling the flow, but this covers most of it. But keep in mind, at no point along the water system is the water being pulled or sucked to its source, rather it is the upstream pressure pushing the water.
    Last edited by Jakey; 08-01-2013 at 01:03 PM.

  23. #23
    RC Turnbuckle Jr. spenniepoos's Avatar
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    Thanks guys this 101 has helped me understand more about the good stuff.........peace
    Famous last words...watch this!!!!

  24. #24
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    For some reason i thought the voltage pushed the start off current to the motor, but then once rotation starts, the motor then draws the current
    Problems.The manual's good starting point. Simple

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