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  1. #1
    RC Competitor
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    Run Time - 2s vs 3s, same mAh

    Both batteries are 5000 mAh (Traxxas batteries both having the same C rating). One is 2s, the other is 3s. Which is the correct answer, or is neither correct?

    1. Both batteries contain the same amount of engery and both will propel the same car the same distance, just the 3s battery has the ability to get the car there faster (becuase it will turn the motor faster). The 2s has two cells each containing 2500 mAh when combined total 5000 mAh. The 3s has three cells each containing 1667 mAh when combined total 5000 mAh.

    2. The 3s battery contains more energy and will propel the same car a further distance. The 2s has two cells each containing 5000 mAh when combined total 10000 mAh. The 3s has three cells each containing 5000 mAh when combined total 15000 mAh.

    Thanks for your help.
    _
    Stampede 4x4 VXL; Slash 2wd VXL; MJ Grinder VXL

  2. #2
    RC Turnbuckle Jr. JimmyNeutron's Avatar
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    Neither are correct.

    The only time mAh adds up is when cells are in parallel. If cells are in parallel voltage remains the same. Because of this, both instances have contradicting information within. You cannot have a 2S pack if each cell is in parallel... same with the 3S. If you want to add capacity, the packs would be 1S.
    Whatever it is I just typed... could be wrong.

  3. #3
    RC Competitor
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    Thanks, I think I figured it out (but I still can be wrong):

    SIMPLE ANSWER
    The simple answer is that the 3s 5000 mAh battery can produce 50% more energy than a 2s 5000 mAh battery. Thus, the 3s battery should propel a car 50% further than the 2s of equal mAh (all other variables being equal). In practical terms, the 3s battery will last 50% longer than the 2s battery if the car with the 3s battery is driven as if it had a 2s battery.

    COMPLICATED ANSWER
    "mA" is the measure of current that is being delivered (i.e., the number of electrons moving through the gate at any time). The higher the “mA”, the more electrons that are moving through the gate at any time. “mAh” is the measure of that current over a 1 hour period. Volts is the measure of the strength of the current (i.e., how fast those electrons are going through the gate).

    The greater the number of electrons moving through the gate (i.e., the higher the mAh) and the faster the electrons flow through the gate (i.e., the higher the voltage), the more energy is produced.

    Thus, “mAh” is useful in comparing two batteries of equal voltage. The battery with the higher “mAh” has more energy as it can deliver more electrons over a 1 hour period than a battery of lesser “mAh”.

    But when two batteries have different voltages, “mAh” alone is no longer a valid basis upon which to compare the two batteries. Rather, consideration needs to be given to voltage.

    The following formula indicates which of the two batteries will produce more energy:

    Watts = Amps x Volts
    -or-
    Watt Hours = Amp Hours x Volts
    Watt hours = the number of watts produced over 1 hour.
    Amp Hours = the current in amps produced over 1 hour (1 Amp = 1000mA)

    Thus, the formula can be used to compare the total energy produced by two batteries whether the “mAh” is the same or different, and whether the voltage is the same or different.

    Applying the formula to the example:
    -the 3s battery (having a nominal voltage of 11.1) will have a watt hour rating of 5 x 11.1 = 55.5
    -the 2s battery (having a nominal voltage of 7.4) will have a watt hour rating of 5 x 7.4 = 37

    In order for a 2s battery to produce the same amount of total energy as a 3s battery, the 2s battery needs to have a current that is 50% greater than the current of the 3s battery:
    -3s 5000 mAh battery will have a watt hour rating of 5 x 11.1 = 55.5
    -2s 7500 mAh battery will have a watt hour rating of 7.5 x 7.4 = 55.5
    _
    Stampede 4x4 VXL; Slash 2wd VXL; MJ Grinder VXL

  4. #4
    RC Qualifier
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    Simple answer, its a mix of 1 and 2 although the logic to get there is wrong. The posts above to a good job of explaining the specifics.
    If you could magically drive the EXACT same with both batteries, the 3s will give 50% more run time as it has more watts available. However in the real world, the 3s batteries will give more speed/torque and slightly more run time.

    If you having difficulty choose lipos, base volt (cells) on how much power/speed/torque you are running. Base mAh on how LONG you wish to run between charges.

    In your example, if you were to find a 2s, 7500mAh lipo, it would have longer run times than either battery.

  5. #5
    Traxxas Employee TireSlinger's Avatar
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    I've found my vehicles tend to run shorter on 3s vs 2s packs of the same capacity. Several factors I think contribute to this:

    As electronics heat up they become less efficient, 3s produces significantly more heat.

    As the vehicle increases in speed the load on the system increases. It takes more energy to fight wind resistance, tire resistance, rotating resistance of bearings / u-joints, etc. at 60mph than 40 for example.

    I tend to be more aggressive when running 3s, knowing the truck has massive power on tap. Just like a real car, harder acceleration uses more juice. Also, braking uses power since the motor is used to stop the vehicle and it doesn't put power back into the battery. So stopping from 60 takes more energy than from 40. I've found I get much longer runtimes and cooler temps if I coast as much as possible rather than slamming the brakes.

    Just my two cents. I don't have any hard data to back it up ...
    Last edited by TireSlinger; 02-19-2014 at 07:02 PM.

  6. #6
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    Thanks, this is good info for us noobs. 😀


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    4x4 Pede VXL Brushless, 2S LiPo. Bought Feb. 2014

  7. #7
    RC Turnbuckle Jr.
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    TireSlinger posted the correct answer for the OP question.

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