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Thread: amps & volts

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    Default amps & volts

    i thought volts were like the power & amps how long it lasts, wrong?

    i have 5 rc car batteries here

    say one is 4000mah with 11.1 volts

    another is 5000mah with 7.2 volts

    will one be more powerful or last longer?
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    Depends for what you use them.
    You are comparing apples with oranges so to speak.
    With the higher voltage the general power is higher for the 11.2V battery but you can not use it to power a 7.2V RC car.
    In perfect conditions you can say one delivers 4amps for an hour before the voltage goes down to unhealthy values, while the other delivers 5amps for an hour but at a lower voltage.
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    ok, so amps are good, rc car

    although i disagree on the volts
    wen i was young i had a cheap 12v car (1 square 9volt + 2 aa batteries
    you could fit 2x 9 volts in it & rig the connection.
    it use to fly
    till it burnt the motor out
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    +1 for DU post#2

    Phil you need to match the voltage of the battery with that of the motor & or controller of the RC toy.

    Then you choose how many mah you want, which roughly gives you;
    More mah = longer run time.
    Less mah = less run time.

    Then there is the C Rating (eg;c5 or c25) this gives you a rating of how much power you can draw in a specified time without causing a premature low voltage or overheated battery.

    C5 is for slow draw of power, C20 & above for aggressive usage of power in short bursts without overtaxing the battery.
    Hope that helps.

    Note if you are using LiPo (lithium polymer) batteries there are some precautions you should be aware of.
    Read my thread,
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    Quote Originally Posted by Philquad View Post
    ok, so amps are good, rc car

    although i disagree on the volts
    wen i was young i had a cheap 12v car (1 square 9volt + 2 aa batteries
    you could fit 2x 9 volts in it & rig the connection.
    it use to fly
    till it burnt the motor out
    Yep that's exactly what will happen if you power a 7.2v RC toy with a 11.1v battery as per post #1.

    It will appear to be more powerful & go faster until the motor or other circuit burns out, that is only if it doesn't burnout the power distribution board of the toy first.
    Could last a month or a minute, you a gambler?
    Last edited by Tiny; 08-01-15 at 06:25 PM. Reason: Dislexic numbers
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    haha, that was a $20 car in 1980

    i scored a hpi e-firestorm flux for $200
    says 11.1 volt 5000 mah battery as a option so? comes with 2x 11.1 lipo, 2 x 7.2 volt nicads
    it flys
    i had to buy a $70 stick controller, lucky just swapped the receiver in the car, old servo's fitted it
    & its got emh? speed contoller & a traxx extreme battery charger
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    In terms of the maximum energy available from the 2 example (fully charged) batteries:
    4000mAh 11.1V = 44.4 watt-hours
    5000mAh 7.2V = 36 watt-hours

    But the actual peak power each can deliver and time required to recharge them comes down to the type of cell. Lithium cells are 3.7V each so the 11.1V pack sounds like 3 lithium cells which have superior characteristics to NiCd or NiMH which I suspect the 7.2V pack is made of IE 6 x 1.2V cells.

    A motor designed for the 7.2V pack would be a nominal 6V motor so running it flat out on 11.1V will cause its early demise (at almost double the design voltage). The type of speed control will determine how long it lasts.
    Last edited by Skepticist; 09-01-15 at 11:13 AM.

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    Looks like a fun toy.
    Looking at the specs for the ESC (electronic speed controller) for that car, it can handle both size batteries.

    You will need a fairly high C rating on the battery used.

    It's also programmable; which means you need to set it for the battery & motor size used.



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    Smart toy too


    The motor cables would have to be substantial to handle those quoted currents without melting the insulation but probably not a problem with NiMH which wouldn't be able to supply that sort of current without cooking.

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    Quote Originally Posted by Philquad View Post
    i thought volts were like the power & amps how long it lasts, wrong?
    i have 5 rc car batteries here
    say one is 4000mah with 11.1 volts
    another is 5000mah with 7.2 volts
    will one be more powerful or last longer?
    Ok... we need to start from the top.
    The first is voltage. Given the two batteries are different voltages, you're basically dealing with slightly different things.
    And, there are things you've forgotten about. Starting with the motor.

    So... lets fix a value for the motor. Lets just say it's a 1 ohm load.
    Now if we connect the motor to each battery. (V^2/R)=P and V/R=A
    [2S] (7.2)^2/1 = 51.84 Watts 7.2V/1R = 7.2A
    The other battery
    [3S] (11.1)^2/1 = 123.21 Watts 11.1V/1R = 11.1A

    Now for the "Storage" which is in amp hours.
    [2S] (4000mAh)=4Ah/7.2A = 0.55hours or 33 minutes.
    [3S] (5000mAh)=5Ah/11.1A=0.45Hours or 27Minutes.

    So the answer is the the 3S can produce more power for a shorter period than the 2S.
    Assuming your motor can handle the 2S and the 3S lipo's.
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