Voltage,amp,wattage,range relationship

Discussion in 'Electric Bicycles' started by dougsr.874, Nov 16, 2011.

  1. dougsr.874

    dougsr.874 Active Member

    Obviously, from the question I am a novice with electric hub motors. My question is: for the most range with SLA batteries, what is the best size kit to buy. I understand that the more watts the higher speed..whats best all other circumstances being equal...a 500watt motor on 36 volts , or a 500 watt motor on more or less volts...I did a search but found nothing...

  2. loquin

    loquin Active Member

    P = I x E
    P = Power in Watts,
    I = Current in Amps, and
    E = Voltage​

    For a given wattage, the higher the voltage, the lower the current.

    This means that, with all other things being equal, if the system/motor wire gage is the same, the I*R heat losses will be less, and the system efficiency will be higher. If, on the other hand, the motor/system is sized with a smaller gage of wire, so that the percentage of power loss is approximately the same, the system will be lighter (there's less copper) and with less weight to haul around, the efficiency will be greater.

    Also, any semiconductor forward voltage drops (silicon diodes have a 0.6 volt drop, for instance) will be a lower percentage of the system voltage, again raising efficiency by a bit.
    Last edited: Nov 16, 2011
  3. jaguar

    jaguar Well-Known Member

    I had an electric bike. More voltage gets you better power and speed but the range depends on the rating of the batteries. One way to get more range is increase the voltage but don't ride full throttle.
  4. Ypedal

    Ypedal Member

    Voltage determines motor RPM, a specific motor will spin faster with higher voltage...

    so a 36v kit, meant to do 20mph in a 26" wheel, will go roughly 25 mph at 48v

    Amps determine how much pull / acceleration ability / total wattage your kit will consume..

    Volts x Amps = Watts

    a kit has a " controller " that allows you to use a throttle to regulate your speed, this controller has a fixed " Max Amps " setting that it will not permit the motor to exceed.. most are 20 amps max..

    so in this example, 36v x 20 amps = 720w peak power, this is roughly called a " 500w " kit.

    Take the exact same kit, and run it on 48v, you get 48v x 20 amps = 960w peak power.. some vendors will call this a 1000w kit.. while others call it a 750w kit... varies a lot.

    Most hub kits that run on SLA *( sealed lead acid ) should be using either 7ah or 12ah bricks... i highly recommend avoiding 7ah, they are too small to deal with the load.. get 12ah bricks ( also available are 10ah .. )

    Lead acid batteries are rated, for example 12ah, to deliver 12ah over a 20 hour period.. but on a bike you drain it in under an hour and at this faster discharge rate they can only deliver half their rating.. so you only get 6ah of range from a 12ah battery.

    Lithium, Nicad, and Nimh dont suffer this effect ( Peukert effect ) .. these chemistries deliver their rated capacity ( or close to it ) wether you drain them over 20 hours .. or 30 minutes.. weigh less.. and are smaller in size..
  5. HeadSmess

    HeadSmess Well-Known Member

    nicd and nimh have "self discharge" and will lose a large percentage of charge just sitting there.

    great for an electric bike with big currrent demands, some idle time, big current demands, etc. or cordless power tools, where they can go on a charger every night.

    theyre easily charged.

    and discharged :) this is important.

    lithium polymer and ion batteries have huuuuge current capacities, with pretty good shelf life, but have very complicated wiring systems because each cell in a pack needs to be individually charged (or balanced) else they EXPLODE!!! :) (poly more so than ion)

    so as the voltage goes up, with 3.7 volts per cell, the wiring and charging gets proportionately harder...

    same time, you only need half as many cells as a nickel based type of the same voltage.
    (and really, even nickel packs should be balanced...they never are)

    lithium being the third element is extremely light as well.

    the lithium types also cost more.

    they also cant be discharged beyond a certain point so require a fancy expensive speed control with BEC, otherwise, you guessed it...they EXPLODE!!! :D

    for now id stick with nickel based types. cheap, easy, do the job and are a good entry level product...

    sla will only bring you tears in the long run :( may seem cheaper but they add up after being replaced a few times...

    sla despise being discharged too quickly, they despise being charged too quickly, they just dont cope with huge current demands.

    yep. chemistry :)

    zinc, copper and salt water ;) thats the way to do it ;)
    Last edited: Jan 4, 2012
  6. Ypedal

    Ypedal Member


    Nimh/ nicad = 1.2v per cell

    Lithium- Ion ( they are all lithium ion ) ..

    - cobalt / manganese = 4.2v per cell
    - LiFePo4 ( iron phosphate ) = 3.2v per cell

    Each behave differently during charge and discharge.

    The foil pouches used on RC trucks and planes refered to as LiPo, are mostly cobalt types, and yes they demand proper charging know how.. not for everyone but once you go lipo.. you never go back..

    instead of writing a huge long post.. just see:

    this thing weighs 13 lbs. .. 48v 15ah

    The equivalent in SLA would weigh well over 50 lbs ! and be HUGE>....