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Help- using NiMh batteries for high load applications (150A peak)

  1. Oct 17, 2011 #1

    taylaron

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    Gold Member

    Greetings, I’m constructing a machine that involves a linear actuator powered by a 12vdc motor. My linear actuator has a foot that crushes an aluminum soda can until the motor’s torque maxes out and it stalls. The machine is intermittent duty at about 6 seconds of use every 3 minutes. My machine is powered by a 110vac 60hz outlet. The motor I’m using has the following specs:
    http://www.robotmarketplace.com/products/0-PDX16.html
    Speed : 1500 rpm @ 12Vdc
    Amps @ nominal: 1.5 Amps
    Efficiency: 45.33%
    Peak Power: 0.55 hp
    Stall current: 148 A
    Stall torque: 1466 oz-in

    Due to the very high stall current of my motor (148A), my budget prevents me from using a 2,000 watt AC-DC power supply.
    **I don’t understand why the rated peak power is 0.55hp (410 watts) does not correspond to the calculated peak power at stall torque of ~2,000 watts (148A * 12v = 1776 watts).
    I presume my only other alternative is to use batteries capable of that kind of output. I cannot afford LiPo batteries and they are also dangerous. Unless anybody can suggest an alternative power source, I have resolved to use NiMh or NiCd cell batteries. However, I am concerned about the lifetime of NiMh batteries in this solution. I was hoping this machine to be service free for a few years.
    I’ve posted this because I’m having difficulty finding an alternative power source. Also, because I’m having difficulty calculating how many amp-hours I need to have while being able to recharge my battery before the next motor cycle. Also, can anybody tell me if NiMh batteries can supply 150A peak current safely without damaging the battery?. I intend to automatically disconnect the charger while the machine is running, then automatically reconnect as soon as it has stopped drawing power.

    The automatic battery charger I would like to use is this:
    http://www.robotmarketplace.com/products/0-DYN4077.html [Broken]
    manual: http://www.dynamiterc.com/ProdInfo/Files/DYN4077_manual.pdf
    Specs: dynamite prophet sport II AC/DC peak charger.
    • 1,2, Amp charge currents for AC input
    • 110-240vac input
    • 1-8 cells (1.2v to 9.6v) ***Does this mean the whole battery pack has to be between 1.2v and 9.6v or is this the voltage per cell? Because my battery pack needs to be 12v
    Battery Capacity Charge Rate Charge Time*
    1500mAh 2A, 40–60 Min.
    3300mAh 3A ,60–80 Min.
    5000mAh 4A, 60–80 Min.
    * Charge times based on fully discharged
    batteries.

    Thank you for your time,
    -Taylaron
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Oct 17, 2011 #2

    jim hardy

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    2016 Award

    is this a hobbyist project or something you intend to mass produce?

    for the former i would suggest a $25 lawn tractor battery , look in battery section at walmart.
    they're good for a couple hundred cranking amps read label on side.
    People use them for the trolling motor in small boats so in sporting goods you can get a nice plastic battery box for it.

    and to keep battery charged,
    an old fashioned battery charger the kind with a transformer that weighs about five pounds.
    They use a ferroresonant transformer that is inherently current limiting so it wont hurt anything to leave it connected as your machine goes through its cycle.
    Tractor Supply sells the Schauer 6 amp for about $26 last time i looked, 4 amp a bit less.

    """**I don’t understand why the rated peak power is 0.55hp (410 watts) does not correspond to the calculated peak power at stall torque of ~2,000 watts (148A * 12v = 1776 watts).""

    horsepower is what the machine delivers out the shaft
    and at stall the power out is zero
    because
    horsepower = 2*pi*torque*rpm/(550*60) , when rpm is zero so is horsepower out.
    That 1776 watts input is just heating your motor so dont leave it stalled.

    i'd measure current and stop motor before stall.
    you'll need a stout switch to interrupt that stall(or near stall) current, an automobile solenoid perhaps.

    have fun!
     
  4. Oct 18, 2011 #3

    russ_watters

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    Staff: Mentor

    Looking at the problem from another angle: if you are only trying to crush a can in 6 seconds, your motor is at least an order of magnitude larger than it needs to be.
     
  5. Oct 18, 2011 #4

    AlephZero

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    Homework Helper

    When the motor is stalled, it is not rotating so it is delivering zero mechanical power. (Power = force x velocity, or torque x RPM) The 1.8 kW is just being dissipated as heat in the motor.

    You don't want the motor to be continuously stalled for any length of time, unless you want to use it to start a fire. So you don't need a power supply that can provide 1.8 kW continuously at 12V. You want a supply that will limit the output power to a lower level (e.g. about 400W if you need to use the peak power output, but I would guess you don't need that much power to crush a can in 6 seconds), with an automatic power cut-off if the motor is stalled for longer than say 1 second.
     
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