Li-ion powering a DC motor

Discussion in 'General Electronics Chat' started by gigabyte, Aug 12, 2010.

  1. gigabyte

    Thread Starter New Member

    Aug 30, 2009
    2
    0
    Hello, I'm new here. I was searching for a circuit to power a dc motor with a li-ion battery. I tested the dc motor with 3.7V DC form an ac-dc adaptor (3A) and it works...but when I try to connect the li-ion direct to the motor it don't starts and the voltage drops to 1.2V. I was thinking for using current limiter, MOSFET, or something else but didn't find out how to do it.
    Can anybody help me please?
    Thanks

    PS: The DC motor is from this grass shears:
    http://www.keengardener.co.uk/light...products/EdgingShear.jpg&width=230&height=220
     
  2. bradstormer

    Member

    Aug 6, 2010
    65
    1
    maybe a simple voltage buffer circuit might do it. an example of a voltage buffer would be just an operational amplifier with a gain of 1. check out this from wikipedia for more circuit information http://en.wikipedia.org/wiki/Buffer_amplifier...
     
  3. gerty

    AAC Fanatic!

    Aug 30, 2007
    1,153
    304
    What kind of battery did you use? You'll have to give us more than
    What voltage is the battery? Is it old and dead?
     
  4. pilko

    Senior Member

    Dec 8, 2008
    213
    20
    Check the battery.
    -- Does it have suitable voltage and current rating?
    --Is it fully charged?

    pilko

    oops crossed with gerty
     
  5. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    If the battery is not dead, then try recharging it with Li-on Charger first and then try.
    a 3.7V li-on should be able to run ur motor for quite a while.
    Even after charging the voltage drops below 3.7V at load then the battery is dying.
    On the other hand if the cells AH capacity is rated at less than 1000mAh, then it won't last long or may even not start a 3A motor.

    That motor will run best at 2Ah cells. Found in laptop battery packs. Try to get a laptop battery and rip it open. Inside you will find plenty of cells that would do your job. Does not matter if the battery is dead. Some cells inside will work fine once recharged using a 3.7V Li-on charger
     
  6. gigabyte

    Thread Starter New Member

    Aug 30, 2009
    2
    0
    Sorry...the battery is brand new...charged...3.7V...and I thing it is size DD (cylinder)

    The problem is that something in the grass shears burned and it didn't worked any more and the battery becomes dead....so I decidet to buy new battery and to do a simple circuit to power the motor. I didn't know what burned in the original circuit...it is SMD and is complicated circuit.
     
  7. sceadwian

    New Member

    Jun 1, 2009
    499
    37
    It sounds like the battery monitoring circuit for your device has short circuited internally. DO NOT use lithium batteries in this device again until it has been replaced by a qualified person, you're lucky so far that you haven't had a fire.

    Do NOT try to hack this device to work from a lithium battery yourself, again you'll either risk a fire or you'll simply go through batteries all the time.

    Lithium batteries can not be discharged bellow approximately 2.7 volts or their lifespan will drop dramatically, if they're left in this state for any length of time they will become 'dead', there is nothing that can be done about this aside from having circuitry that will turn the device off before this occurs. This is what the SMD board in your device did.

    Without knowing what you're doing continuing to play with your device is going to coast you time and money and leave you with a device that still isn't functioning.
     
  8. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    There are several writeups out there on "The care and feeding of LiIon batteries" and they are best aid attention to. Amid a lot of other variables they will last the longest if stored at only around an 80% charge and in the refrigerator.

    Anyhow, now that we don't throw as many parties anymore since we've become serious the extra fridge in the garage is good for something again. :)

    They have a limited life then they are usually internally disabled by some complex circuitry. Most monitor charged and discharged voltages along with charging rate curves but others actually just have a counter that keeps track of the number of charge/discharge cycles.

    They are without a doubt the highest energy level per weight batteries on the market but the technology still has a ways to go.
     
  9. bradstormer

    Member

    Aug 6, 2010
    65
    1
    why not use a Ni-Cad battery, they are far more forgiving than Li-Ion batteries...
     
  10. sceadwian

    New Member

    Jun 1, 2009
    499
    37
    They also contain toxic metals, 4-7 times less energy by the same weight, and have a memory effect problem.

    Although I do still wonder about the long term effects of the use of lithium in batteries... It's a very active metal, environmental impact in a throw away society is a little on the unknown side.
     
  11. sceadwian

    New Member

    Jun 1, 2009
    499
    37
    Weiiiiiiiiiiiiird....
    Looking up the typical energy density of lithium ion vs NiMh batteries I found.
    http://en.wikipedia.org/wiki/Energy_density

    Apparently an efficient steam generator is actually higher per weight than NiCad =\
     
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