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Which battery is suitable?

Discussion in 'Power Electronics' started by DanielAlmeida, Jan 20, 2015.

  1. DanielAlmeida

    DanielAlmeida

    13
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    Jan 20, 2015
    Hey everyone.
    As i previously said in my presentation, i'm a tattoo artist. One of The annoying things of The trade is when the machine cable stands in the way, rubs on people's skin, makes a bad connection. I feel like, with so much delevopment going on, this would be a nice thing to see solved.
    However, i dont know how i would power an engine remotely and make it run max. 8hours without charging. There is also the battery weight issue.
    I wonder if anyome could give me any hints, best suitable battery to power an engine that runs at 7 to 14 volts for thet period of time.
    Thank you in advance, hope someone finds they can help on this one.
    Best regards
    Daniel
     
  2. BobK

    BobK

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    Jan 5, 2010
    Are there commercially available battery operated tools? If not, I would think that means it is not feasible. If there are, what type of batteries do they use?

    In any case Lithium batteries have the highest energy density of any common batteries, so they would give you the lightest and smallest battery you could get to do the work. If you used removable batteries you could go for a shorter run time and swap batteries every 2 hours or so while additional batteries are charging.

    Edit: To size the batteries we need to know the current drawn by the tool.

    Bob
     
  3. DanielAlmeida

    DanielAlmeida

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    Jan 20, 2015
    Hello Bob, thank you for your answer. There are none that i'm aware of. I dont think it's not feasible, i believe it can be done with positive results. However, i know i'm not the first to have the idea. But i want to test it, and see if it is viable.
    The possibility of multiple batteries that charge while one is being used is the route i've thought to follow if i cant get a decent autonomy from one battery.
    The battery would have to power a 4.5 watts engine that would run fom 7 to 12 volts ideally.
    Hope this info is enough..
     
  4. BobK

    BobK

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    Jan 5, 2010
    Hmm. Are you sure it is only 4.5W? That should be no problem. 4.5W at 12V is 375mA. An rechargeable AA cell has 2000mAH so it would run up to 5 hours (realistically 3-4). It would take 10 of them to make 12V (rechargeables are 1.2V.)

    Lithium batteries such as 18650s can be found with up to 5000mAH. Three of them would give you 11.1V and should last more than your 8 hours.

    Given this info, I am very surprised no one has made a battery operated tool.

    On second thought, 10AAs or 3 18650s is a lot of weight to be holding continuously for hours. How much does a corded tool weigh?

    Bob
     
  5. davenn

    davenn Moderator

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    Sep 5, 2009
    Seeing the tattoo machines in use at the local place, they have quite a substantial power supply for them
    I too would be surprised if it was inly 4.5W --- 45W would have been closer
    you sure it wasn't 4.5 Amps @ 7 - 12 V ???? that would also be more likely

    please check and confirm the requirements

    The real problem I see is weight .... the machine hand unit already has significant weight and
    adding a battery pack to that is going to increase the stress on your hand and arm to support it whilst doing delicate drawings

    Dave
     
  6. DanielAlmeida

    DanielAlmeida

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    Jan 20, 2015
    Bob, thank you again.
    There are two types of machines, coil operated machines with a typical iron frame, and more recently rotary machines, powered by an electric engine.
    The weight of the machine i'm currently using is under 200g, i wouldnt want to increase it too much. I dont know the weight of the engine, and the frame would have to be of a light material.
    But those are good informations!

    Hello Dave, thank you for your participation
    The tattoo machines are connected to a DC power supply where the voltage is ajustedaccording to the functio .The power is sent to the machine trough a pedal that works as on/off. I dont know the watts of the power supply by memory. I know the engine of my machine is 4.5W, and runs between those voltages.
    As you said, it's not practical nor comfortable to have a lot of weight sitting on my hand for hours as i do detailed work.
    Im absolutely oblivious about battery types so i dont know which ones i could use. I though about cell phone like batterys, but dont know if they ate pouerful enough.

    Thank you alot, both of you have been very helpful!
     
    Last edited: Jan 20, 2015
  7. DanielAlmeida

    DanielAlmeida

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    Jan 20, 2015
    I have a question, i havent quite understood how the power of the battery translates in time. Is it related to the amps?
    Edit: read some info online, then Bob's info again, let me see if i am right.. A 4,5w engine running at 12V 'wastes' 375mA per hour? And if so, what does the 'm' stand for?
     
    Last edited: Jan 20, 2015
  8. DanielAlmeida

    DanielAlmeida

    13
    1
    Jan 20, 2015
    This is what i understood: 4.5w at 12V rate uses 375mAh.
    To cover this, i combine 3 18650's that result in a total of 11.1V and 7200mAh. With this battery combo i can power the engine for 19.2hours, am i correct?
    The thing is, it has a lot of amps, more than needed although the more autonomy the better.
    I cant produce more volts than the total of the 3 batteries combined, can i? I'm just wondering if i could compromise battery autonomy ( hours) to produce more voltage..
    Am i talking nonsense? Sorry mates, i never studied this :s
    Thank you!
     
  9. BobK

    BobK

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    Jan 5, 2010
    mA is a measure of current. This is an instantaneous value that changes with the load.

    mAH is a measure of how much current can be supplied for 1 hour. This is a measure of battery capacity.

    Theoretically, a battery that has 1000mAH can supply 1A (1000mA) for 1 Hour or 100mA for 10 hours. But in real life it is different. The ratings really only apply at some specific discharge rate (i.e. mA). When lower or higher than that the capacity is actually reduced. So your 1000mAH battery may well be able to supply 100mA for 10 hours but only supply 1000mA for 1/2 hour. There are special batteries that are rated for high discharge rates, often used on battery powered model aircraft for instance, but your application (if 4.5W is correct) should not need these.

    Another thing to consider is that when you put batteries in series, their capacity in mAH does not add up. The capacity remains the same as a single cell. So if you have 3 18650s that have 2400mAH, they battery that consists of 3 of them in series still has 2400mAH, not 7200mAH. It sounds like you are losing something here, but you are not. The batteries in series are putting out 3 X the power of a single battery because power = current x voltage.

    Hope this helps.

    Bob
     
  10. DanielAlmeida

    DanielAlmeida

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    Jan 20, 2015
    That was a great lesson, thank you once again!
    I undestood the first part, just got a little curious about why does the same battery discharges at different rates and not proportionally...
    As for the second part, i dont know if i got it right. If so, how does it translate to my case.
    You said previously that 3 18650's would supply for more time than i needed, but not how i calculated it. However, the volts add up. You say that the batteries running in parallel supply power all at the same time, instead of what i previously thought, like after o e is used it passes to the next, wich after thinking about it, doesnt make sense because that way, voltage would decrease after the first went off.
    I read an analogy about this regarding a water hose, that when added pressure at the exit, water comes out with more power. Does it apply to this case? If so, how does the increase in power by 3times reflects on performance? Does it make the engine run faster? That way, would i not need so much voltage to achieve as many rpm's? What i mean is does it compensate the voltage?
    Best regards
    Ps: you guys rock, im registered in some forums from other categories and its hard to find such helping people so thank you!
     
  11. BobK

    BobK

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    1,662
    Jan 5, 2010
    At any given voltage your motor is going to spin at a certain speed.

    Batteries in series: the voltage adds but the capacity in maH does not.

    Batteries in parallel: The capacity in maH adds but the voltage does not.

    If you use 3 batteries in parallel, the motor will run at the same speed, but for 3X as long as if you used one battery. But in this case, it is usually better to just use one larger battery.

    Also, beware of 18650 batteries bought on Ebay. I bought some cheap ones, and they had nowhere near the stated capacity, more like 1/10 of it. Expect to pay $10 a piece for genuine, quality 18650 batteries.

    Bob
     
  12. DanielAlmeida

    DanielAlmeida

    13
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    Jan 20, 2015
    I guess the solution im searching for then is connecting the batteries in series to add up the volts...
    About the batteries, any recommended brand?
     
  13. Gryd3

    Gryd3

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    Jun 25, 2014
    Would you be opposed to a small wrist-band that has the batteries fastened to it?
    You could then use a little coiled pig-tail to connect the gun to your wrist mounted power pack.

    One other questions.. if you currently have a pedal to control the gun, what will you substitute it with?

    Here is a good resource for 18650 battery comparisons : http://lygte-info.dk/review/batteries2012/Common18650Summary UK.html
     
  14. DanielAlmeida

    DanielAlmeida

    13
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    Jan 20, 2015
    Damn had a giant response, and my phone lost its battery.....
    Gryd3, thank you so much for your participation.
    One of the possibilities i thought about was ased on something i saw as a kid sitting by the TV at christmas time. It was the advertisement for a spiderman glove that threw web-like stuff, it had a can located in the inner forearm and a button on the palm of the hand. It could be a possibility because stress would be moved from the wrist to the arm, i just dont know how comfortable it would be...
    I still plan on using the pedal. Instead of sending power to the cable connected to the machine, it sends a signal to a bluetooth device that signals to the machine.
    Thank you for the link, i'll take a look at it!
     
  15. Gryd3

    Gryd3

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    Jun 25, 2014
    In addition to bluetooth, you could also look into using an 'XBee'. Keep in mind that using a radio in your device will increase the current draw and decrease the battery life. Of course there are lots of wireless options all with pros and cons but it MUST be reliable and tamper resistant... you don't want the gun doing anything unexpected.
     
  16. DanielAlmeida

    DanielAlmeida

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    Jan 20, 2015
    Cool looking small thing, just couldnt figure if the prices are for one of them or for the pair, antenna and receptor. I have to study thw best option, i need a simple device that sends/ receive only a I/O signal.
    Depending on the consumption of the device, maybe it could have an independent powering battery.
     
  17. Gryd3

    Gryd3

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    Jun 25, 2014
    Any transmitter / receiver pair will work for you but regardless of the device make sure you can make it reliable. The XBee products range from $25 to $60 depending on model (range, and other features)
    Some other devices labelled as a 'transceiver' will also work. They can communicate two-ways, but you only need to communicate one way.

    One more item to work out...
    Do you want your pedal to operate in a linear region? ie... can control a range of speeds from 0% to 100% by pushing a little harder or softer... or will is be a simple on/off switch?
     
  18. DanielAlmeida

    DanielAlmeida

    13
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    Jan 20, 2015
    Well, what is the name of both the bluetooth and xbee devices? Transceivers? Its easier for me to search for it. The 25$ is for the pair?
    You are right, i only need one way communication.
    The pedal is basicaly an on/off swich that do not require hands. The speed of the machine is regulated in the power supply.
    Perhaps i could add a little wheel to the machine to regulate the power...
     
  19. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    Look at sites like 'sparkfun' or 'adafruit' and you will find a 'wireless' section.
    You would simply be looking up rf transmitters and receivers. The parts sites will usually include a drop down or other selection box for what 'kind' of radio frequency you want. (Bluetooth, WiFi, ZigBee, etc.) Simply select which one you want and you get a new list with available parts.
    $25 for a pair will be a lower end RF pair, but will still work. A simple on off pedal will be easier to implement than a pedal that adjusts its speed like a sewing machine pedal.
     
  20. DanielAlmeida

    DanielAlmeida

    13
    1
    Jan 20, 2015
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