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Battery advice for size and weight sensitive project

Discussion in 'Power Electronics' started by Delx, Mar 15, 2017.

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  1. Delx

    Delx

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    Mar 15, 2017
    Hi All,

    I am building a project but am struggling with the power aspect of things.

    I have a micro fan but am not sure how to power it. It is 4.5V/5.5V with a current rating of 0.04A and I want to use the smallest and lightest batteries that I can get away with. My initial thought was to use a set of three 1.5V button batteries in series but their current rating is not anywhere near high enough.

    The circuit, for now, will be an extremely simple switch to power on/off the fan.

    I am fairly new to electronic project building and my battery knowledge is quite lacking so any advice, or a point in the right direction, will be greatly appreciated.

    If any other information would help just let me know.

    Thanks
     
  2. BobK

    BobK

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    1,686
    Jan 5, 2010
    Batteries are rated in maH or milliamp hours. That translates to the number of milliamps you can draw and expect the battery to last an hour. Draw half that, and it will last 2 hours, etc.

    Your battery needs to be at least 4.5V, so 3AAA or AA primary cells (non-rechargeable) would just suffice.

    AAA batteries are rated at about 1200maH. Since you fan draws 40ma that would translate to about 30 hours.

    Bob
     
  3. Delx

    Delx

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    Mar 15, 2017
    Thanks for the reply Bob.

    As my project is extremely size and weight sensitive, using three AAA or AA batteries is vastly too big. Also, as the draw from the fan is on 40mA and the power would need to last an absolute maximum of an hour, I am hoping to find a solution that would be much more compact.
     
  4. BobK

    BobK

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    1,686
    Jan 5, 2010
    Well, as you found, button cells are not designed for 40mA.

    Most primary batteries, in fact, are not designed to discharge over 1 hour or less.

    A lIthium Ion battery (like in a cell phone) would be the best bet. You can get industrial ones in practically any shape and size, and, of course they are rechargeable, and they are designed to discharge at that rate.

    The problem with these is that they are nominally 3.7V.

    Bob
     
    Delx likes this.
  5. Delx

    Delx

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    Mar 15, 2017
    Sorry, I didn't mean that it needed to last less than an hour, just that it wouldn't necessarily need to be any more than that.

    I will however look into the lithium ion batteries and see if I can find one that may suit my purpose.

    Thanks again for your help Bob.
     
  6. BobK

    BobK

    7,682
    1,686
    Jan 5, 2010
    No, you didn't. But if you want the smallest battery you can get, you would want one that lasts no longer than necessary.

    Also, the fan will not run directly off a LiIon battery. You would need a boost converter as well.

    What kind of size are you talking about? I think you might have boxed yourself into an impossible situation.

    Bob
     
  7. Delx

    Delx

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    Mar 15, 2017
    True :)

    I didn't know that about lithium ion batteries. I could probably deal with the size of a single AAA battery at most.

    This project is still in the planning stage so I know a re-think might be on the books.
     
  8. Alec_t

    Alec_t

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    Jul 7, 2015
    Can you test your fan to see if it will run satifactorily on 3.7V (or even less)? Try it with two AA cells, for example.
     
  9. Audioguru

    Audioguru

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    Sep 24, 2016
    A lithium-Polymer (Li-PO) battery is the same as a Lithium-Ion except it is smaller and lighter because its case is a plastic film instead of metal. Electric model airplanes and quad-copters (small drones) use a single cell that is 3V when dead to 4.2V when fully charged. Hobby stores have them in hundreds of sizes.

    Here is one that is very small and lightweight used in a small quad-copter. It is rated at 35mAh but can power your 40mA fan for almost one hour. It needs a circuit to detect low battery voltage then disconnect it. It needs a special charger that the hobby store will have:
     

    Attached Files:

    Delx likes this.
  10. Delx

    Delx

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    Mar 15, 2017
    It is en-route from China at the moment so will test this when it arrives.

    I will have a look into these specialist battery packs and see if I may be able to source something that would work. They definitely seem like the right kind of size for my requirements.

    I did know my project may require some extra circuitry and am not opposed to that as long as I can keep it all compact, so possibly focusing on model aeroplane/quad-copter equipment sounds like a plan as size and weight is obviously important to them as well.

    I may also look into a slightly different fan that can run on lithium-ion/lithium-polymer voltages if the 5V fan isn't going to be viable.

    Thank you all for your help and suggestions, they are all greatly appreciated and I have some directions to go in. A great introduction to a site I didn't know existed until yesterday! I will definitely be sticking around :D
     
  11. Delx

    Delx

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    Mar 15, 2017
    This looks like it might be exactly what I need if I want to use a 3.7V lipo battery.

    1S Lipo 3.7V to 5V Synchronous Step-up Converter Module

    I see that a lot of lipo batteries come with the under/over voltage protection circuitry integrated into them so as long as that is the case for the one that I use, I shouldn't need anything more than the step-up converter, should I?
     
  12. Delx

    Delx

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    Mar 15, 2017
    After some more research, I think I misunderstood the function of the circuitry integrated into the batteries and that they do not, in fact, protect against under voltage.

    I have been able to find a number of instructions on how to build a circuit to do this function but there doesn't seem to be any OOTB solutions easily available. Is this something that is not that common, or are people just building their own?
     
  13. Delx

    Delx

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    Mar 15, 2017
  14. Audioguru

    Audioguru

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    Sep 24, 2016
    Beware of Hobby King. Some of their Li-Po batteries are very cheap and seem to be missing any quality control or even any testing to see if a battery works. On airplane forums people say some of the Hobby King Li-Po batteries work fine, but only for 10 times instead of for hundreds of times like good batteries.

    Larger heavier batteries for phones and laptops have protection circuits that prevent over-charging and over-discharging.
    Smaller lighter batteries for airplanes and quad-copters do not have protection circuits, a circuit in the airplane causes the high current motor pulse then turn off to warn that the battery is low. But the battery still powers the receiver and servos so that the airplane can make a controlled landing. My quad-copters blink an LED and have a loss of power when the battery voltage is low, then the motors turn off but the battery is not disconnected.
     
  15. Delx

    Delx

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    Mar 15, 2017
    That is good to know about Hobby King, thanks. I will probably source the batteries from somewhere more reputable then. Any suggestions?
     
  16. Audioguru

    Audioguru

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    Sep 24, 2016
    I buy E-Flite and Thunder Power Li-Po batteries from my local hobby store. They are American companies, not Chinese. The Thunder Power batteries can supply pulses of current that are 140 times their mAh rating, or 70 times continuously. There is a Japanese brand of Li-Po that is recommended, but in Canada they are sold only by a Chinese guy with a price much lower than they should be (fakes?). Did you see this video?
     
  17. Delx

    Delx

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    Mar 15, 2017
    WOW that is crazy. Yeah there are a few things that I would not buy from eBay, and batteries is definitely one of them!
     
  18. tedstruk

    tedstruk

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    Jan 7, 2012
    Nothing new...
    Foreign countries have been doing this to US citizens for about 75 years...
    and they have petitioned the government to do something about it...
    as for your problem...
    Build an aircraft aluminum framed generator with an internal stator, and use some capacitors like spark plugs. Then use a smaller specially made battery with high tolerances for charge and discharge... or just use big capacitors and multiple light generators? A battery is really just a capacitor...
     
  19. Audioguru

    Audioguru

    2,864
    643
    Sep 24, 2016
    The voltage of a discharging capacitor drops quickly at first, then it levels out when it is almost dead. The voltage of a discharging battery stays up, then drops quickly when it is almost dead. Don't you want the fan to run properly?
     
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