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Using a DC Motor as a Generator

  1. Sep 19, 2016 #1
    So I'm working on a senior design project and was trying to use a DC motor to charge a battery. The point is to allow for quick use to charge a phone or a laptop in circumstances where you need a quick charge. We have lead-acid batteries connected in series and parallel to give us:
    14,000mAh with an output of 13.2V and 15.2A
    I was wondering what specifications we would need on the DC motor to allow for adequate charging. I'm a mechanical engineer and much of this is beyond my basic knowledge.

    From my understanding, correct me if I'm wrong please:
    We need a voltage of at least 13.2V coming out of the motor, not its rating, to begin charging and the current needs to be 1/10 the total capacitance to charge in 10 hours. If the DC motor were to be connected to a crank to spin the motor we would need to match the rated RPM's to produce the rated current and voltage of the motor (ignore efficiency for the moment). If the motor is geared so that the crank's desired RPM's were 60 then the torque required to turn the motor would also increase. I'm not sure exactly what torque is considered easy to hand crank but I assume anything less than 8ft-lbs would be decent.

    What kind of DC Motor (Brushless, PM Brushless, etc), what Voltage and Current does the Motor's rating need to be to allow for ANY charging, and what ratings would be best for charging?

    We're having the battery connected through regular switches to prevent unnecessary discharge and have a switching regulator connected to a USB Outlet to allow for USB charging. We also have a 300W inverter to allow for traditional wall outlet charging.

    Side Note: Since the motor will be hooked up directly to the battery it is my understanding that a zener diode(help please?) needs to be wired in to prevent from the motor to spin when not cranking. Input on anything is greatly appreciated. If any of this is overkill, inefficient, not enough, or just wrong please correct me.

    Thank you, Chuck
  2. jcsd
  3. Sep 19, 2016 #2


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    Hi ChuckStr. Are you planning to hand crank a generator to charge your lead-acid batteries so they can recharge the phone's lithium battery?
  4. Sep 19, 2016 #3
    Yes, we have to worry about cost more than anything.
  5. Sep 19, 2016 #4


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    Those are vehicle lead-acid batteries, it seems. I think the most practiceable way to manually recharge a phone battery would be to use a commercial hand cranked generator, or for regular use something like a pedal bicycle turning a generator. It is not efficient to charge batteries which are then used to charge another, you are likely to run out of puff before the job is even half done.

    Are you wanting to fully charge the phone battery, or just deliver enough charge to make a quick emergency call? What exactly is the situation you are planning for? —there may be alternative solutions.

    The lithium batteries in phones can be recharged in much less than 10 hours, but you must use the right charger.
  6. Sep 19, 2016 #5
    There is also a wall charging system to get a full charge, but the crank is used in case of emergencies. We are encasing it inside of a toolbox that way if a car battery were to die in a camping situation you could use the batteries to charge the phone. Or if the battery dies, then you can crank it to provide enough energy to charge a phone or small device.
  7. Sep 19, 2016 #6


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    Last edited by a moderator: May 8, 2017
  8. Sep 19, 2016 #7


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    The 10 hr rate would be 1.4A, but would take rather more than 10hr to charge and you would need to reduce the rate later.
    If your battery is suitable (consult manufacturer's data) you could charge at a higher rate (maybe 2 or 3 hr). You would need to be more careful about accurately timing/controlling the charge rate. If the battery was charged from partial discharge, care is also needed.
    The voltage needed for charging is more like 13.5 - 14V, maybe higher for fast charging.

    So the power you need is about 1.4A x 14V, around 20W.
    8 ft lb of torque gives about 60W at 1rev per second, so you'd have plenty of power. (If you can persuade someone to keep turning it for several hours?)
    A WP article says, "adults of good average fitness average between 50 and 150 watts for an hour of vigorous exercise. A healthy well-fed laborer over the course of an 8-hour work shift can sustain an average output of about 75 watts." But arms are probably lower power than legs, so I'd guess the lower values would apply, unless you use a cycle arrangement.

    I do like NascentO's idea of just charging your small Li batteries directly on demand. You could probably get away with higher human power output for a short time, fast charging a smaller battery.

    As a mech. eng. I suppose you're mainly interested in the mechanical design & manufacture, but maybe also, ergonomics, project management, research skills, ...?
    Maybe you just want the electrical design off the shelf, but constrained to be a DC motor and whatever sort of storage battery you've been given?
  9. Sep 19, 2016 #8
    If size is a must, you can use one of those brushless motors used in RC cars that came with neodidium magnets. They will give AC current so to convert to DC you will need a tree phase rectifier made up from 6 power diodes. A reduction gearbox can help to produce high rpm from the crank to the motor. Once you have a DC voltaje upper than the volts needed to charge your batteries, a voltaje regulator will be required. Take a look of cheap solar charge regulators.
    hth (hope this helps).
  10. Sep 19, 2016 #9


    Staff: Mentor

    To avoid damage, phone and laptop batteries must be charged slowly, no matter how much power is available. That is what a charge controller circuit does. My phone requires 2 hours for a full charge of an empty battery.

    Your OP is confusing. I interpret it to mean that you want to charge the phone battery, not the 12V lead-acid battery. I don't know why you mentioned the 12V lead-acid battery at all, if your project involves only the generator and the phone.

    To charge a phone, your generator will have a 5V USB output. See here for USB power specifications:
    However, you could probably power several USB ports in parallel from a pedal driven generator.

    Charging laptops directly from DC is difficult. Different brands of laptops use different DC voltages. The usual solution is to generate 120VAC and plug the laptop's charger into that.

    Good luck on your project. Take this advice from an engineer. "Make sure that the requirements are clearly written before thinking about design." In other words, define what it must do before worrying about how to do it. Maybe you could try again on this thread. Post a statement of the requirements of your project.
  11. Sep 19, 2016 #10


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    If I remember correctly a rare earth motor will typically have a lower motor constant than a ferrite. This may reduce the gearing up you will need to do to get from hand cranking rpm to what the motor needs.

    I think I would start by experimenting with a few cheap toy motors around 12-15v. Use one to spin another and measures the rpm and voltage with the generator motor connected to a suitable load. That should get you data on how fast the motor has to be spun to generate the output required. Don't worry if you can only get 12v motors to experiment with, if you just spin them a bit faster they should cope with generating say 14v.
  12. Sep 23, 2016 #11


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    You will want to use a schottky diode. An ideal diode circuit would be better but is more complicated and probably no suited to your project unless you need to maximize efficiency.

  13. Sep 24, 2016 #12
    Definitely most efficient to use an AC generator. A small permanent magnet BLDC motor could do the job with the correct gearing and motor Kv. For rectification smart diodes eliminate the rather significant losses of Schottky diodes at higher currents; http://www.ti.com/lsds/ti/power-management/oring-and-smart-diodes-overview.page

    Just about all Li-Ion batteries can handle a 1C rate which is a one hour charge time. High drain Li-Ion batteries can accept higher charge rates, but they are not typically used in consumer electronics. So for example a 2 Amp-hour battery can accept a 2 Amp input for a one hour charge. Though 2 Amps at the typical 19V for a Laptop computer would be quite a bit of power required by human input. For a phone that typically uses a 5V source to charge that would be a lot less. Still a lot of power for a hand crank, but probably do-able.
  14. Sep 24, 2016 #13


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    They are very different requirements. Phones all (for the past few years, at least) require the 5V from a USB output and have their own internal charging circuit. It would be best to have a 5V 'Power Bank' for that purpose. Laptops vary, I believe but the required DC voltage is much higher than 5V.
    Hand of foot cranking is very hard work and whatever system you choose has to be pretty efficient without any intermediate voltage changer. Even carrying two separate systems would involve less hard work in the long run than a voltage change.
    Did you consider a flexible PV panel? Mounted on your backpack all day, it could probably give you enough for emergency use. Do you really need a laptop - rather than a low power tablet? Time was when people just didn't need a computer when they were wild camping.
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