How to use an Electric Motor to recharge a battery.

  1. Does anyone know how to use an electric motor to charge a DC battery? I read on that all you need to do is find a way to spin the shaft and it produces electricity that will charge the battery? Is that true, is it that simple?
  2. jcsd
  3. Electric motors aren't good for that because the voltage is tiny. Motors are high current low voltage transducers, so you'll have to spin it VERY quickly to charge a battery.
  4. Pengwuino

    Pengwuino 6,942
    Gold Member

    In theory it's quite simple. Typically a battery spins a motor using the Faraday effect. If the battery is rechargeable, you can basically put the process into reverse and spin the battery to put energy into the battery.

    However, like Curl implied, the practical issues at hand may be too much.
  5. Here is why I ask the question. I am building an electric bike from random parts and I have 2 permanent magnet motors. So I was thinking if I were to hook up the second motor as part of the gear system I could use it as a trickle charger. Does that make sence? It would be spinning around 900 rpm would that be fast enough to do anything?
  6. No. Your bike would be better if you left all that off. It will be lighter.
    You won't gain anything from your charger (are you thinking of recovering energy when going downhill?)
  7. uart

    uart 2,776
    Science Advisor

    An electric motor can certainly be used as a generator (or alternator depending on the motor type) to charge a battery, this is how "regenerative braking" works on electric vehicles.

    Your idea of adding a second motor to act as a full-time generator while leeching power from the main motor is completely without merit. Say for example that your "generator" was producing 1.0 Amps into your battery, then the primary motor current would increase by about 1.2 Amps (or more) just to overcome the extra drag caused by the generator. There is no way around this, the efficiency of both machines is than unity.
    Last edited: Oct 1, 2011
  8. vk6kro

    vk6kro 4,058
    Science Advisor

    One problem with regenerative braking is that the motor which normally drives the wheels of the car produces less voltage than it was getting from the battery when it becomes a generator.

    Theoretically, it would be possible to use a voltage boost circuit to step up the voltage from the generator to a high enough voltage that you could charge the battery with it. Boosters like this are available on Internet's Ebay.
    This is just an example. It needs at least 10 volts in to produce up to 35 volts out.

    To do this, you would need to disconnect the battery from the motor.

    This would require a switch that was controlled by the mechanics of the machine.

    An example of this would be the chain on a bicycle. If the chain is loose, it loops downward at the bottom when it is being driven by the rider, but it becomes loose at the top and tight at the bottom if the back wheel is driving the pedals. (Modern bikes make this impossible, but old, direct drive bikes used to do this.)

    So, you could have a toothed wheel pressing on the chain and operating a switch if the chain is pulled tight. This could then switch in the voltage booster which could charge the battery.

    Note that this is discussion material only and not a how-to-do-it guide.
  9. NascentOxygen

    Staff: Mentor

    Incredible! Whatever will they invent next? :wink: :wink:
  10. DaveC426913

    DaveC426913 16,537
    Gold Member

    Concur with uart.

    Regenerative braking, maybe. In theory.

    But you cannot try to recover energy while your motor is trying to move the bike. The recovery process will put an additonal load on the battery, leeching off it, draining it as fast as (actually faster than), you can recover it, netting you nothing (actually, a loss).

    Classic perpetual motion error.
  11. Im not planning on using charging the battery solely with the generator. Im thinking that since I have the extra motor and the space I could probably use it to be a trickel charger. Not as the permanent charging device but just to extend the battery life while riding.
  12. NascentOxygen

    Staff: Mentor

    The efficacy of regenerative braking (or simply charging a battery) depends on the terrain you typically expect to be riding on. If it comprises long flat stretches, and you are energetic and would welcome putting some extra effort into your peddling, then go for it. Similarly, if it involves long downhill glides where you need to wash off speed anyway, again, go for it. But if you are peddling around town, stop, go, up short hills, then down a bit, repeatedly, you won't have energy to spare. You'll want the downhill stretch to take a breather and gain a bit of speed to get you up the next rise. (I'm picturing "battery-assist" bike riding, here.)

    From some years back I recall that regenerative braking on electric cars typically extended their range by merely 10%. All that extra gadgetry, expense, and weight for a miniscule gain meant it was not worth the effort. Hopefully, maybe it is not so uneconomical today.
  13. NascentOxygen

    Staff: Mentor

    If you have some way to mount a solar array, that would give you trickle charge, both while riding and while parked! Providing you were out on sunny days.
  14. I don't know what a "trickle charge" is, but however much extra you will charge the battery, that's at least how much extra the battery will drain in order to charge itself.

    Runnign this proposed process:

    Battery chemical energy > electrical energy > mechanical energy > electrical energy > battery chemical energy

    is clearly not as efficient as just leaving the battery chemical energy alone.
  15. DaveC426913

    DaveC426913 16,537
    Gold Member


    You are trying to use the energy in the battery to recharge the battery. You gain nothing. Period.

    Unless ... plan to recharge while under pedal power with motor off. That would work - you'd be using human muscle power to recharge the battery.

    But let me tell you, there's nothing that sucks more than trying to ride a bike that has a generator attached. It is hard work. The bike will not coast; it will slow to a stop rapidly unless you work to keep it moving - and that's tiring. Defeats the purpose. (Ultimately, what what is the point in an electric bike if it is actually powered by you? Skip the whole contraption and use the bike as a bike.)
  16. vk6kro

    vk6kro 4,058
    Science Advisor

    When the vehicle is moving and you have to stop, you can use conventional brakes and burn up the kinetic energy in heat, or you can use the kinetic energy to run the motor as a generator and use the power generated to charge the battery.

    Nobody is talking about perpetual motion. This technique is standard in electric and in hybrid cars.
  17. sophiecentaur

    sophiecentaur 14,712
    Science Advisor
    Gold Member

    In addition to all the theoretical objections to some of what you are planning, there is a practical issue. The motor /generator you want will not be designed the same as the motor you have. You can buy bikes which charge a battery to help you up hills but their motors are specially designed. I think you would find one of them (plus a good battery and the right control circuitry) may be as expensive to buy (one off) as buying one of those fancy bikes already made up for you.

    What's wrong with hanging onto the back of a truck to get you up hills anyway? haha
  18. It's not so clear to me that this discussion is not about perpetual motion..unless the term "trickel charger" has some special meaning that I don't know about.
  19. sophiecentaur

    sophiecentaur 14,712
    Science Advisor
    Gold Member

    The OP seemed to imply it but the conversation has gone beyond it without shooting anyone.
  20. NascentOxygen

    Staff: Mentor

    I used to ride a bike with, for its time, the standard dynamo friction-driven against the side of the rear tyre. 6vac 1.2amp it was rated, and on callow fingers would give a fair tickle.

    I knew when it was operating that I had to put more effort into peddling, but it wasn't a showstopper. (Except when I came to really steep hills.)

    How does the OP view 6 vac at 1amp for his application? (You did say "trickle" charger. :smile: )
  21. sophiecentaur

    sophiecentaur 14,712
    Science Advisor
    Gold Member

    The Dynohub was a bit better than that. Not much friction loss - pretty well perfect for a cycle lamp. There days, they could be a lot better, with the posh magnets that are available. You would need to find a way of making it into a motor but the gearing (1:1) would make it pretty unsuitable.
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