Electric motor + 12 volt battery ?

  1. I admit it, I am a complete and utter noob when it comes to electrical components. I need help. I am trying to run an electrical motor on a regular 12 volt car battery. Can this even work? If so, how much money and work needs to be done?

    This is for a little..."project". If anyone can give me a hand or point me in the right direction, I would appreciate it.

    Koop
     
  2. jcsd
  3. Integral

    Integral 7,351
    Staff Emeritus
    Science Advisor
    Gold Member

    You have not provided sufficient information for anyone to help you.

    As long as you have a DC motor there should be no problems. Please provide more information. What is your power source? What motor are you driving? What is the project?
     
  4. Well, I am trying to test out a motor run on rechargeable power (basically a fully electric car), but only using the battery to "start" the motor...if that makes any sense.

    I would like to use a motor that is large enough to provide enough force to the wheels of my project cart, but can still be started by a regular 12 volt battery.

    I apologize for the lack of better information, but, my motor will depend on whether or not the battery (or some home modifications) can actually "start" it.

    Koop
     
  5. chroot

    chroot 10,426
    Staff Emeritus
    Science Advisor
    Gold Member

    You don't "start" electric motors. You have to provide them a continous source of power, i.e. current.

    Your motor should be designed with a specific supply voltage in mind -- if you overpower it, you could well destroy it. A 12 V car battery can actually supply an incredible amount of current, and it could be dangerous.

    - Warren
     
  6. Right. Yeah, that makes sense.
     
  7. Cliff_J

    Cliff_J 789
    Science Advisor

    koop - check out eletric golf carts as a source for common (and therefore typically cheaper with higher production and competition) source of parts like motors, gearboxes, controllers, and so on.

    At only 12V you have a large restriction in terms of delivering power to/from the battery because as current flows you end up with a lot of wasted energy as heat that is the square of the current times the resistance of the wire. Low resistance means big wire and big wire means lots of money and weight, bad for an electric car project.

    The hybrid cars like a Toyota Prius or Honda Civic operate in the hundreds of volts for their battery packs but since this is a dangerous level of voltage (lethal!!) there is a lot of protection involved in case of an accident and so on.

    But golf carts work at like 24V-36V and this is easy to achieve at roughly the same size and weight as a car battery by using something more like motorcycle batteries instead.

    There was a documentary about making a drag racing electric car and the guy featured has his own website where you can buy parts. His personality made the show amusing but not too educational, especially when it had so many mechanical problems. The website below links to his, and a google search will find many more.

    http://www.suckamps.com/
     
  8. didn't he break an axle?
     
  9. Cliff_J

    Cliff_J 789
    Science Advisor

    Well, as I recall he broke off the key that tied the pulley to the shaft, and then after the pulley was welded to the shaft he shredded the belts. Impressive since that size of timing belt can drive a supercharger that might require 100HP. Not so impressive he thought he could run a 2+ ton car with that much weight from 40 batteries into the 12s with that setup.

    After I watched the show I went to his website and he had put up an update that they converted the belt/pulley over to triple chain/sprocket drive and that broke a half shaft. Again not too impressive that he's welding up Subaru parts and trying to make them work, used Corvette half-shafts or even better 3/4 ton Chevy pickup front half-shafts are much more heavy duty out of the box. Neither would be easy to adopt since he has already setup the car with the Subaru suspension, but even with a really tough half-shaft the CV joints are going to be the weak link.

    http://www.suckamps.com/index.php?page=life_of_the_postal_van

    Not trying to be overly negative but going quick in a drag race isn't new, why deviate so far from the norm and overstress parts?

    Now, the old RX-7 they have is quite impressive, its suppossed to run 11s. Its in the top pictures:

    http://www.suckamps.com/index.php?page=build_team_vehicles

    He does have a challenge on his hands to beat any production car though, the Enzo and Carrera GT run low 11s.
     
  10. Motorcycle batteries? Got it.

    That site is great, and the store has some great stuff in it. Thanks for pointing that out, I have been looking for days for a motor, but all I could find were industrial sites.

    Thanks again,

    Koop
     
  11. Cliff_J

    Cliff_J 789
    Science Advisor

    Well I'm not saying to go run out and buy 3 motorcycle batteries instead of 3 car batteries to get your 36V - they're almost similar in price but their size and capacity is very different!

    For a small electric car of short range, the motorcycle/golf cart batteries would be fine. In fact, the golf cart batteries are likely the deep-cycle variety. Deep-cycle means the batteries are designed to last after being drained, a regular lead acid battery looses capacity after it has been run down. In fact, run it dead more than a few times and a regular car battery won't hold much of anything.

    So rather than just looking at the CCA (cold cranking amps, how much current at something like freezing and 7V) you should be looking at the ampere-hour rating and only at deep-cycle batteries unless you plan on using them for only a short period of time.

    Once you determine about what you need after compromising on size/weight/duration/capacity/cost then you have a few batteries to select from like some for electric wheelchairs or golf carts or trolling motors on fishing boats etc. The latter are likely the cheapest for the money at quite a few stores, the others are more specialized and will cost more per ampere-hour.

    Its all compromise koop, just try to find the best choice. :smile:
     
  12. Right, that makes sense.

    I do have two deep cycle batteries...but they weigh about 30lbs each, and are for our travel trailer.

    I think motorcycle batteries sound like the way to go.

    Now for a potentially ridiculous question:

    Can I hook up an alternator to the shaft of the motor? :confused:

    If that is a retarded question, try not to laugh too hard.

    Thanks for your help so far.
    Koop
     
  13. Is it possible? Yes. Will you gain anything? No. The power required to drive the alternator will come from the battery. Alternators are not perfect devices---they have both electrical and mechanical sources of power loss or rather some of the energy put into the alternator is converted to heat. Additionally, hooking your little electric motor to an alternator will increase the load on the motor which increases the current requirements of the motor which increases something called the "I squared R losses". The motor will convert more of the electric energy from the battery to heat energy than it would without the alternator. Essentially you will be adding more heat to the environment with the alternator than without---and heat equals power lost that you could have otherwise used to scoot around with.

    [edit] If you want to return some of the energy to the battery then you should look into regenerative breaking.
     
    Last edited: May 17, 2005
  14. That's what I was looking for, to a point...

    EDIT:

    (What if) I use small solar cells for that task (I know, energy would be used faster than converted by the panels, but it could possibly lengthen battery life per charge, right)?
     
    Last edited: May 17, 2005
  15. Yeah, you can use solar cells. You'd need enough cells to supply sufficient current to be useful. Matching solar cells is not easy or cheap either. You can't simply group 10 solar cells together and expect to charge your battery.

    Just to give you an idea of the costs involved in using solar cells: http://www.engin.umich.edu/solarcar/donate/buycell.html

    The kind that you would need would not be cheap at all. You can't get these at radioshack.
     
  16. I was thinking more along the lines of multiple cells that are smaller, rather than a large, single cell.
     
  17. That's not a large single cell. That cell is about 1" by 2". You can find lower quality/effeciency cells for $6-10 a piece but you still can't connect 10 and expect them to work. Cells do not all produce the same voltage for a given amount of sun light so connecting an unmatched pair means that one of the cells becomes a load on the other. Cells are usually grouped in small matched arrays and each array is connected to a grid. The arrays and the grid all have little controllers to prevent a single cell from becoming a load on the others thus releasing the black smoke the manufacturers installed at the factory. Aside: all electronic parts are made with a small puff of stinky smoke inside. If you do something like overload an electronic component then you let the factory installed puff of smoke out=>the component wont work anymore.

    You'de be better served looking into simple regen breaking rather than trying to make a solar car IMHO.
     
  18. Point taken.

    I was thinking more along the lines of that in this little drawing of said "car".
     

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  19. Cliff_J

    Cliff_J 789
    Science Advisor

    IF they can be properly setup (no releasing magic smoke as faust9 pointed out) they would assist a little yes, but the cost is typically prohibitive.

    As said, the alternator is a bad idea and would only waste energy.

    And as far as rengenerative braking is concerned, from what I've read its not a very efficient method of energy capture. Maybe if you could just always find a way to go downhill...because otherwise it will take far more power and create more losses going up the hill then could ever be recovered braking on the other side.

    Here's enough info to keep you busy for a while:
    http://www.4qd.co.uk/faq/index.html
     
  20. I read in a DOE report, just a few minutes ago, that regenerative braking is only 4% efficient.

    Good site, thanks for pointing it out.

    Koop
     
  21. Another few things you'll need to consider if you get serious about an electric car are the materials you build out of (light as possible yet strong), aerodynamics of the vehicle (wind resistance obviously), and rolling resistence of the tires (the bigger the contact patch for control/braking the more rolling friction).
    I remember back in the late 70's there was either a Popular Mechanics or Popular Science article about building a 3 wheel 2 seat car (possible source reference for you). You could power it by gas or electric.
    I found it in 1984 in my H.S. electronics class. I remember thinking why not both? A mostly electric powered car with a gas generator that kicks in to power the vehicle and recharge the batteries when necessary.
     
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