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Time of charging

  1. Feb 27, 2013 #1
    Can I ask how long a dynamo of 12 V, 6watts to charge a 12 V, 100 Ah battery?
    And by connecting the battery to a air compressor working on 1.2 hp and 12 V, max amp draw = 18Amps, how fast the battery will become flat?
     
  2. jcsd
  3. Feb 27, 2013 #2

    NascentOxygen

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    Sounds like your little dynamo puts out ½ amp. You will need more than 12V to fully charge a 12V battery, but let's assume your dynamo can deliver 13.5V because ratings are often conservative. (Otherwise, you'll have to step the voltage up to 14V by some means.)

    To deliver 100 ampere-hours at just ½ ampere will take 200 hours, but battery charging is nothing like 100% efficient, so realistically you'll be looking at around 300 hours to fully charge it, give or take a day or two.

    That's a lot of peddling, if this is the dynamo on a pushbike. Have you got some really long downhill runs that you can take it on? :smile: Perhaps mount it on an exercise bike?
     
  4. Feb 27, 2013 #3
    I just search a 12v 55A dynamo, is it workable by human peddling on bicycle?
     
  5. Feb 27, 2013 #4

    NascentOxygen

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    I used to find that a 6V 1A dynamo slowed me down and made it significantly harder to pedal my bicycle. A 55 A dynamo would require an engine or a windmill to turn it at the required speed. You'd be looking at at least 5½ hours to fully charge that battery at 55A, though that may be too high a current for best efficiency. 12V batteries come in various types, optimized for different applications.
     
  6. Feb 27, 2013 #5
    So now i need to charge a 12v battery in a shorter time by bicycle dynamo, any ways can make it not longer than one hours?coz my battery later need to supply a 12v load...any better suggestion to make my design feasible...coz i juz left this electrical prob...thx!
     
  7. Feb 27, 2013 #6

    sophiecentaur

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    Only for a few seconds, unless you are super super fit. That's nearly 1HP!!!! You would have no power left to cycle anywhere. 50W would be quite hard work for continuous pedalling. You need a hundred hamsters and a hundred wheels.

    I reckon you would be better to use a hand compressor in the first place - far fewer losses that way.
     
  8. Feb 27, 2013 #7

    NascentOxygen

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    Sorry, can't be done. If it were possible, we'd have solved the world's energy needs!

    Of course, you could discard your big 12V car battery and substitute small flashlight cells (rechargeable ones). These could be recharged in one hour by a bike dynamo, but they hold far too little energy to drive your compressor.

    Would solar cells be practicable?
     
  9. Feb 27, 2013 #8
    My design shud be human power..mon is due date ady...i still haven solve the problem..now i chg my compressor to a 12v portable air compressor...i found a hub dynamo that can output 50 v and 35 amp, can i directly connect it to the compressor to drive it directly?
     
  10. Feb 27, 2013 #9
  11. Feb 27, 2013 #10
    And add a voltage and current regulator to ensure that my air compressor will not burn...is it feasible? coz i cant find any ways to charge a battery in short time also =(
     
  12. Feb 28, 2013 #11

    NascentOxygen

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    That's an interesting motor. Are you thinking of propping the bike up and using it as a stationary generator? I don't think you have any chance of pedalling it to many amps, not even briefly. I don't know anything about hub motors, so I don't know whether it is DC or AC, but I see it needs a controller. You would have to do a lot of homework to determine whether it could be made to function as an efficient generator. If you could get it to work, I'm sure you couldn't connect it directly to your 12V compressor. A regulator would in theory allow it, but you aren't going to have enough power to be operating a compressor. Does the compressor say how many amps at 12V it draws?

    I think you'd be lucky to be able to power a fish tank air bubbler using pedal power.

    Maybe someone else can cast a more rosy light on your plan?

    As sophiecentaur suggested, a manual air pump sounds more feasible.

    Good luck with your project!
     
  13. Feb 28, 2013 #12
    Nascent, I suspect it's somewhat worse than your estimate. As with charging a capacitor, the higher the charge of the battery, the slower it will charge. If we look at this as a typical RC charging problem then R can be determined by the short circuit current which I believe is 0.5 amps resulting in the resistance of 24 ohms.

    Farads is equal to the number of coulombs needed to raise the voltage of a capacitor by 1 volt. A coulomb is an ampere per second so the equivalent number of farads of a 100 AH battery would be (100 AH * 3600 sec/hr)/12 volts = 30,000 Farads. One RC time constant is equal to 30,000 * 24 ohms or 720,000 seconds. For 5 charging time constants we're up to 3,600,000 seconds. Dividing by 3600 to get the charging time in hours results in 1000 hrs. charging time. This of course is assuming 100% efficiency.
     
    Last edited: Feb 28, 2013
  14. Feb 28, 2013 #13
    Finally I use a (12V, 4.5 amps air compressor), (lifepo4 battery 12v 2ah, max charge current 3.5A), a (12V 3A continuous duty Permanent Magnet DC Generator), hardly match a 35 mins pedaling to charge the battery. It compensate 25 mins of relax when air tank discharge and drive my air muscle, the 0.8bar output gas pressure acts a car turbo for car, so i think it is pretty much for my bicycle.
    I think some technologies can be use to refit these all devices, but these are what i can find from internet sources, hope prof will examined my design proposal from a highly-advanced technological view.
    Thanks ur help
    I really appreciate it=)
     
  15. Feb 28, 2013 #14

    NascentOxygen

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    It's not clear what you need the air pressure for, in your homework. But if it was for something like inflating a dozen bike tyres in some remote location, I was going to suggest that you consider taking along a well-inflated car wheel and bleed off the air from that. Would save a lot of trouble. :smile:
     
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