Charging 12vdc battery with Bike Generator Hub

Discussion in 'The Projects Forum' started by Justinwat, Mar 11, 2010.

  1. Justinwat

    Thread Starter Member

    Dec 15, 2008
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    I am powering a bike stereo I built with a LiFePO4 12.8V 6.8Ah battery. (http://www.batteryspace.com/lifepo4...wh25arateinaluminum-boxwpcbandt2terminal.aspx)

    I would like to buy a bike generator hub to slowly charge the battery while I ride to get more power and time from it. http://www.sheldonbrown.com/harris/lighting/shimano.html or the DC equivalent: http://www.bikewagon.com/Wheel-Goods/Hubs/Front-Hubs/PedalPower-Red-DC-Output-p7819760.html

    Questions:
    What would the output of the generator hub need to run through before connecting to the battery? The charger I have for it obviously uses 120VAC power, is there a charger that I could buy or make that could work with 6VDC?

    Is this worth it? Can a 6V generator provide enough power for a worth while trickle charge?

    Would this damage the battery? The battery itself is pretty smart and I don't know if it was made to be charged this way.

    Thank you all so much for reading this. Many of you have helped me in the past with other 12V project problems and I really really appreciate it.
     
    Last edited: Mar 16, 2010
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    sorry buddy, In order to charge a 12V SLA, you need at least 15V minimum @ at least 5 amps to get a decent power to charge ur battery.

    If you plan to use a 6v hub then the hub should be at least capable of supplying more than 10 amps. but for this you need to built a dc to dc converter.
     
  3. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    The best hub dynamos (like Schmidt) are rated about 6V 3W.

    You can use a voltage doubler rectifier to get over 12V; basically two half wave rectifiers for opposite polarities.

    The actual voltage will depend on the load, as alternators tend to give higher voltages at low currents. You are still limited to the 3W rating, or whatever the device is made for.

    You could trickle charge the battery, but forget going via 120V it would have to be something suitable for around 12V or 6V input.
     
  4. Justinwat

    Thread Starter Member

    Dec 15, 2008
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    Thanks guys!

    So if I was able to get the voltage higher does anybody know of a charger that could be bought or altered to work with this kind of power supply?
     
  5. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    first thing is to get a voltage and from that we can find the charger
     
  6. retched

    AAC Fanatic!

    Dec 5, 2009
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    If you had a trike, and used 3 hubs, you may be able to keep the battery charged.

    If you used a lead-acid battery there would be a way. but in order to use Li-poly, you will need a smart charger IC that will also draw some of the power from the hubs.

    It might take you 24 hours for a charge, if you didn't use the radio.

    A stronger dynamo would be the way to go. Opposed to the hub, if you mounted one to behind the seat, and ran a chain to the dynamo, you could produce the power...slowly.
     
  7. Justinwat

    Thread Starter Member

    Dec 15, 2008
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    So they do make 12v Dynamos. If this can generate enough power to work with a charger how would I make it work with a smart charger? http://www.peterwhitecycles.com/dymotec.asp

    I have a 400 watt DC to AC inverter in the stereo for an outlet on the side and I use the built in USB to charge my iPod while riding. I don't know if this would help the situation.

    (forgive me I don't know many of the technical terms for electronic components, just of their existence.) If I had something that only allowed a full 12v at a higher amp from the Dynamo to pass through to the inverter and then had my smart charger hooked up to the AC output and that to the battery would that work? Would the inverter itself eat up too much power for it to be worth it?

    Can anyone think of a way to use straight 12VDC with a charger that will work with this battery?

    Thanks again!
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    One horsepower = 550 foot-pounds per second = 745.7 Watts

    If your 400W inverter is 90% efficient, it takes 444 Watts from the battery.

    An average healthy human can produce 1/10 horsepower indefinitely.

    How long do you think you can continue to lift 55 lbs per foot in 1 second?

    That's just to power a generator that won't come close to keeping up with your inverter. You'll still need to provide additional power to move yourself on the bike.
     
  9. retched

    AAC Fanatic!

    Dec 5, 2009
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    You may want to look up simple windmill charger circuits. There are many online that use a dynamo. You can simply build the "windmill" with the bike wheel as the propeller. That will give you a small charge that will keep the battery topped off. You will need HUGE legs to power the 400w inverter. So you should stick with DC all the way.
     
  10. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    And start dead lifting as well
     
  11. Justinwat

    Thread Starter Member

    Dec 15, 2008
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    Thanks Retched. I could also use a low RPM permanent magnet motor like they talk about using for homemade windmill generators.

    So I have several ways of generating 12VDC or more. Now the tricky part: How do I put that power through a smart charger or is there another way to get that juice into that battery?

    http://www.batteryspace.com/lifepo42...2terminal.aspx

    Thanks Sergeant and Rif!
     
  12. retched

    AAC Fanatic!

    Dec 5, 2009
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    Absolutely. Get yourself a battery charging IC. That will take care of everything.

    You will want to use the IC opposed to rolling your own, there is just too many variables involved for it to be economical to build. So you can use the PM DC motor, and a voltage regulator IC, and charger IC.

    That is the safest way with lithium based batteries, in my opinion.

    Here is a single cell charger from Maxim-IC you can usually get 4 cells to an IC, but you will see what is packed into the one chip.
    http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2217
     
  13. trader007

    Active Member

    Feb 27, 2010
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    to be honest, i dont think its really feasible to charge li-ions this way (what retched posted would probably work, but the general idea of powering the charger from a bike just isnt ideal by any means). basically all you can do is trickle charge it and have a voltage cut-off at what you need, but it would take a long time to charge. much longer then you probably ride your bike. if you tried charging the pack at .5C or more, and the charging is turning on and off alot because youre starting and stopping the bike, the battery pack's hysteresis will prevent the charger from monitoring it properly.

    with lead-acid batteries, its pretty hard to screw up. you really dont even need a charger with them, just limit the current to 14v/2a or so and youre set...
     
  14. retched

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    Dec 5, 2009
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  15. Justinwat

    Thread Starter Member

    Dec 15, 2008
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    So here is what I've decided. Tell me if it works or not. I'm going to connect a small 12V 3.2AH Lead Acid battery (http://www.batteryspace.com/sealedleadacidbattery12v33ahs.aspx) to my LiFePO4 12.8V 6.8Ah battery in parallel. (http://www.batteryspace.com/lifepo42...2terminal.aspx)

    I will send the 12v generated by the bike dynamo directly into the lead acid battery to charge it as I ride.

    Side note: When I use too many of the electronics at the same time everything shuts down and the voltmeter reads 0.00. I think the LiFePO4 battery has some kind of over load protection built in. So I think that means I need to add more Amps into the equation which is why another small battery would help.

    This way I can get more power and store the power I generate from riding!

    Question: Will I need something to block the voltage from the generator from going into the LiFePO4? This is a pretty smart battery, much smarter than I so it might have something in it already to prevent damaging it in a parallel situation. Does anyone know if this is the case?

    Thank you all for your help so far, I think we are almost there!
     
  16. SgtWookie

    Expert

    Jul 17, 2007
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    Even if a charging system itself were 100% efficient, you lose somewhere between 20% to 30% of the power used for charging, as the chemical process for lead-acid batteries is in the range of 70% to 80% efficient. You would be better off using the power you generated directly, and use the battery power for when you are riding slowly or stopped.

    You won't be able to produce much of a charge for the batteries while you are riding and using electricity. You simply won't be able to generate the amount of power necessary.

    I tried to explain this previously.
     
  17. Justinwat

    Thread Starter Member

    Dec 15, 2008
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    Nooooo! This breaks my heart. Sorry it took me so long to realize it's an unrealistic dream.

    One more question: So if I was to use the power directly from the generator to power the stereo is there something that exists that would switch the power over to the battery when I slowed down and the voltage dropped? If there is such a thing would there be a brief delay in the power to the amp during the switch that would make the music cut out for a second. (I need to do some more research)

    Is there something that could do this or I could make (in kit form) so my dream can live on?

    Thanks.
     
    Last edited: Mar 19, 2010
  18. Justinwat

    Thread Starter Member

    Dec 15, 2008
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    So after a little searching this is what I found:
    http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1142,C1079,P2220

    or maybe something as simple as this (without the charging)
    http://www.ezacdc.com/Scripts/prodView.asp?idproduct=106

    Or!!!! Could I just put diodes on the battery so that the power from the generator wouldn't affect the battery that way I could just connect the battery and generator output directly to the amp! This would make the battery last much longer because the generator would help with the load...right?
     
  19. SgtWookie

    Expert

    Jul 17, 2007
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    Right - diodes would work.

    Schottky diodes have a low Vf (forward voltage). However, you would need some rather large diodes, since you seem to be using quite a bit of power.

    Let's say you could divert 1/20 horsepower to use for power generation. That's 37.3 Watts, and at 13v that's 2.87 Amperes - if the generator/alternator is 100% efficient. However, they aren't that efficient - it really depends upon what the individual rating is. Some of the power is dissipated in the bearings of the generator. Some is used for cooling. Some is overcoming the air resistance from the turning rotor. Some is dissipated as heat in the windings. If an alternator, a good bit will be lost in the bridge rectifier. So, just as a guess, you might get 60% out for the power you put in, or about 1.72A out.

    Actually, if the generator is an alternator, you probably won't need diodes, as an alternator would have a bridge.

    If a DC generator, you probably would need a diode between your generator and electrical system.
     
  20. Justinwat

    Thread Starter Member

    Dec 15, 2008
    17
    0
    I'll probably buy this cheaper Dynamo. http://socalicustom.com/bikes/product_info.php?products_id=2034&language=1
    There are no specs available.

    Should I put the diodes on both the battery and generator to be safe or just the battery or vice versa?

    Do you know what kind of diodes I would need?

    Battery: 12.8VDC Ah6.8
    Generator: 12VDC 6W

    Thanks again. I know I said that would be my last question but I'm so close to understanding what I need to make this happen, so close!
     
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