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  1. #1
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    bike dynamo on a bike trailer

    I'm looking to try and put 2 dynamos (1 on each tire of a bike trailer) and i want to stay away from the friction style dynamo. They would be used to charge a car battery and operate/charge other small electrical devices. What I have been thinking about doing is using a 24 VDC motors and have then belt or chain driven from the tires. I am hoping someone out there has done something similar and might be able to give me some pointers on what to do and what to stay away from doing

  2. #2
    Senior Member metro2005's Avatar
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    If you want to charge a car battery using leg power you will have a hard time riding your bike. the human body generates between 200 to 400 watts of energy.

    To charge a car battery (say a 35Ah 12v battery = 420 watts) you would like to charge it at 10% of the power, so thats 42 watts of power, give or take a bit of energy loss during the charging process and you are looking at 50watts of power for 10 hours straight.
    To give you an idea: A standard bike generator generates 3 watts of power.

    It can be done but you would loose most of the power you generate in charging the battery instead of going somewhere or you would have to ride your bike for hours on end to charge the thing

    Not really a good option, You should keep it small (small 7ah SLA battery for example) or go with a solar charger for charging the battery, I personally would opt for a small solar panel with a small battery.

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    Senior Member Looigi's Avatar
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    Although some DC motors can be used as generators, they will be comparatively inefficient compared to a purpose built generator/dynamo or alternator. I also agree with metro2005, charging a car battery is a tall order.

  4. #4
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    i actually need to use PM DC motors as they will act as a brake assist for the trailers and the plan with the trailers is for camping gear and my daughter we plan on taking week long bike trips across the US so it would be biking a majority of the day and camping at night i am just wondering if someone has done a set up like this before having it be chain or belt driven vs. a hub or friction style

  5. #5
    Randomhead
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    you could probably use a hubmotor from an electric bike

  6. #6
    coprolite fietsbob's Avatar
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    most hub dynamos are 6v 2.4 or 3 watt, and made for 26~28" wheels ,
    there are some for smaller wheels , but folding bikes
    have a short 74 vs 100mm wide .. front axle width
    Oh and they are Alternators , put out an AC sine wave, frequency increases with speed
    so you need a rectifier circuit, to make it into DC, then, maybe charge a smaller amp/ hour
    Motor-scooter sized gel cell 6v , after a week of riding..

  7. #7
    Senior Member dougmc's Avatar
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    Quote Originally Posted by metro2005 View Post
    If you want to charge a car battery using leg power you will have a hard time riding your bike. the human body generates between 200 to 400 watts of energy.
    More to the point, a racer may average between 200 and 400 watts of energy produced during a race, but most normal cyclists don't produce that much energy. 100-150 watts is a more reasonable average for an "average, but serious" cyclist.

    To charge a car battery (say a 35Ah 12v battery = 420 watts) you would like to charge it at 10% of the power
    You butchered the units there, even if you ended up with the right answer. A 35 Ah 12v battery has about 420 Watt * Hours of energy. So if you wanted to charge it at a rate of C/10 (i.e. going from fully discharged to fully charged in 10 hours) that means 42 watts of power -- which is 28-42% of the power generated by an "average" cyclist (and that even assumes that everything is 100% efficient -- which is far from the truth.) Very few people would tolerate that sort of drag on their bike.

    Also, car batteries are designed for high discharge rates, not for high capacity. And leaving them discharged for any length of time wears them out. At the very least, you want deep cycle batteries, not car batteries, but considering that you're pulling this around with you, going with some sort of LiPo battery might be better.

    To give you an idea: A standard bike generator generates 3 watts of power.
    They're also only about 50% efficient from what I've heard, so they put 3 W into the battery (at around six volts, so you'll need to step that up) and draw about 6 W of power from the bike's motion (i.e. from your legs.)

    Not really a good option, You should keep it small (small 7ah SLA battery for example) or go with a solar charger for charging the battery, I personally would opt for a small solar panel with a small battery.
    Solar is a better option if you need high power. If it's on a trailer and you have a 3'x3' solar panel on top, with the sun beating directly down on it you could possibly get 100 watts out of that. (Of course, that's a maximum, requiring that everything be just right.)

    I'd suggest (to the original poster) to work out just how much power is needed and try to go with a much smaller battery. If you do want to go with hub generators, you'll need to bump up the voltage somewhat to charge a 12v battery, but that shouldn't be terribly difficult. I wouldn't suggest going with larger generators to get more power -- you'll be too annoyed by the drag caused by them, and they're also likely to be less efficient. Perhaps a solar panel (for when the sun is available) and one hub generator (when it's not.) Then you'll have enough to keep your phone and lights charged all the time, and run your laptop for a little while each night if you've been riding all day. (Not that I know what you want power for, that's just my guess.)

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