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Thread: Hub dynamos

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    Hub dynamos

    Hi all,
    I'm in the process of building a custom trailer for my bike and would like to run 4 hub dynamos to a central point to charge batteries or run a fridge for example. The dynamos are 5v3w each. The trailer has 4 wheels for stability and length. What do you think would be the best corse of action with regard to the connection. Could I connect the 4 in parallel to create 20v12w or would it be better to charge 4 batteries and power the devices from them?

    My bike already has a hub dynamo connected to my tablet and phone.

    Any and all thoughts would be appreciated.

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    Quote Originally Posted by Tsmoreland324 View Post
    . . . Could I connect the 4 in parallel to create 20v12w or would it be better to charge 4 batteries . . .
    Not exactly. Driving loads directly is better than via batteries.
    BTW, ever seen a 12w refrigerator?

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    First of all it would depend on whether the direct output is AC or DC. Multiple DC power sources are easier to integrate, while AC id much more complicated because there are phase considerations.

    Even DC isn't that easy if there's any difference in voltages.

    I'm sure there is circuitry that would make combining multiple dynamos possible, but it's not a plug and play deal.
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    What situation are you in with a bike and a working fridge paired together? Seems odd.

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    Maybe the OP is tired of drinking hot water from his water bottles.
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    Consider the drag of running 4 dynamos, plus the weight of the fridge. That's a lot of pedaling for a cold beer.

    I'd look at solar panels myself.

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    The Improbable Bulk Little Darwin's Avatar
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    There is a forum here that specializes in electronic questions... there may be someone there who has tried to combine dynamos.
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    afraid of whales Mr IGH's Avatar
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    Quote Originally Posted by Tsmoreland324 View Post
    ....Could I connect the 4 in parallel to create 20v12w or would it be better to charge 4 batteries and power the devices from them?...
    How many watts and is what input voltage for the fridge?

    You'll need to find someone with advanced electronic skills, this isn't trivial. The hub dynos are AC with no way to synchronize.

    OK, the coffee's starting to work. Here's what I'd do if the fridge is 12Vdc:

    Use two batteries. Connect a pair of dynos to each battery summed via diode bridge for each dyno. Take care of the grounds and use newer hub dynos that don't short one side of the dyno to the axle (and the frame once the wheel is in place).

    With the grounds properly isolated, you can stack the two 6v batteries to generate 12V for the fridge. Only question is can you generate enough energy to run the fridge directly or do you have to synchronize the battery charging with the fridge's on-time to optimize everything.

    Example: If you can generate 6Wx4 hubs that's 24W. If the fridge takes 48W, you could run the fridge 50% of the time you're riding and keep everything cold. But you'll have to keep the fridge off while you're charging the batteries up, that will take a little smarts.
    Last edited by Mr IGH; 04-24-14 at 05:55 AM.
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    Senior Member dsbrantjr's Avatar
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    "The hub dynos are AC with no way to synchronize. "

    One could run the dyno outputs through full-wave bridge rectifiers to convert them to DC, which could then be combined. However, the user would experience 2X diode voltage drops (2X 0.7V for silicon diodes) which would reduce the voltage from 5 volts to roughly 3.6V. If as suggested by Mr IGH non-grounded hubs are available all 4 could be put in series to yield 14.4V, which would charge a 12V battery. I am not certain that there would end up being enough power for any meaningful refrigeration system; perhaps one of the small solid-state Peltier module-based coolers could be accommodated..

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    Dometic portable refrigerators are the way to go.

    There's no way I'd do dynamo hubs to power a fridge. Go solar. You might also consider a home-built 12v generator involving an old lawn mower engine and a car alternator. In any case, add a battery into the system.
    I know next to nothing. I am frequently wrong.

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    afraid of whales Mr IGH's Avatar
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    Quote Originally Posted by dsbrantjr View Post
    "The hub dynos are AC with no way to synchronize. "

    One could run the dyno outputs through full-wave bridge rectifiers to convert them to DC, which could then be combined. However, the user would experience 2X diode voltage drops (2X 0.7V for silicon diodes) which would reduce the voltage from 5 volts to roughly 3.6V. If as suggested by Mr IGH non-grounded hubs are available all 4 could be put in series to yield 14.4V, which would charge a 12V battery. I am not certain that there would end up being enough power for any meaningful refrigeration system; perhaps one of the small solid-state Peltier module-based coolers could be accommodated..
    You can't put the AC sources in series, they would need to be synchronized to work properly.

    You need a separate bridge for each hub, floating input, sum two bridges into a single battery. Two hubs could charge a single 6V battery providing ~500mA x 2 or 1A @ 6V.

    The hubs are really an AC current source, while the diodes add to the amount of power the rider needs to provide, they don't limit power to the load for all real-life cases.

    I just looked at some coolers, the Peltier coolers need 5A @ 12V, that would limit the battery based system to 20% duty cycle at max power. I'm not sure if that's enough on a hot summer day.

    edit; I'm finding some 12V, 3A Peltier coolers, that's starting to get reasonable but they're not big enough for a keg
    Last edited by Mr IGH; 04-24-14 at 11:04 AM.
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    Quote Originally Posted by Mr IGH View Post
    . . .You need a separate bridge for each hub, floating input, sum two bridges into a single battery. Two hubs could charge a single 6V battery providing ~500mA x 2 or 1A @ 6V.

    . . . the Peltier coolers need 5A @ 12V, that would limit the battery based system to 20% duty cycle at max power. I'm not sure if that's enough on a hot summer day. . .
    Just a suggestion: compound the losses, end to end, in your proposal.

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    Quote Originally Posted by Sullalto View Post
    What situation are you in with a bike and a working fridge paired together? Seems odd.
    I'm looking to go on a cycling holiday around the lakes and would like a small fridge to keep things like milk and bacon cold during the day between villages and stops.

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    Senior Member Bill Kapaun's Avatar
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    Quote Originally Posted by Tsmoreland324 View Post
    I'm looking to go on a cycling holiday around the lakes and would like a small fridge to keep things like milk and bacon cold during the day between villages and stops.

    An ice chest makes more sense.

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    Quote Originally Posted by Tsmoreland324 View Post
    I'm looking to go on a cycling holiday around the lakes and would like a small fridge to keep things like milk and bacon cold during the day between villages and stops.
    You might look at these micro fridges made for cars. The power needs are very low and you could probably get it lower yet by improving the insulation. Possibly you can power it with solar. Or go the old fashioned route with a small igloo box, and fresh ice daily.
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    Quote Originally Posted by Tsmoreland324 View Post
    I'm looking to go on a cycling holiday around the lakes and would like a small fridge to keep things like milk and bacon cold during the day between villages and stops.
    On my tours, I don't carry anything needing refrigeration. If there's a camp store I might buy a bottle of milk but only 1 meals worth.

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    Nobody mconlonx's Avatar
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    Hey, what about this:

    Mount an automotive alternator on a rear rack, with a chain drive connected to either a chainring or a cassette cog out back. Run cable to the trailer and a 12v deep cycle battery and then power a 12v fridge.

    or... can you run a generic electric motor hubbed wheel so that it will generate electricity?
    I know next to nothing. I am frequently wrong.

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    Quote Originally Posted by mconlonx View Post
    Hey, what about this:

    Mount an automotive alternator on a rear rack, with a chain drive connected to either a chainring or a cassette cog out back. Run cable to the trailer and a 12v deep cycle battery and then power a 12v fridge.
    Discounting the weight, consider that automotive alternators need fairly high RPMs to produce the rated voltage.

    I think the OP should be able to find a 12v fridge that meets his needs. Then maybe somebody still makes a 12v bicycle generator of the type like the old Sanyo units. He can attach that to drive off either trailer wheel and be good to go.
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    Senior Member dwmckee's Avatar
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    Four dynos is going to cost a lot and generate a heck of a lot of drag to boot. I think you should take a step back and get a clear picture of what you want to solve first and then look at all of the different ways to do it. For example, I have a vacuum insulated cooler that is about a cubic foot and cost about $250.00. That sounds real expensive until you compare it to the cost of four hub dynos all set up. My cooler will keep a half gallon of ice cream frozen solid for five days (sometimes even six) without need to add any ice. As long as what you put in is already cold, it will stay cold for about as long as you need it to be. Would this be a better and simpler solution to what you are trying to do?

    Also, if you want to go the 12v refrigerator route you can get a 12 volt A/C bottle dyno from peterwhitecycles.com. Not sure if this will have the amps you want though.

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    afraid of whales Mr IGH's Avatar
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    Quote Originally Posted by dwmckee View Post
    Four dynos is going to cost a lot and generate a heck of a lot of drag to boot....My cooler will keep a half gallon of ice cream frozen solid for five days (sometimes even six) without need to add any ice....
    You're right about the drag. If the hubs are 50% efficient then it'll take 48W of leg power to run the hubs, that's going to be very noticeable when riding along level ground takes ~200W of leg power.

    And how much stuff is OP going to haul that he needs a 4 wheel trailer? The Surly Bob and Ted have 300lbs max load with 2 wheels.
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