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Old 03-24-16, 11:34 AM   #1
gauvins
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How long will it take to recharge a 10 000 mAh using a dynamo?

Considering building a wheel on a Shimano DH-T780. The dynamo is rated 3W@6Volt. (that would be 0.5A).

So far so good.

Now, how long will it take for such a dynamo to charge a 10 000 mAh battery bank connected through USB? I assume that this is a function of the speed (assume 20kmh) and the efficiency of the transformer (??). (If the bank is 10 Ah, a 100% efficient process would require 20 hours @ .5A)

Anyone knows either the maths or has experience with such a setup?
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Old 03-24-16, 01:03 PM   #2
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It depends on how empty it is, pack charge circuit efficiency, USB power supply efficiency (this varies a lot), what else you're powering at the time, and how fast you ride.

Back of envelope with an empty pack you could assume a minimum of 5V USB * 10,000maH = 50Wh. At 3W it'd take 16.7 hours to charge losing nothing to power your lights.

Google translated Future Bike USB power supply comparison

A Schmidt E-werk is 80% efficient which would yield 20.9 hours, while an original Plug doesn't break 50% which is 33.4 hours.

OTOH, the 3W rating is into a 12 Ohm resistive load at 15 km/h.

Dynamos are current sources which can deliver more power into loads with higher resistance

More speed means more power. The StVZO rating is at 15km/h or 9.3 mph, and you're probably averaging 15 mph which is 24 km/h

With a 24 Ohm load a Shimano DH3N80 delivers 5W at 20 km/h and 6W at 30km/h. That'd drop charge time to 9.8 hours at 20km/h
Google translated Future Bike dynamo comparison

Last edited by Drew Eckhardt; 03-24-16 at 01:24 PM.
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Old 03-24-16, 03:20 PM   #3
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Not disagreeing with Drew's math, but at that point perhaps the better question to ask would be, "where's the nearest plug I can charge this battery up with?"
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Old 03-24-16, 03:38 PM   #4
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Not disagreeing with Drew's math, but at that point perhaps the better question to ask would be, "where's the nearest plug I can charge this battery up with?"
Then you'd be sitting near that wall socket for 5 hours, assuming a 2A/5V (10W) power source.
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Old 03-24-16, 04:51 PM   #5
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You need to figure out your actual power requirements. If they exceed dynamo output during daylight hours and what you can spare at night you'll need a USB fuel cell which runs off lighter fluid. If sufficiently below dynamo output you'll be able to keep any battery full during the day. Where it's getting iffy you'll need a better engineered solution which combines a USB output, battery (Pedal Power has a 6700ma unit which is about the most you'll be able to charge during daylight hours), and electronics in one package with AC dynamo input.
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Old 03-24-16, 05:11 PM   #6
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Back of envelope with an empty pack you could assume a minimum of 5V USB * 10,000maH = 50Wh. At 3W it'd take 16.7 hours to charge losing nothing to power your lights.
Thanks for the reply. I hesitate between building a dynamo wheel VS installing a solar panel (Instapark's Mercury 10) on my rear rack. I'll do a quick assessment of the daily energy requirements, which are certainly less than 10 A.
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Old 03-26-16, 11:42 AM   #7
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I too, was in a quandary between solar and dynamo, ultimately chose the son hub.
In one season of touring, still I find us dependent on edison, although not as much. I often switch off the headlamp in sunny conditions to kick up charging rate and always be ultra conservative in electric use.
R
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Old 03-26-16, 03:52 PM   #8
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Originally Posted by gauvins View Post
Considering building a wheel on a Shimano DH-T780. The dynamo is rated 3W@6Volt. (that would be 0.5A).

So far so good.

Now, how long will it take for such a dynamo to charge a 10 000 mAh battery bank connected through USB? I assume that this is a function of the speed (assume 20kmh) and the efficiency of the transformer (??). (If the bank is 10 Ah, a 100% efficient process would require 20 hours @ .5A)

Anyone knows either the maths or has experience with such a setup?
What are you doing with the battery pack that will need 10,000 mAhr between rides?
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Old 03-26-16, 04:35 PM   #9
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What are you doing with the battery pack that will need 10,000 mAhr between rides?
I don't understand your question. We'll be under way for 10 weeks. I estimate that our daily consumption might be in the 4-5 Ah. A 10 Ah would give us, roughly, two days. A dynamo would probably add 2.5 Ah per day. A 10W solar panel would probably yield twice as much on good days.

Between rides, the pack gathers dust. If this was your question
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Old 03-26-16, 04:48 PM   #10
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The Kemo dyno to USB will output 800mA USB so you'll get your daily power needs with 5-6 hours of riding.
M172N Bicycle Power Charge Controller USB

Charge a buffer battery too for a little extra power.
Kemo Electronic GmbH
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Old 03-27-16, 02:12 PM   #11
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If you're going to spend a lot of time on your bike without needing lights, this is worth considering. You should be ready to buy an expensive regulator and some hardware, because the cheap regulators aren't proven yet, so it's a lot more than just the dynamo wheel. A regulator has to take a variable voltage alternating current and turn it into a relatively smooth direct current capped at close to 5 volts.
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