Fwd: Re: Freerunner-powered bike ride

André Gaul gaul at web-yard.de
Mon Aug 4 00:15:17 CEST 2008

Yesterday Marko Mäkelä sent me some interesting information about a
charging solution for bicycles. He's not on the list and wants me to
forward his mail:

-------- Original-Nachricht --------
Betreff: Re: Freerunner-powered bike ride
Datum: Sat, 2 Aug 2008 22:56:15 +0300
An: André Gaul <gaul at web-yard.de>
CC: community at lists.openmoko.org

I'm not subscribed to the list, but I've been following the
development of the OpenMoko since the Neo1973 was announced.

André, I think I can answer your question:

> The next step would be a dynamo-powered charger for the Freerunner. I read an
> article in the german computer magazine ct [2] about creating a converter for
> charging USB devices while riding your bike with a hub-dynamo.

The c't article features a step-up/down converter that outputs 5 volts
and a battery charger for 4 Ni-MH cells. I suppose that you'd be only
using the 5-volt supply. However, I have designed a much simpler and
cheaper solution that I believe should work at 500 mA. I don't know if
you can convince the Freerunner to input 500 mA instead of 100 or 1000
mA, or if you can tell the Freerunner to adjust its charge current
according to the voltage, as per the USB battery charging
specification that the Freerunner does not comply with.

My solution consists of a MOSFET rectifier bridge and a low-dropout
linear voltage regulator that outputs 5 volts:


The SMD version is about 14mm*22 mm, so it can be fitted inside the
head tube, for example. I know of two switch-mode regulators: the c't
one (which is available as a kit from segor.de) and a commercial
product, the JaWeTec BikeCharger. I don't think that they're
significantly better than my cheap and simple circuit. Fully charging
an old Nokia DCT-3 series phone (about 1 Ah) takes about 50 km when
riding at an average speed of 20 km/h.

I'm going to build a small series (5 or 10 or so) of the circuit and
sell it to fellow hobbyists at about 10 EUR. It'd be nice to see this
being sold as a commercial product (for a similar price, I guess).

Best regards,

Marko Mäkelä

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