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List:       olpc-india
Subject:    Re: [OLPC India] charging XOs using bicycles
From:       "Arjun Sarwal" <arjun () laptop ! org>
Date:       2008-04-27 16:54:16
Message-ID: dc942fa10804270954w4fc50a49g7263a6b8974c929d () mail ! gmail ! com
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Thanks, Richard. I will try and put up these details on the wiki.

On 4/27/08, Holden Bonwit <hbonwit04@yahoo.com> wrote:
>
> Richard and Arjun, thanks for taking some time to help!  I do realize that a
> bicycle isn't the fastest way to get something charged, but I have one  in
> front of me, and nobody has posted about it, and I think *someone* out there
> might actually legitimately want to use this method.  So I'll pursue it for
> now.
>

Sure, please do post your experiences on the wiki as you progress,
we'd  love to hear about them (and help in whatever way we can)

> In measuring the output from my dynamo, it's anywhere from 0-34 Volts, AC,
> which was very surprising.  It is marked simply "Miller 12V 6W"
>
> I've been working to get the dynamo to power a pair of computer speakers --
> when it can do that reliably then I'll try it on an XO.  Right now I've just
> got a diode bridge with a capacitor across it, and that is doing fairly
> well.  A small amount of hissing/vibration noise.  Might go with a higher
> capacity capacitor.  Possibly a supercap?  Not sure what the fix is.
>
> Additionally I'm going to throw on an LM7815 or LM7812 to try and steady out
> the voltage a bit.  The problem is really that the input is such a wide
> range, and yet I don't want to go to a full, proper power system.
>
> Thoughts?
>

*** If you are directly charging the XOs ***
Richard suggested L200, which I find is a  great solution  because its
low cost, robust, requires minimum external components and easily
available too. This circuit regulates voltage and limits current too
http://wiki.laptop.org/go/Image:L200.png
(you might need to recalculate R1 R2 and Rsc values)

*** If you are charging a car battery and then charging the XOs from there ***
I believe this might be a better solution, because car batteries could
charge at much faster rates too when you have more current and would
easily absorb the fluctuations in current.
Regarding the acceptable range of voltages for car batteries, youd
need to check.



Hope this helps.
Let us know how you progress.

best regards
Arjun

> -Holden
>  ==
> Holden Bonwit
> hbonwit@alumni.nd.edu
> (You probably have an email from me in your spam folder.)
>
>
> ----- Original Message ----
> From: Richard A. Smith <richard@laptop.org>
> To: Arjun Sarwal <arjun@laptop.org>
> Cc: Holden Bonwit <hbonwit@alumni.nd.edu>; OLPC India Mailing List
> <india@lists.laptop.org>
> Sent: Sunday, April 27, 2008 6:38:15 PM
> Subject: Re: charging XOs using bicycles
>
> Arjun Sarwal wrote:
>
> > A while ago, I along with Richard did some approximate calculations
> > and as a rule of thumb , you'd need to give the XO about 1A @12V for
> > a little more than an hour to charge it completely.
>
> Arjun since then I've done a bit more in depth measurements:
>
> The later measurements I've taken are 26 Wh to charge the battery from
> empty to full.  Thats measured @ the battery so rounding that up I
> usually say 30 Wh.
>
> Depending output voltage of the setup you use to provide the input power
> the XO is going to draw between 15 and 18 Watts when its charging the
> battery at the maximum rate.  Most of the time it will be closer to 15W
> but I normally just use the 18W number so that any estimates have some
> room for error.  When its drawing 15W power delivered into the battery
> stays the same which is around 15W.
>
> So 15 to 18W @ 12V = 1.25 to 1.5A
> 30Wh / 15W = 2 hours to charge.  The actually measurement when attached
> to the AC adapter is 1:47.  2 hours is a nice easy number to use.
>
> This is with the laptop "off".  If you charge with the laptop on and
> running then its a much more complex calculation.  But its only going to
> increase the time you need for a full charge.
>
> > This means your generator (bike dynamo in this case) would need to
> > supply approx 12W for an hour. I've found bicycle dynamos to usually
> > generate much less power, more in the range of 1-4W.
>
> Columbia is looking at using some bike setups with beefer alternators.
> I'll pass on the details when they actually do something.
>
> > (Richard: please correct me if I have been wrong on any of the
> > charging details, I will make sure to put the details down somewhere
> > on the wiki this time)
>
> Most of this is on the wiki somewhere but we can certainly redo the
> organization so its easier to find and all in one place.
>
> --
> Richard Smith  <richard@laptop.org>
> One Laptop Per Child
>
>


-- 
Arjun Sarwal
Intern, One Laptop per Child
Email: arjun@laptop.org
IRC: arjs on irc.freenode.net in #olpc, #olpc-health, #sugar
Skype: arjunsarwal85
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