[OLPC India] charging XOs using bicycles

[Richard A. Smith]

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 at laptop.org>
One Laptop Per Child