[BULK] Re: harvesting energy

Richard A. Smith richard at laptop.org
Thu Oct 27 23:45:40 EDT 2011

On 10/27/2011 07:16 PM, Hal Murray wrote:

>> Has anyone estimated the work required to charge an XO? Mike Lee gave a
>> demo some time back at the Washington D.C. Learner's Club which seemed  to
>> show that it would be a difficult workload for an adult athlete to  charge a
>> laptop.
> You don't have to estimate.  It's easy to measure.
> If you are interested in things like this, you should get something like a
> Kill-A-Watt.  They are down to $20 (plus shipping and such).
>    http://tinyurl.com/5u3alby
>    http://www.p3international.com/products/special/p4400/p4400-ce.html
> There are other similar units.

Speaking from experience measuring the power draw of a single XO with 
these low cost power meters is tricky.  They can be very inaccurate at 
lower power measurements.  The kill-a-watt for example has a typical 
accuracy of 1% with a max of 4%.  Full scale is 1800W which would be 18W 
minimum measurement.  You can play some games with the gain into the 
ADC's and reduce that but its not clear if the kill-a-watt does that.  I 
can't remember the exact output numbers when I tried a kill-a-watt.

The AC meter I use is a WattsUp .net version which is accurate to 0.5W 
and then I use a bank of at least 10 XO's.

> It takes about 20 watts to charge an XO-1.   Running uses 5 watts.  The
> backlight is a few more.  I think the limit is the front end so it will take
> longer to charge the battery if you are also using the XO.
> If it takes 2 hours to charge the battery, that's 40 watt hours.

In my talks about power to deployments and groups I have a standard 
saying that if the answer to "How much power does it take?" is a simple 
answer with no qualifiers then its wrong.

Its wrong because the environment surrounding the power measurement matters.

Is the XO running or powered off?
Is it for a XO-1.5 or XO-1.5?
Is it DC measured at the input jack or is it AC?
If its AC what power adapter is it using? The 17W, the 24W or the 25W 
What AC voltage and frequency is the adapter running at?

Every different combination of the above will give you different 
numbers.  Many combinations will have similar numbers but some will be 
pretty different.

In this specific case the question was about human power so its 
reasonable to assume that the setup would use a direct DC input. 
Otherwise you are losing 30% or more of your power in conversion losses.

The number I use for the DC input is 25 watt-hours. XO powered off.  Its 
reasonably constant between XO versions and at various DC voltages.

Reflecting that power draw to the actual amount of human work required 
needs some data on how good the human->electrical conversion in various 
setups is.  I don't have any good data on those setups.

As Paul mentioned in his other mail here at OLPC we are trying to 
transition from thinking in terms of how much power it takes to charge 
the battery to terms of how much power does it take to get your task 
done?  Opportunistic charging.

With XO-1 and 1.5 human power was impractical because the run to charge 
ratio was just too low.  With XO-1.75 we think that we are getting into 
the right ballpark.  Since the advanced power management for 1.75 is 
still under heavy development I don't yet have a good number for what 
that ratio is on 1.75.  I don't think we are going make it to our magic 
10:1 ratio but 5:1 is looking pretty good.

As part of my MPPT testing on the latest build of C1 XO-1.75s I have it 
on my TODO to do some measurements while charging from the hand crank. 
I'll have a better idea of where we stand.

Richard A. Smith  <richard at laptop.org>
One Laptop per Child

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