[BULK] Re: harvesting energy

Richard A. Smith richard at laptop.org
Sat Oct 29 04:25:11 EDT 2011

On 10/28/2011 10:10 PM, Hal Murray wrote:

> I haven't carefully checked the accuracy of my Kill-a-Watt, but it's passed
> all my sanity checks.  At $20, it's a useful tool.  (The technology for this
> sort of thing must be reasonably solid.  My new electric meter has an LCD
> rather than a spinning wheel and dials.)

Yes for $20 it can do a lot as long as the limits are understood.  I 
mention it because I've had to have the same conversation with a few 
deployments and some consultant for an education ministry. They were 
taking power readings and concerned that their numbers were much 
different than my numbers.

> The question I was commenting on was roughly: Can a person put out enough
> power to usefully charge the battery on an XO?  In that context, I think
> anything within a factor of 2 is close enough.

Yes, and in that context you are quite close but, I have to deal with 
all the out-of-context uses. :)  So when I see power numbers thrown 
about in a olpc public forum I try to frame them accordingly.

> XO-1, system running, not doing much (no human), backlight off.
> 17W Adapter.  (12V, 1.42A)  running on 120V, 60 Hz.
> The battery says LiFe, 6.5Vdc, 3.1Ah.
> Kill-a-Watt reads 19-20 watts.
> It took 2 hours to charge the battery from 10% to 80%.  The charge% was quite
> linear up to 80%.  At 80%, it started to round off.  After another half hour
> it got to 90%, then jumped to 100%.

80% is roughly where it switches from constant current to constant 
voltage.  The jumps are because all the batteries are not equal yet the 
charging system treats them that way.  When the CV marker or the full 
markers are detected the charge% is set to specific values.

Here's a graph of the battery power on a XO-1 across a few different 


> I'll make a pretty graph if anybody wants.

Here's one for a 1.5:


Its a 1.5 on 220Vac rather than 120Vac but works for illustration 
purposes. Sample time is 10 seconds per sample.  The "Off" curves are XO 
powered off and the "Idle" is with the XO powered on but just sitting 
there.  Automatic suspend is disabled.

An XO-1 graph will look very similar but since the DC input power of an 
XO-1 is capped at 17W instead of 25W the front of the curve will be much 
flatter until the draw decreases below the DC 17W input limit.

If you take the area under the curves then you get 30Wh for the "Off" 
cases and 47Wh for the "Idle" case.  The matching number I have for an 
XO-1 in the "Idle" case is 56Wh.  Its a good bit higher since the XO-1.5 
has more efficient power supplies than XO-1.

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

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