[BULK] Re: harvesting energy
hmurray at megapathdsl.net
Fri Oct 28 22:10:30 EDT 2011
richard at laptop.org said:
> 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.
Good point. Thanks. It's even worse than that. In the power mode, the
Kill-a-Watt only shows whole watts, no fraction. So at roughly 20 watts, it
can easily be off by 5%. (Maybe somebody should add that to a UI lessons for
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.)
>> 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.
;) Thanks again.
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.
> Is the XO running or powered off? Is it for a XO-1 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 adapter? What AC voltage and frequency
> is the adapter running at?
Here is a data point.
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%.
I'll make a pretty graph if anybody wants.
These are my opinions, not necessarily my employer's. I hate spam.
More information about the Devel