# [BULK] Re: harvesting energy

Hal Murray 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
kids.)

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.

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.

```