[Power] SA

[Richard A. Smith]

On 02/25/2010 03:04 PM, Mike Lee wrote:
> You're right. I apologize for my misread. That was build 656. I will
> be sure to have the logging available in 767+ installed going forward.

Now is probably a good time then to mention measurement cases when using 
olpc-pwr-log and solar.

olpc-pwr-log is mostly for diagnosing battery charging/discharging 
issues.  It just so happens that it can also be useful for some solar 
tracking if you understand the limitations.

olpc-pwr-log runs on the host so while its running you are burning host 
cpu power.  On XO-1 that an average of about 4-6W if idle.  In addition 
to using half your available power it make is tricky to measure how much 
power was input since the power use of the system is dynamic.

However by using the suspend feature of the laptop we can take some 
reasonable good readings over a longer period of time.

The way I would recommend using olpc-pwr-log is to start up the laptop 
and then either at a virtual terminal or from the terminal activity run 
olpc-pwr-log and let it snap 1 or 2 readings to let stuff stabilize.

Then either close the laptop or press the power button to put the laptop 
to sleep.  In sleep the laptop uses very little power.  Then every 15 or 
20 minutes or so open up laptop or press the power button to wake it 
back up. Leave it running for long enough for it to take a reading then 
put it back to sleep.

There's also the low maintenance 2 point version where you take one 
reading and then forget about it until you are done for the day and take 
another. That gives you a good overall net number but not a lot of 
dynamic info.

Battery WattHours is an iterative measurement because the voltage 
changes during the measurement period. So samples with a long period 
in-between will have a lot of error in Wh.  But the EC SOC should still 
be accurate.

I also have a forth script that will do auto-wakeups snap a reading and 
then go right back to sleep but it still needs a bit more love before I 
can send it out to the wild.  You also have to have an unlocked laptop 
to use it.

You could do the same from Linux and sugar if you use rtcwake and a 
script that bangs the right controls to turn off the display and then go 
into suspend but that's more config work than most people want to 
undertake.  If anyone wants to attempt such a script then let me know 
and I'll tell you the right knobs.

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