XO power draw
Contents
XO power draw
The XO laptop has many modes of operation so calculating the power draw can be complex. This page will try to describe the various modes and the methods of measuring the draw for each mode.
Tools
The following tools were used to take readings from the gas gauge chip housed inside the battery. Therefore these readings are valid running on battery only. Duplicating these readings with measurement tools using external power and no battery will end up sightly higher due to the efficiency loss of the DC/DC converter that powers the system power rail. If the XO is trying to charge an installed battery then it will be much greater.
olpc-pwr-log
This is the monitoring script that waits for some specified interval to pass and then takes a reading of information available from the battery. The sample interval is changed by editing the script. The default to around 12 seconds per reading
Where to get it
Since Joyride-2301 olpc-pwr-log is included in the build you no longer need to download it. If you have an older build you may download a copy here:
The legend for the output of the script is:
<Timestamp>,<SOC>,<Voltage>,<Current>,<Bat Temp>,<ACR>,<Status>,<Overall Level>,<Net ACR>
- Timestamp
- POSIX time stamp of number of seconds since Jan 1, 1970
- SOC
- State of Charge. A % number between 0 and 100% indicating how charged your battery is. This number is normally between 7% and 97%. Due to variations from battery to battery its difficult to get an exact measurement down to 1%. Therefore low is anything less than 15% and full is anything greater than 93%.
- Voltage
- Battery voltage in microVolts ( x 10E-6 ) The resolution does not really go down to microVolts but it's easier not to do floating point math in the kernel driver.
- Current
- Signed number indicating the current into or out of the battery. Value in microAmps.
- Battery Temp
- Temperature of the battery in 100ths of degree Centigrade. The battery can have quite a bit of rise above ambient temperature during charge and discharge. In a normal 25 degree room its not uncommon to see values up in to the 30 C range.
- Accumulated Current Register (ACR)
- ACR is a 16-bit 2's complement value that counts up or down depending on the amount of charge into or out of the battery. The value is in units of the counter inside the battery but it can be converted to milliAmpHours (mAh) by multiplying by .4167. A single value does not tell you much but the difference between any 2 readings gives you the net charge or discharge during that period.
- Status
- "Charging", "Not Charging", or "Discharging". "Not Charging" occurs when the battery is full but you are on external power.
- Overall Level
- "Low", "Normal" , or "Full". A basic indicator of where your charge level is.
- Net ACR
- The net ACR since the script was started. Handy for looking at the data prior to processing. Net ACR is in mAh. Positive numbers indicated a charge and negative discharge.
process-pwr-log.py
http://dev.laptop.org/~rsmith/process-pwr_log.py
Python script that takes one of the log files from olpc-pwr-log and crunches measurements of power consumption between each reading. It creates a file of the same name as the input file with the text 'processed_' prepended to the filename.
The legend for the output of the script is:
<Net Time>,<Avg Current>,<Net ACR>,<Delta T>,<Avg Voltage>,<Watts>,<Net Wh>
- Net Time
- Relative time in hours from the start of the logfile.
- Avg Current
- Difference between the 2 ACR readings / difference in Time
- Net ACR
- Relative ACR reading from the start of the logfile.
- Delta T
- Difference in time in seconds between readings
- Avg Voltage
- (Previous voltage reading + Current voltage reading) / 2.
- Watts
- Avg Voltage * Avg Current
- Net Watt hours
- Iterative summation of power consumed for that reading times duration in hours for the reading
Modes
The following are definitions created by Richard with how I view their meaning. Probably subject to change and additions in the future. Some modes have lo and hi power levels.
- Sleep
- WLAN device disabled via no firmware. Lid Closed (Display off)
- Ebook
- WLAN device disabled via no firmware. Lid Open, Display Monochrome, Backlight Off
- Mesh
- WLAN enabled, Lid Closed (Display off)
- Suspend Lo
- WLAN enabled, Display Color, Backlight at minimum setting
- Suspend
- WLAN enabled, Display Color, Backlight Auto-dimmed by ohmd
- Idle
- WLAN enabled, Display Color, Backlight full, CPU on, no load
- Camera
- WLAN enabled, Display Color, Backlight full, CPU on, high system load. Running the camera in full screen exercises nearly every bus on the XO so it draws a lot of power although it does not use a lot of CPU time.
- WLAN disabled by moving the firmware so it would not load. This was done because 'olpc-control-panel -s radio off' does not actually turn off the WLAN processor therefore it still uses a lot of power.
The Numbers
- Notes:
- Due to the way the logging works the wattage numbers are not exact because gas guage chip does not track the battery voltage with the same precision as the current (there is not an AVR register). So the voltage used is the midpoint between the sample from each reading. For the suspend tests this was less than .1V between readings. The exception is the Idle and Camera modes where the sample interval was set to 10 seconds so the voltage did not change much from reading to reading. This has the nice side effect that a few peak measurements showed up. During suspend peak readings are not possible with the battery logging script.
- All the suspend modes (Sleep,Ebook,Mesh,Suspend) have zero wakeups during the measurement so they represent a lower bound for the power draw.
2008-2-25 By rsmith
Sleep .25W Ebook .71W Mesh Only 1.1W Suspend Lo 1.7W Suspend 1.9W Idle 3.9W 4.9W Peak Camera 6.5W 8.7W Peak
