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Sunday, November 16, 2014

Windows 8 tablet (battery) dies in 2 days because of Instant-On

I have a Windows 8 / 8.1 TW100 10" quad core tablet that leave at work as an inexpensive internet device.  The device is always dead with 0% battery when I arrive at work Monday morning. The thing has good battery life when I'm actively using it.  I have to charge the device and reset the clock every Monday. (The clock is dead because it relies on the main battery as the clock battery).  My IPad 2, by comparison, sits standby for days/weeks without draining the battery.

The TW100 uses Microsoft Windows 8 new Instant-On or Connected Standby to stay connected to the internet. This new SoC compatible standby mode uses a lot more energy than the standby on other tablet.  Note: This tablet has no applications installed.


You can use the "powercfg" command to analyze battery usage. It generates a nifty HTML file that describes battery usage across the last 3 days.
powercfg sleepstudy
 This chart shows the continual power drain on this tablet across the weekend.





The table after the chart shows that we were in the low power state 99% of the time.

powercfg creates a chart that shows the activity intervals on the tablet.  Windows 8 tablets wake up every 32 seconds.  Values other than 32 seconds mean some program is causing activity and using power. This chart shows that my tablet has no other activity.


The offender section shows which hardware causes the activity. The machine is 99% idle. We can see here that the I2C controller and SD card account for the < 1.5% active time.

powercfg also shows the programs that cause the activity. It finds no programs as the cause of my battery usage on this TW100 Windows 8 tablet.  This means all of this activity is related to the core functions of the Windows operating system, the SoC and this standby mode.

Conclusion

Windows 8.1 InstantOn (formerly InstantGo) provides a high level of connectivity for windows tablets when they are idle.  This is great when you are actually using your tablet or have it plugged in continually. InstantOn can completely eat the battery when the tablet is not actually in use, draining a full battery within a couple days.

The only way to guarantee a working tablet on Monday morning is to either leave it plugged in or power the machine off on Friday.  The TW100 cold boots in 5 seconds so a full powerdown isn't that big a deal.

Comment

I don't need for my tablet to communicate continually with the network when I'm not using it.  I wish Microsoft provided a way of adding additional power plans to tablets. They currently limit you to the InstantOn if it is installed by the OEM.