XO 1.75 battery life?

Richard A. Smith richard at laptop.org
Fri Dec 2 16:12:03 EST 2011

On 11/29/2011 05:11 AM, Peter Robinson wrote:

>> How's the battery life on the 1.75 going for Read, web browsing&
>> suspend? (3 of the most important functions) Anyone doing tests on
>> these yet?

I have begun doing battery life testing.  But aggressive suspend/resume 
is not 100% functional yet (but its very close).  Until aggressive 
suspend resume (ASR) is 100% I can't do a full round of usage mode testing.

I do have some data points and I can give you an minimum value.  One of 
the nice things about the 1.75 is that its maximum power draw from the 
battery is coming in at 5W regardless of how loaded the machine is. 
This makes a minimum battery life value much easier to predict.  The 
average value of the minimum for battery juice is 18Wh.  So 18Wh/5W is 
3.6hours.  So I feel safe in making the claim that regardless of what 
you do [1] with a 1.75 you can run for 3.5 hours.

That said you have to have a lot of stuff going to keep that at constant 
5W.  Certainly using Read or browsing the web you won't be at 5W so the 
battery life will be longer.

My existing data points have been collected under the following conditions:

  - Machine idle at sugar home screen (zero user input) with power 
management off.
  - Backlight at our default of 80%.

This is of course an artificial test since zero user input is not very 
useful as a battery life estimate but it allows me to do OS to OS 
comparisons and represents and upper bound of non-suspend runtime.

In that mode we I see abot 2.75W which on the above 18W est is 6.5 hours 
of runtime.  I have run logs that range from 6-7 hours for that test.

So I'm expecting to see in-the-wild numbers for people who disable ASR 
to be in the 4-5 hour range.

If you are outside and can turn off the backlight then that lower bound 
drops to 1.8W. With the backlight off I've got idle runs that show 
between 8.5-10 hours.

Throwing ASR into the works adds a _huge_ dynamic.  During ASR our low 
power drops from 2.75W to about 1.2W and even lower if the backlight has 
a chance to dim or turn off.  So if you can spend a lot of time in this 
mode then the battery life will increase quite a bit.

We don't have any good estimates of how much suspend time you can get 
out of things like Read or web browsing.  Powerd can now collect this 
sort of info but unfortunately a kernel bug in build 860 prevents the 
information from being accurate.

If you wanted to help and you have a 1.5 you can.  People with 1.5s can 
provide accurate usage info they install the latest release, make sure 
the date is set correctly, and then allow powerd to do ASR (ie don't 
disable it).  Then use the XO as you normally would but use it on 
battery as much as possible. Only charge up the battery when it gets 
low.  Powerd logs are kept in /home/olpc/power-logs .  After a few 
cycles of the battery send me all the logs in that directory.

[1] There's always a disclaimer. :) Thats without any USB devices 
plugged in.  the USB subsystem can provide up to 5W to an external 
device.  If you were to also draw 5W from USB then the draw would be 10W 
and your battery life will be much less.  In between 1 and 1.5h. (The 
harder you draw a battery the less capacity it can provide so its not a 
straight /2)

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

More information about the Devel mailing list