OLPC battery life--an update

After my Monday-morning blog post reporting on some preliminary battery-life testing for the XO laptop from the One Laptop Per Child project, I was contacted by Jim Gettys, vice president software for OLPC.

This early testing showed that the current Beta 4 development systems achieve only a little over 5 hours of operation from a single charge of the lithium iron phosphate battery. According to Gettys, these batteries deliver about 20 WH (watt-hours) of energy.

Gettys told me that the XO's power consumption in those tests was about 3.81 watts, and provided me with a list of pending software changes and alternative usage models that will improve this figure:

A closeup of an OLPC laptop's power and battery indicators.

(Credit: One Laptop Per Child)

• Configuring the DCON (display controller) chip to refresh the display whenever possible, so the primary display clock source can be shut down (saving about 0.52 W)
• Turning off the backlight (saving about a watt)
• Optimizing the wireless firmware to reduce power consumption (savings unspecified)

The DCON and backlight changes alone would bring system-level power consumption down to about 2.28 watts, which would translate into a battery life of about 8 hours and 45 minutes. Saving another quarter-watt on the wireless interface, if that's possible, would bring the XO up to the 10 hours of "heavy use" predicted by OLPC's Walter Bender.

This configuration, however, is only suitable for using the XO as an electronic book reader--and outdoors, at that. That isn't what I call a "heavy use" workload, but when I said as much to Gettys, he insisted that this is what the OLPC project regards as heavy use--so by the project's own standards, it looks like the XO may be able to reach its promised goal.

But not by my standards, for whatever that's worth. I think that the usage model for a classroom environment should assume that the backlight is on and that students are typing, drawing and making their way through computer-aided learning programs. In such an environment, the figures from OLPC suggest to me that the XO will run for only 4 to 6 hours per charge.

But that's probably a good day's use for the students at the heart of OLPC's plans...good enough, I think. The OLPC project ought to stop spreading around battery-life estimates that many users won't see in real-world operation. This is one of those cases where it's better to underpromise and overdeliver.

[idsantos]

Here's a question for Mr. Bill Gates!..

When will it be posible to do that with Windows Vista installed?..

[Peter N. Glaskowsky]

Windows also has power-management features

Windows already provides similar power-management features. Windows
laptops need much larger batteries to get similar battery-life figures because
they run faster, higher-power processors, more memory, rotating hard disks,
etc.

In short, they're more complex and more capable, and that requires them to
consume more power.

I will note that the power consumption of most laptops is less than
proportional to their relative performance vs. the XO. You can get a notebook
with a dual-core 1.6 GHz CPU and a 12" LCD that consumes only about 8W
under normal use-- about 2.5 times as much average power consumption as
the XO, but with over 10 times the CPU performance potential.

Of course, that system will consume much, much more power if you actually
try to use all that extra performance. The XO maxes out at about 6 watts for
433 MHz of CPU frequency (roughly the performance equivalent of a 300 MHz
Core 2 Duo processor), according to Mr. Gettys of OLPC.

A 1.6 GHz Core 2 Duo low-voltage notebook can consume 25 watts or more
(I'm just guessing here, I've never tried to measure it), but you're still getting
ten times the performance. So in fact a PC notebook is more energy efficient.
This should come as no surprise, because it uses more modern technology
than the XO.

The XO's advantage as an educational system is that it delivers enough
performance and a lower minimum-power figure at the lowest possible cost.
That's more important than energy efficiency for these emerging markets.

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[Peter N. Glaskowsky]

Oh, one more thing

When I was working at Microprocessor Report, Bill Gates once told me that he
read our newsletter, and I thought that was pretty cool. But if he's reading this
blog, I would be very much surprised. Flattered, of course, but surprised. :-)

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