The Final Word on Battery Life

In our iPhone 4S review I mentioned that our web browsing battery life tests were in dire need of an update. The move to iOS 5 increased the aggressiveness of browser side caching, which impacted our old tests significantly. We still have plans for a much more significant departure from our existing battery life testing methodology, however as an interim solution I developed a modified version of our traditional smartphone battery life test.

The old web browsing test lacked any significant cache busting features and the HTML workload itself was more representative of mobile sites than a full desktop experience. The modified test does a much better job of not allowing its components to be cached by the browser, and includes both lighter and full desktop websites. Since the workload has changed, these numbers are no longer comparable to those running the older version of our test. The new web browsing test is also stressful enough to span both smartphones and tablets, whereas before we had a separate version just for tablets.

Finally, I must mention that we ensure all tablets tested here are normalized to the same display brightness setting of 200 nits for a true apples to apples comparison. As always we rely on the stock web browser that ships with the platform and manually quit all background tasks before running our tests. In the case of the Transformer Prime, I was able to run our new battery life test using all three power profiles to get a better idea of the range of battery life. As a recap, here are the details on the power profiles.

ASUS and NVIDIA have defined three power profiles on the Prime: Normal, Balanced, and Power Saving. Normal allows the CPU to run at up to 1.4GHz with only a single core active, and 1.3GHz otherwise. Balanced is the default setting and it drops maximum CPU clock to 1.2GHz and favors lower clock/voltage targets on the curve compared to Normal mode. Power Saving caps CPU frequency at 1GHz with 1—2 cores active, 760MHz with 3 cores active and 620MHz with 4 cores active. It's unclear what the impact is on GPU clocks/performance. I've also noticed that the Power Saving profile also enables a dynamic contrast control that you can't override.

Web Browsing Battery Life

The shipping Prime does much better than the original tablet I reviewed a couple of weeks ago. It's clear that whatever was impacting WiFi performance also took its toll on battery life. What I suspected might be the case ended up being true: the implementation of Tegra 3 in the Transformer Prime delivers better battery life than Tegra 2 in the original Eee Pad Transformer. There are too many variables here for me to attribute the gains to NVIDIA's SoC alone, but seeing as how battery capacity hasn't changed it's likely that we do have Tegra 3 to thank for better battery life in the TF Prime.

Note that even running in Normal mode and allowing all four cores to run at up to 1.3GHz, Tegra 3 is able to post better battery life than Tegra 2. I suspect this is because NVIDIA is able to parallelize some of the web page loading process across all four cores, delivering similar performance to the original Transformer but at lower frequency/voltage settings across all of the cores. Additional performance gained by supporting NEON likely helps improve efficiency, not to mention higher single-threaded performance thanks to Tegra 3's higher clocks. Combine all of this with a more mature 40nm process and a more mature Cortex A9 implementation on that process, and the power improvements make sense.

The Transformer Prime is still unable to dethrone the iPad 2 here, which just goes to show you how efficient Apple's platform was to begin with. That being said, the Prime is within striking distance of Apple's tablet. The race is now close enough where you won't notice too much of a difference between the two.

There's also not a huge difference in battery life between the various power profiles. I do applaud ASUS and NVIDIA for giving us control over the CPU governor, but unless you're trying to make the tablet last over a very long flight you're probably fine just leaving it in Normal mode and enjoying the extra performance.

I included results from the Prime running with its dock attached. The additional 22Wh battery improved battery life by 51%. The scaling isn't linear because the dock itself draws power. Once the dock's battery is depleted, the tablet needs to power both itself and the dock. The improvement is tangible though, if you need to do a lot of writing on a flight around the world the dock will help you get there.

Video playback battery life is much better than the original Eee Pad Transformer. Tegra 2 was clearly broken when it came to its video decode abilities and as a result we saw significant limitations on both what video you could play and how long you could play them. Tegra 3 fixes all of that, and the result is battery life that's getting closer to the iPad 2:

Video Playback—H.264 720p Base Profile (No B-Frames)

Getting 11 hours of continuous video playback battery life is something to be proud of. Being able to watch 4-6 full length movies on a plane without worrying about your battery is a pretty nice feature. Unlike the active use case, video playback seems to see a bigger benefit from switching away from the Normal power profile.

 

Super IPS+ & Color Gamut Changes to the Browser & Performance Analysis
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  • kishorshack - Thursday, December 15, 2011 - link

    That graph is still a bit difficult to understand
    a typical graph comparing two transformers would have been great :|
    Reply
  • freedom4556 - Thursday, December 15, 2011 - link

    While your video reviews are really cool and you should definitely keep doing them, I think you should have some sort of a sign-off at the end of them. The way it just cuts is really abrupt. You know, something simple like along the lines of "I'm Anand Shimpi and this has been the Eee pad Transformer Prime" or something. It'd add a little polish.

    Good review though, I much prefer listening to reading
    Reply
  • Matias - Thursday, December 15, 2011 - link

    Sign in too, something like "Hi, my name is Anand from Anandtech and today I'll be reviewing..." you know?

    20 minutes is ok, good video.
    Reply
  • Andrew Rockefeller - Thursday, December 15, 2011 - link

    I think this passage from an interview with Anand may sum up his feeling on extranious self promotion:

    http://www.theverge.com/2011/11/14/2560801/5-minut...

    "...when I first started attending industry events I noticed everyone always introduced themselves as Name from Publication. It always rubbed me the wrong way. If I'm asking a question, and my question is appropriately phrased, the publication I write for is irrelevant information."
    Reply
  • Belard - Thursday, December 15, 2011 - link

    In asking the question at an industry event, I agree with your view and Anand's. Its a really KISS MY BUTT thing to do, like should you get a difference answer because who is asking?

    But this is about the video. A proper closing (perhaps with a fade to black) would be nicer and more polished. Even the video editor in Windows7 can handle such jobs easily - and I think his Macbook can as well.

    Also, his video can be embedded onto other sights, people may not know WHO is talking or where it came from.

    It is the end that is at issue.
    Reply
  • bungfinger - Tuesday, January 03, 2012 - link

    "Also, his video can be embedded onto other sights, people may not know WHO is talking or where it came from."

    yeah! because it's unfair to all the illiterate folk... who cruise geek sights for reviews on tech.. who can't read... that small inconspicuous sign over your left shoulder... wait... what?
    Reply
  • MrTeal - Thursday, December 15, 2011 - link

    I know I always enjoy being told to stay classy. Reply
  • Graag - Thursday, December 15, 2011 - link

    That would be a *great* sign off line.

    "That concludes our review. This is Anand Shimpi for Anandtech.com reminding you to stay classy."
    Reply
  • Belard - Thursday, December 15, 2011 - link

    Also... OMG! He lost his hair! I remember about 10 years ago when he had... hair, on HIS HEAD!! Put a wig on man, please!!

    Don't take it personal Anand... I knew I would be balding when I was a teenager and my hairline started thinning in my mid-20s. I tried that Rogaine (sp?) crap for about 2 days, but that sticky nasty stuff felt horrible and the IDEA of putting sticky crap on my head every day for the rest of my life (at about $25 a month) seemed stupid.

    I shaved my head that day. And I keep it shaved. I've haven't seen my curls in years, I don't need a comb. Makes me look a bit tough too :)

    PS: Asus has done a great job with their tablets... It amazing how HP, RIM, Toshiba and others have screwed up.
    Reply
  • Anand Lal Shimpi - Thursday, December 15, 2011 - link

    To be honest, there was about another 30 seconds at the end of the video that ended up being cut out which contained a more gradual sign off. I appreciate the feedback for sure :)

    I try to do these things in one take, it significantly reduces editing time and helps me get them done given the already insane schedule we have to work on at times. This one suffered as a result but I'll do my best to avoid the abrupt ending happening again in the future :)

    Take care,
    Anand
    Reply

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