Absolute Power


Mystery solved, and FREE iPhone 4 batteries?

This blog originally posted on the Low Power Engineering Community 10/7/10. 

Our cliffhanger from last time left us with the open question, “where does all of the energy in a 5.25Wh (watt-hour) iPhone 4 battery go during normal daily operation?”  Watching Star Trek as a test vehicle seemed amazingly efficient on iOS devices.  Our “Star Trek Inefficiency” metric (STI – energy used to watch Star Trek) showed STI=1.1Wh on an iPhone 4 in its most efficient configuration (airplane mode, display minimum, sound off), and STI=1.7Wh with the display and sound at maximum.  And further experiments demonstrate that energy used for sound via speaker, headphones, or Bluetooth are all small contributors to the energy equation.  So we’ve fingered the display.

But amazingly, that’s not the biggie!  As I’m sure you’re all screaming in your head by now (aren’t you?), we have conveniently ignored the contribution of the thing that makes today’s portable devices so compelling – COMMUNICATION!  Without the communications link, email is just a file of printouts, social networking is only possible within the range of your voice, and phone calls – well, when’s the last time you saw a phone booth?!  We are now completely dependent on wireless voice and data communications.  Remember when your parents used to complain to the phone company when there was some static on the phone lines?  Today, we pay nearly $100/month per person, and we live with service that cuts off 20% of all phone calls!

OK, rant finished – we’ve established that we can’t live WITHOUT it.  Now let’s see how much we’re paying to live WITH it – in today’s most important currency – ENERGY.

iPhone 4 STI=1.1Wh in its most efficient mode.  STI=1.7Wh with brightness and sound at maximum (it’s a great movie player).  Now, instead of playing the Star Trek movie on the device, let’s stream the movie via 3G!  The iPhone 4 STI with Star Trek streamed over 3G, STI=3.4Wh.  Wow!  Streamed (at a much lower resolution) via Netflix, Star Trek now requires twice as much energy as the full brightness case!  I ran this experiment at my house in Palo Alto, where my reception is pretty consistently between 3-4 bars.  Remember, battery capacity is 5.25Wh, so we’ve gone from 1/3 of the battery capacity to watch Star Trek locally in high resolution to 2/3 of the battery capacity to watch it streamed in low resolution.

And for my final trick, I wanted to find the perfect spot to rerun this test – 3G reception good enough to maintain the stream of data, but otherwise right on the edge.  A place of great 3G signal suffering.  Not a dead zone, but something like eternal reception purgatory.  No bars…one bar…two bars…one bar…  Eureka!  On my desk in my office!!!  The perfect spot (and I’m SURE one of only a few spots on earth) where you can be tempted, enraged, encouraged, and frustrated all while trying to read your email.  In this “mystery spot”, where good 3G reception and evil 3G reception wrestle to the end of time, the STI on my iPhone 4 came out to be 5.3Wh!!  Woah!  You’re probably thinking, “Hey, I thought he said the battery capacity on the iPhone 4 was only 5.25Wh?”  And you’d be correct – in this magical spot (where I sit everyday), I can drain a fully charged iPhone 4 battery in less than two hours, and not even see Spock do his little telepathic speech at the end!

The revised list of ways to improve your iPhone 4 battery life:

3. Forget everything else on the lists you read on the internet

2. Turn down the brightness on your display

1. Improve your reception

It turns out that “improving your reception” isn’t so easy to do – you could be promoted and move to the corner office, sell your house and choose your new one by “closely watching the little bars”, or for no money and little fuss, simply get a case for your phone!  Remember iPhone 4 “antenna-gate”, resulting in the Apple free case program?  It turns out that using a case for your phone not only avoids the “death grip” problem, but also moves your hand (which is mostly water) a little bit away from your phone, slightly improving antenna performance and reception in both cases.  The result is a one or two-bar improvement in signal reception, especially in areas of generally weak reception.  And we’ve already seen that one or two bars of reception can have a profound impact on battery life – in our case, a jump in the STI from 3.4Wh to 5.3Wh just based on those one or two little bars.  So, for the purposes of streaming Star Trek over 3G, that free case may just contain a supplemental 1.9Wh battery!  But don’t let Apple know, or the prices will be going up…

And yes, I got my case through the free case program, but I’m not using it.  Why not?  I just don’t like the way it looks. (Go figure!!!)

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