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Disabling Turbo Boost to get more MacBook Pro battery life

The best battery life in the 15-inch Retina MacBook Pro line is the base model (without the high-powered GPU), but Apple’s stated 8–9 hours of battery life is only achievable with very light use — if you’re using tools like Xcode, Logic, or Lightroom, you’re lucky to get more than 4 hours.

I used to eke more battery life out of my 2008 MacBook Air by underclocking and undervolting its CPU to a lower speed with CoolBook, but that’s no longer possible on modern Macs.

Modern Intel CPUs use Turbo Boost to dynamically increase CPU speed, depending on workload, up to their thermal limits. The advertised clock speed — 2.2 GHz for the base-model 15-inch — is only the guaranteed minimum.

Turbo Boost Switcher can temporarily disable Turbo Boost, and I was curious to see if disabling it would meaningfully affect battery life.

I devised a simple battery-testing script to simulate light and moderately heavy loads on my brand new, mid-2014-generation, base-model 15-inch Retina MacBook Pro: it loads some popular websites in Safari over Wi-Fi, spending some time scrolling down each one, while playing music in iTunes, then appends the time and battery status to a file in Dropbox, which promptly gets synced. This cycle repeats indefinitely. For the moderately heavy load, the cycle also includes a clean and full compile of Overcast’s iOS app using xcodebuild every few minutes.

I ran this test at 50% brightness (with auto-adjustment disabled), no keyboard illumination, and sound muted until the battery ran out and it shut down. Before running the test, the battery’s cycle count was just 2.

When plugged in and fully charged, I also ran the Geekbench 32-bit single- and multi-core benchmarks, and observed the power range drawn from the wall with a Kill A Watt.

Results

My “Heavy” numbers with Turbo Boost line up well with the very similar 2013 model in AnandTech’s “Heavy” battery test, which gives me confidence in my test. While AnandTech’s “Light” test appears lighter than mine, neither of us could reach Apple’s “8 hour” claim for this model, which is disappointing.

Turbo BoostOnOff
Idle power21W21W
Geekbench power58–75W36–45W
Peak CPU temp.90°C60°C
Geekbench single3,1302,143
Geekbench multi11,7668,369
Average Xcode build22 sec30 sec
Average web loop133 sec134 sec
Battery Life, Heavy4:025:10
Battery Life, Light5:407:07

Disabling Turbo Boost hurts performance of CPU-intensive tasks by about a third, but doesn’t significantly slow down lighter tasks. The MacBook Pro also runs noticeably cooler, and gains about 25% more battery life.

This won’t be worth doing all the time, but it’s nice to have the option.

And I’d still like to see higher battery capacity in normal operation. This is far from a solved problem, and far from any reasonable definition of “all-day” battery life. I hope Apple agrees.

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