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Mine is late 2012 and has 8GB RAM. His is late 2014 and has 16GB RAM. Both have an SSD (although of different size) and both are running Yosemite. We set the brightness to 6 pips each and turn off Bluetooth and the keyboard light.

Using Activity Monitor we both ensure that our %CPU is very low (<1%) with no visible apps running (apart from Finder and Activity Monitor). We then launch CoconutBattery and check out the Battery Usage therein.

After allowing a few minutes for the usage to settle down his MB idles at 3.8W. Mine idles at 7.6W, leading to his battery life being around twice mine.

What could be causing this disparity?

EDIT: I'm also pretty sure I used to get much better battery life on mine that this when I first bought it. Maybe I need to recalibrate my battery - it has spent a lot of time plugged into the charger so maybe it has got out of whack?

EDIT: Tried this, to no effect, so I'm back to square one.

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    If you put your friend's rMBP in target disk mode and then boot your Mac with the option key held down, with a thunderbolt cable you can boot his OS on your hardware and determine quickly if it's software or hardware causing your idle to be higher in watts. – bmike Jan 30 '15 at 14:48
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    What processors do the different machines have? – Don Simon Apr 15 '15 at 17:19
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    Don't forget to consider the different generations of processor (I believe in your cases it's Ivy Bridge vs. Haswell). Intel's microarchitecture changes follow a "tick-tock" (en.wikipedia.org/wiki/Intel_Tick-Tock) pattern where performance/energy usage are optimized alternating with new features. The amount of energy used by the newer system to do the same amount of work is much lower. That said, it could also be a background process. – David Apr 15 '15 at 21:10

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