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How to keep a MacBook battery charged to only 70%?

A chemist told me that a battery should be charged to and used between 30-70%, and its very bad for battery life to completely load or unload it.

He uses a utility program that always keeps the charge of his Lenovo laptop between 30% and 70%.

How can I achieve the same effect using Mac OS X?

Unlike a Thinkpad, there is no way to control the firmware on a Mac. The only way I have found to stop the charge while running off the line power is to insert a thin piece of paper between the plug and the laptop. If you block the middle conductor, the laptop will be unable to identify the power supply, and will therefore refuse to charge the battery, which is the desired result.

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I didn't find a pure Software solution, so I've set up a quick hack based on the Belkin WeMo Switch and a Node.js app. The WeMo is a power outlet that can be switched on and off via Wifi. So you can plug your laptop charger on the WeMo, and a script or an app running on your laptop monitor the battery and switch the outlet on and off accordingly.

Edit: more explanations from the original blog post (the link is dead.):

My inspiration came from an Article on Wired, explaining that to improve your battery life, you must keep it charged in the 40% - 80% range.

In fact I really doubt the Wired article really make sense. "Not charging the battery up to 100% increase the discharge cycles", ok it's probably true. If the laptop is almost always plugged in and the battery is at 100%, you can expect 300-500 cycles, but your are only "using" a cycle once in a while. If the laptop is permanently cycling between 40-80% you can get 1200-2000 cycles, ok it's a lot more, but you are now using 10 cycles a day.

I'm not chemist and I don't know what's really going on in the battery. What happens if instead of doing 40%-80% cycles, we do tens of thoushands 79%-80% cycles ? What percentage variation is considered a cycle ? Does 0%-80% cycles are better than 20%-100% ? As always the evil is probably in the details, and all batteries, even among the LiPo familly have different specifications.

I think it would be more useful to just pop up a notification if the battery get lower than x%. Just to remind you that if you are close from a power source that would worth it to plug your laptop.

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Based on this article:

  • Use partial-discharge cycles
  • Avoid charging to 100% capacity: why don't you unplug the charger now and then? Or try that software : http://fruitjuiceapp.com/ (Note: I haven't tried it myself...)
  • Limit the battery temperature: make sure your laptop is well vented, and do not forget your laptop in your car during the summer.
  • Do not charge your battery in a cold environment (<0deg Celcius). The charger circuitry is supposed to monitor the temperature but just in case...
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