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Title says all. My friend told me that I need to disconnect the power cord when the battery is fully charged. Otherwise, I will shorten the life of the battery.

Is that true? I am going to use my laptop at home 90% of the time. So, I was going to have the power cord connected all the time.

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6 Answers

up vote 27 down vote accepted

You do not need to disconnect your MacBook Pro's battery. Your battery will stop charging once it is full. Apple's modern batteries are much smarter than previous designs.

To get the most out of your MacBook Pro's battery, follow the Notebook Battery advice from Apple: unplug and use your battery until empty about once a month, then charge back up to full.

If you need help following Apple's advice, use Battery Guardian; it is free and will remind you when to deplete your battery.

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Your response seems contradictory to what is on the Apple Notebook battery site you linked to: "For proper maintenance of a lithium-based battery, it’s important to keep the electrons in it moving occasionally. Apple does not recommend leaving your portable plugged in all the time." Are you suggesting that material is out-of-date? –  codinguser Apr 18 '11 at 14:10
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If you read the next sentence "An ideal use would be a commuter who uses her notebook on the train, then plugs it in at the office to charge", the key words are "all the time". Don't treat it like a desktop. Periodically use it the laptop while unplugged. This is not the same a unplugging it every time the battery is full. –  Anm in LA Apr 18 '11 at 15:37
    
If you always keep your battery topped off, then it's almost as bad as long term storage when full. That decreases the capacity between 6 and 35% annually based on temperature (these numbers are for 0 C to 40C). Discharge cycles also wear down a battery - so it's safe in the short term, but not ideal in the long term. They are replaceable so if you prefer to have shorter life by always topping off - you can pay sooner for a replacement. –  bmike Jun 10 '11 at 20:39
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No. The nature of the lithium battery chemistry does not require periodic cycling to maintain capacity.

Furthermore, the periodic cycling described in the apple documents listed above (@I-M-JM's Post) does not have an effect on the battery chemistry. It only serves to allow the battery capacity meter to accurately track the battery capacity.


Periodically draining the battery only serves to allow the Battery metering systems to accurately estimate the remaining battery capacity. It is not needed to maintain the battery itself, and actually wastes a battery cycle.

Basically, if you don't run your battery down for a long time, the next time you disconnect the laptop from AC, the estimated time renaming value will be incorrect. However, the actually battery capacity and runtime will be unchanged from normal.

The statement "For proper maintenance of a lithium-based battery, it’s important to keep the electrons in it moving occasionally." from Apple's site is actually incorrect regarding the lithium battery chemistry at worst, and an oversimplification at best.


Furthermore, The battery metering and battery charging systems are separate, so inaccuracies in the battery metering system will not cause errors in charging the battery (before you ask). Lithium cells are actually quite simple to charge.

Ref:
http://en.wikipedia.org/wiki/Lithium-ion_battery
http://batteryuniversity.com/learn/article/lithium_based_batteries
http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries

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Wow - I figured the long term storage implications of a full battery to be the same as always having it charged and topped up (and risk 20% permanent capacity loss annually). The new unibody batteries last so many more cycles I'm less concerned about duty cycles - perhaps the old data with simpler battery controllers is less relevant? –  bmike May 17 '11 at 18:22
    
You get the 20% (or similar) capacity loss no matter what you do, as it is a function of breakdown of the battery chemically. There is a slight increase in capacity loss if you keep the battery charged all the time (most manufacturers actually recommend long-term storage at ~40% charge). However, either way, actually using the battery is worse then keeping it charged. –  Fake Name May 18 '11 at 0:59
    
Thanks - I saw table 3 of batteryuniversity.com/learn/article/… and was amazed that they quote only 4% annual loss if the battery is at 40% charge instead of 20 % annual loss 100% charge at 25C. Makes sense why service batteries are usually 50% or less. –  bmike May 18 '11 at 3:16
    
Thanks a ton, Fake Name. –  Undo Apr 24 '13 at 17:46
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I keep my macbook pro plugged in all the time with no adverse effects. And if you want to use your mac laptop in clam-shell mode (ie lid closed, connected to an external monitor), then you will need the power supply plugged in anyway.

Still, I (and Apple) recommend to unplug it every month or so and use the battery a bit, just to remind it that its a battery ;) Keeps it healthy.

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I would suggest that you read following 2 articles:

  1. http://www.apple.com/batteries/
  2. http://www.apple.com/batteries/notebooks.html

From my personal experience, don't charge fully, and you can disconnect it after you charge till 90% or more.

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I use Watts which tells me the current state of my battery (in terms of total capacity) and has a regular calibration process to improve its lifetime. It costs $6.95.

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From Apple's Notebook battery site (emphasis mine):

For proper maintenance of a lithium-based battery, it’s important to keep the electrons in it moving occasionally. Apple does not recommend leaving your portable plugged in all the time. An ideal use would be a commuter who uses her notebook on the train, then plugs it in at the office to charge. This keeps the battery juices flowing.

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