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I have a problem with my 3 year old MacBook. The battery has "swollen up" (I could post a picture if that is of help).

One side of the battery is much bigger than the other. Since it no longer works, I opened it. Inside it is 6 cells covered in metal envelopes. 2 of the 6 cells are about twice as big as they ought to be.

What do Apple's batteries contain? What could be the reason for battery cells to swell up?

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Apple battery packs contain a set of 6 lithium Ion (Li-Ion) cells (the metal covered envelopes), connectivity circuitry, and a small logic board.

The main cause for battery cell swelling is what is known as thermal runaway.

If a cell reaches a certain temperature then a chemical in the cell (usually Cobalt Oxide for Lithium Ion batteries) starts an unintended reaction which is exothermic in nature (i.e. it creates heat). More heat raises the cell temparature further which causes more of the same chemical reaction

This creates a runaway thermal overload which can end in one of two ways, the cell and therfore the battery pack, catches fire or the reaction fizzles out with the cell in a swollen state.

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    This. But I would add that it is (obviously) dangerous, and you should dispose of it properly. I would suggest taking it to an Apple Store, and asking for them to dispose of it for you. It is entirely possible they will offer a free replacement, I seem to recall a swap programme for this a couple of years ago and it may still be open to you. Also, check that the rest of teh Macbook hasn't been warped by the swelling, or a replacement battery may not sit properly inside.
    – stuffe
    Jan 31, 2012 at 13:40
  • @Olmar, just stuff I happen to know from being an Elctronics Engineer and having had to change the cells in a MacBook battery a few years ago.
    – Stu Wilson
    Jan 31, 2012 at 15:46
  • @stuffe According to the explanations I received at 3 different Apple Stores, the battery swelling is the expected behaviour of Macbook batteries and will not replace them free of charge. But they told me that it is dangerous to use the Macbook with a swollen battery.
    – xpereta
    Sep 23, 2013 at 17:09
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    Just happened to my 2011 MBP 15 inch. Apple Store says its the protection mechanism of the battery from leaking acid. But this mechanism damaged my computer. There was no warning sign to replace the battery. I think its design defect and Apple should own it. replace the battery and fix the computer for free. I was told it is safe but was asked not to power it up or plug it in. Initial response is that I need to pay for the battery replacement and fixing the back cover of my computer as it is seriously warped.
    – krishna
    Feb 25, 2018 at 6:01

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