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I brought my 4 year old - one year past AppleCare - MacBook to the Apple Store yesterday with its second bulging battery.

Happily, Apple lived up to its reputation for excellent customer service and gave me a new battery at no charge despite the fact that there's no recall in effect.

But the real reason I'm posting is to share some info the Genius gave me:

1. MacBook batteries are most likely to fall victim to bulging if, as I do, you run your machine on "wall" power all the time.

2. If you run your Mac that way, leaving the battery out of the machine - charged to about 50% - is the way to go, and, in fact, he sent me home without even unwrapping my new battery (which he said is charged to 44% when it leaves the factory).

3. I can store the battery that way indefinitely with no loss of functionality.

PS: A new battery, which cost $100 a bit more than two years ago now cost $130...pretty steep increase.

Batteries WILL lose charge over time. Apple ships them usually with 2 lights, because it's better not to have them sit unused with a full charge for any length of time. We have to top them off every month or so if we have stock that hasn't moved. Otherwise the battery will go into safe hibernation and that can be risky. (see below)

This is necessary. Today's "smart batteries" must communicate with the computer (and magsafe pack, AND the magsafe end that plugs into the computer!) and all agree everything is cool before power goes, in either direction. That means electronics have to be running IN the battery BEFORE it will accept power. If it doesn't have any charge, it can't accept any charge. Catch-22.

This creates a chicken/egg problem if the battery COMPLETELY loses all power. It can't turn on the gate to let charge in to get charging again. It's bricked, for good. As a precaution, when a battery gets to critically low, (past the one flashing light) it will fall into safe hibernation. The circuitry sets an alarm and goes to sleep, drawing close to nil power. If you plug it into a computer, the computer will see the battery, but can't charge it because it can't talk to it because it's hibernating and can't be woke up.

Every so often, (nobody will tell me how often - 20 minutes to an hour by my experience) the battery BRIEFLY wakes from hibernation to see if a computer is offering power. If so, score, charging starts, and the battery is recovered.

If not, it winds the clock and resets the alarm, and goes back to hibernation.

If it sits like that for several months, it will have lost all power and be unable to wake from hibernation, and then you have a $130 brick. The only way to salvage it is to charge the cells directly.

If you have a battery that shows 0% and is refusing to start charging, let it sit on the charger for 24 hrs (per apple's recommendation) You'll either come back to a fully charged battery, or a battery that's still dead. If it's dead, it's passed beyond the vale and is bricked.

Thanks for the clarification; the Genius's statement that I could allow the battery to sit indefinitely didn't ring true.

I don't like having an empty battery bay in my deuced Mac(hina), so now I've got to decide whether to save the new Apple battery to maybe resell the machine some day and fill the bay with a 3rd party battery or install the Apple battery and risk its being my third "air-bag."

Hmmm... "Tis a puzzlement,"

Not all geniuses are.