Laptop Batteries: "I was having the same problem mentioned where my iBook would go from a 90% charge to nothing very quickly and what I did was turn off the battery status monitor in the menu bar. My battery has been fine ever since. I have not tried turning that status indicator back on since. I imagine the code that causes this error may be in the menu bar widget code. Just a thought."
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I'm seeing a similar thing, battery gets to 30% then dies without warning. I've seen a few mentions implicating bad bugs in the menu bar status indicator.
I've also seen mention of improvements with booting to OS 9 then charging there. I'm going to try removing the battery status indicator from the menu bar. (I've already tried Apple's tip to reset the PMU.)