Powerbook battery jumps remaining life VERY quickly

Question

I recently got a powerbook from a friend, and noticed some pretty wierd stuff going on with the battery. first, when I completly charge it, it will stay at about 98% charged for maybe 45 minutes saying there is 70 minutes remaining the entire time, never higher than that. then it will suddenly drop to 4% every time and give me about 7 minutes remaining. then I will charge it and it will go from 4% all the way up to 9% normally before it jumps back to 98%. I was looking through the posts on this forum and did not see anything like this, nor did I know what to search for, so I don't know where to start. any help would be greatly appreciated.
Thanks

Answer 1

Apple talks about a properly-maintained Powerbook or iBook battery being designed to retain 80% of its original charge capacity after 300 charge-discharge cycles (see the lower right sidebar on this page). The meaning of "properly maintained" is not explicitly spelled out anywhere that I know of, though one can infer some of it from the other contents of that article. A battery that has ever been exposed to intense heat (e.g., left in a closed car in the summer sun) is likely to be permanently damaged, and I wouldn't expect it to behave as intended thereafter. When buying a used computer (or battery), one has no way of knowing what abuses it may have suffered in the past.
It's one of the hallmarks of a damaged or defective battery, or even just a well-maintained battery approaching the end of a long and productive life, to become unable to report plausible or accurate information about itself: its current charge level, its charge capacity, or the estimated running time remaining on a charge. All of these readouts, whether displayed in Apple System Profiler, through a third-party utility, or on the menu bar, can become wildly inaccurate, and it's very commonplace for the OS's weak-battery warnings to disappear, so that the Powerbook simply goes to sleep without any advance notice. When these things happen, replace the battery, no matter how many or few charge cycles it reports. If the battery is still new enough to be under a warranty, by all means appeal to the manufacturer, but I suspect the circumstances would have to be unusually clear-cut for such a claim to succeed.