Continuing our coverage of unexpected (without the typical "you are now running on reserve..." message) sleep in some PowerBooks and iBooks - which has generated a tremendous response - MacFixIt reader Brett Buck notes that this behavior is not atypical of lithium-ion batteries, and offers some technical explanation:
"This has been possible with essentially all Powerbooks whose batteries had internal microprocessors. The low-power warning is a function of the reported %remaining. It gets to something like 6%, and then it gives the warning. But the actual battery voltage determines whether the computer will run or not.
"If something should cause the battery voltage to drop unexpectedly from above 6% to too low to operate the machine, the warning may be skipped. As it turns out, there is a well-understood phenomena that causes precisely this. If the battery's cells have differing capacities, one cell may exhaust itself long before the others. Since they are wired in series, the overall battery voltage will change by a large amount in a very short period, and since the battery microprocessor is calibrated for all the cells being equal, the low-power warning may not be given. So the machine shuts off with no warning.
"The cells can get imbalanced for many reasons, but if they come from different lots, or are of significantly differing ages, it's entirely possible that one cell may degenerate before the others. And of course it would be expected to be more likely to happen under heavy loads where the capacity is being expended faster - like watching a DVD, burning a CD, etc. [...]
"I've also seen a virtually identical thing happen in aerospace battery operations - leading to some very complex operational machinations in satellite operation. Individual cells in series packs inevitably become mismatched if you use them long enough, and the result is sudden drops in the output voltage at far less than full capacity of the entire stack.
"There is another potential problem, as well. If one cell exhausts itself but DOESN'T trigger the machine to turn off, the cell is then reverse-charged as the rest of the pack continues to supply current through the depleted cell. This hastens it's demise. One thing for almost certain is that this state if very likely to occur on any Li-Ion battery given enough time. Whether it gives a warning, or skips it, depends on how depleted the cell is, how good the rest of the pack is, and how much load you have."