Mac Forum / General / Portable Macs / February 2006

It is likely that his is happening because the battery is dying before
the computer expected it to. The computer doesn't actually know how
much power is left in the battery. When it registers that you have 10%
left it does not know that. It is only assuming that. The way that it
comes up with its assumption is that from past experience (last time
you calibrated the battery) it learned at what voltage level how much
of the batteries capacity reamined. But, over time the characteristics
of your battery change. Your battery is not as spry as it used to be.
Therefore your computer is making incorrect assumptions about its
capacity.

The solution to the problem is to recalibrate your battery. To do this,
charge it fully. Then discharge it by operating the computer until it
shuts down. Then recharge it fully and the computer should now have a
updated, correct measure on how much power is left at any given
measured voltage level. It should not run empty unexpectedly.

Another possibilty is that your batteries are nearly worn out meaning
its discharge curve may be much too irregular. If this is the case it
may be impossible for the calibration process to produce usable data
for the computer to work with.