Sorry, you all gave a bad answer!
The advice to allow Windows to "hibernate" when battery level hits 2% (the lowest available "critical" battery level) is flat-out WRONG.
The correct answer: modern batteries require recalibration at regular intervals (typically 2-3 times a year) to stay smart about their own capacity. If you do not do this, the calibration will go out of whack, leaving you with a battery that appears to have gone bad when it has not. I've seen plenty of batteries that quickly go down to a critical level (typically 1-3 percent) but if the right settings are made, can continue to operate for at least an hour longer without trouble! This is one evidence of an uncalibrated battery.
What to do? Learn proper recalibration, and learn how to get around the bug (!) in Windows 7 that prevents recalibration.
Proper recalibration:
- The goal: help the battery understand its max possible power that can be used.
1) Charge the battery ALL the way. In fact, unplug for a minute, then replug and let it charge more. Keep charging until it won't hold more.
2) Unplug, let it drain. Use the laptop "lightly" -- don't try to hurry this step.
3) Of course, save your work and exit all programs when you suspect the end is near...
4) Wait for the computer to completely run out of battery... and shut down
5) Try to power on again, at least into the BIOS. Again, let it run out...
6) I even try pulling the battery and reinserting... and repeating...
At this point, you've done everything possible to drain that battery. Recalibration is done. Now you can recharge and use as usual.
Important notes:
a) Windows 7 makes the above process impossible, because "Do Nothing" is not a valid choice for critical battery action. What to do?
- Create a new Power scheme, called something like "Recalibrate"
- at an administrative command prompt, do the following:
“powercfg -setdcvalueindex SCHEME_CURRENT SUB_BATTERY BATACTIONCRIT 0"
(without the quotes). This will supposedly set the Critical Action to "Nothing" even though not available!
b) To better monitor what is going on with your battery, try the free trial version of BatteryMon from PassMark. I actually paid for a copy long ago... it is quite helpful.
c) There is NO WAY you will do "serious damage" to your laptop hardware by running it until the battery forces a shutdown. Think about it: your BIOS allows this at any time. Windows 7 arbitrarily shuts down early... to the detriment of your battery calibration.
d) I have NOT yet fully tested the above "powercfg" method...and do not know for sure if it will accomplish the goal. Feedback welcome! (My first Windows 7 laptop is being tested as I write

)
Blessings,
Pete