Detecting laptop battery status... at BIOS and OS level?

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I am trying to squeeze some more longevity out of my Compaq Presario
1600i (no, the Smithsonian isn't asking for it) but I am having a
curious problem:

* The battery gets and keeps a full charge, and can run about 2 hours,
which is normal.
* The OS (previously Windows 2000 Pro, now Windows XP Pro, cleanly
installed)  constantly thinks that the battery is about to die, and:
  * if Advanced Power Management is on, it tries to shut down or
  * if APM is off, it constantly beeps. (Note: This is not a Control
Panel->Sound setting - is it the BIOS beeping?

Anyway, I don't want to spend $100 to replace the battery, and I guess
I could get by with the speaker muted, but can anyone help me

* Is battery power detection peformed at the level of the BIOS, OS, or
* Is there any way to get Windows to stop beeping even if it thinks
the battery is low?
* Any way to 'rejuvenate' an aging battery, e.g. clean the contacts,

Thanks in advance for any advice.

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