Laptop Battery-OT???

Daughter finally whined long and loud enough to get us to cave and buy her a
new whiz bang laptop. :\

So we' ve got the old Dell with a battery that's at least 3 years old.
Think I bought her a new battery before she headed to college.

It's used mainly as a desktop replacement, so ac is available, and we
normally just leave it plugged in. Had it in the kitchen for a recipe, and
held it's juice nicely for about 20 minutes.

After 20 minutes (2 open windows) it read 75% then dropped precipitiously
about 15% every 5 minutes.

Dumb question is this normal or is the battery about had it? Inspiron P3
600mhz, win xp, with a wireless card.

Thanks
Mark