Battery or power management?

[rks]

According to Power Manager (2.5) my battery is in good condition. But, when I'm running on the battery fully charged, it all goes off after a few minutes when the gauge says that the battery is about 85% charged. Then I plug it in, turn it on, and the battery gauge says that it's 8% charged and charging. This is extremely bothersome. It happened to me before and I updated the power management software and things got better. I updated again (to 2.5) and I have this problem all over again. What to do?
Richard