T43p not powering up when the battery fully discharged

[Hans Gruber]

One of my T43p's does not power on after the battery is fully discharged. I have to hold down the power button and pull the battery to get it up and running. I replaced the bios battery with one that was fairly new many months ago. I think that battery was 2 or 3 years old. Do I need a new bios battery? What is causing this anomaly?