Well, a couple of things could be causing the problem:
1. SSD doesn’t not automatically mean better battery life. Recent tests have shown that some SSD drives actually consume more power than traditional hard disks due to the fact that they have a higher idle power draw. Do you have the specs on the drive you purchased?
2. The operating system and/or HDD may not be configured to use all of the possible power saving features. I assume you’re running XP.