The differences between Bipolar Disorder and Borderline Personality Disorder come up from time to time, so I found an article from NY Presbyterian Hospital written by an expert on BPD on the differences. I find it to be both accurate and fascinating. I would suggest reading it for anyone curious about the differences:

The difference between bipolar and borderline personality disorder

Oops, I'm not sure the link works. If not, try Googling Distingishing between BPD and Bipolar Disorder, like I did, and see if you come up with this article from NY Presbyterian Hospital.

I hope this is helpful!