My great, great uncle was a sergeant with Co. C of the 45th NCI. I have a copy of a letter that he wrote with a buddy after the Battle of Cold Harbor.
My uncle's name was Samuel D Knight. Most of the unit was from Guilford County. They fought a few skimishes before Gettysburg, but it was their first major battle.
They were heavily engaged in the assault on McPherson's Ridge on the first day at Gettysburg. Subsequent to Gettysburg, the 45th fought in a number of major battles up until the end of the war. What was left of the unit was with Lee at Appomattox. My gg Uncle was captured at the Battle of Cedar Creek in October 1864. He died two months later at the Federal P.O.W. camp at Point Lookout, Maryland.

You may want to contact the Sons of Confederate Veterans(I can't find their web address right now, but Google it) for more information. The SCV has a Genealogist who has a database of Civil War service records. He may be able to track what happened to your relative. I got quite a bit of information from him.

Best Regards

Mike Lyon