I have a 2007 Honda Fit. The battery light came on. The battery was charged and placed back in the vehicle. The car started fine but the light was still on and their was no increase in voltage at the battery as it was not charging. I purchased a brand new alternator and replaced it. Same issue. A brand new battery was purchased and replaced the old one. Same issue. What else could it be?

 

dead cell in battery

 

Battery cables, fuse, or relay.

Sadly, a mechanic maybe could have saved you some money. There may not have been a problem with the parts you replaced.