replaced battery, get battery light until approx. 3 miles driven, then it goes on 2008 Mazda Mazda3

out. why? do I need an alternator?

Asked by for the 2008 Mazda Mazda3
You need to have the charging system tested.
Thank you, I will. Is this expensive also?
If you can drive the vehicle, drive it to a Auto Parts House that does it free. Like Advance, Autozone or just call or google it. The power of the internet is mighty.