Have a problem with my 2002 pontiac sunfire 2.2l SFI engine. Cannot get the battery to charge up. Replaced alternator and has a new battery. Help?

New alternator and new battery?

First, go to an auto parts store and have them test your alternator. It may not be working correctly, even though you replaced it. You can’t tell unless you have it tested. Ask them to “load test” the battery, too.

If the alternator checks out, I’d test the continuity of the battery cables.

“Cannot get battery to charge up.”

You didn’t say you have trouble starting the engine. Why do you think the battery is not charging?

Don’t forget to check the "fuse linked " wire that runs from the battery to the alternator. If the link has blown, there won’t be the 12 volts at the alternator it requires for operation.

When I put in the new battery and alternator the vehicle starts up ok. After about 30-45 seconds the little red light(symbol of a battery) comes on.

It appears that the alternator is not charging the properly. I would take it back to where you bought it.