Nissan Micra alternator not charging

My Daughter initiated the first problem by not fully replacing the dipstick, which gummed up the alternator! We got a replacement one and everything was fine for 14 months.

Then the lights dimmed and air bag light came on-it wouldn't start. The garage eventually agreed it was the alternator and replaced it at a reduced cost. This failed on test in the garage so they installed another one. This worked for 5 months then the car wouldn't start.

Check battery -flat-still under warranty so I replaced this. One month and 1000 miles later-difficult to start, the air bag light comes on and electrics faded. Only way to restore power was to rev hard, when in a traffic jam. Fine when travelling at speed, but nearly died lights dim,radio fades every steep hill and at roundabouts-foot down, radio starts working and air bag light goes off.

The car is a 52 Micra and has only done 55k. Any ideas?