Quick check on alternator:
Use a multimeter to measure battery volts, ignition off, should be 13.5v. Record it.
Start engine, battery volts should be >14.5v, engine running. Record it.
Switch dipped headlights on, battery volts should still be >14.5v. Record it.
If battery volts is >14.5v under load (lights), alternator is probably OK.
If battery volts, ignition off, is < 12v, then battery is very suspect.
This is a basic check only, but will avoid buying an alternator, only to find it's the battery, or vice versa.
Mac.