Alternator Or Battery Confused

[WHITTALL]

So couple of weeks ago whine appears from Alternator which disappears with the smart charge disconnected. Investigation battery low on voltage so I charge her up and output is normal for a couple of weeks and now she's died. So I swap out the battery for an old one which could be faulty but is the same voltage so I charge over night to make sure and the cars dead after 3 starts with voltage readings 12v off and under 10v running. I know your not meant to disconnect the negative on modern but I did so anyway and the car died so I believe it's the Alternator and not the battery would I be correct? Ford Focus 1.8 TDCI 56 plate. Smart charge fuse is also fine.

Sent from my iPhone using Ford OC

[alexp999]

I think disconnecting the battery breaks the circuit so the car cannot run anyway.

Most garages can do a battery/alternator condition check. Some even do it for free.