Let’s try again 

I’ve got a 2012 Focus 1.6 tdci, 

recently had both battery and alternator changed, but now 1 minute after starting the car the battery light comes on and the auto stop/start turns off. I’ve tested the battery with engine idling and revved, both show battery at 14.5 V. Does anyone know what the possible issue is, everything was ok before having to change both

not sure if this helps but check the voltage when the battery light is on, could be a worn belt slightly slipping. also make sure you plug is on securely, ive seen broken plugs that connect fine most of the time but come slightly loose every so often. also maybe a faulty battery

If you disconnect the smart charge plug on the alternator does the charging voltage fall to the default of 13.6V   ?

You can use FORScan to both check for any Ford specific DTC's and also use it to monitor live data. My guess at the problem is that the earth bond straps between the engine and the chassis are either: Missing, Corroded, or high resistance.