Problem on altenator or the battery

[GeorgeFocusmk3]

My focus mk3 1.0 ecoboost, has recently develop start stop malfuction. First it start to misfires on random cylinder 2 or 3. Now the misfire is not present but on idle the s/s indicator goes on but no always.No coolant loss or oil despite the  175.000km.  After a research i saw that when these happens the battery voltage is 11.8v(headlights start to flicker) and the car runs on idle for a long time due to traffic. I am reading this voltage through the cluster menu. Do you have any ideas or other methods to determine if it is my altenator or the battery that is causing that. I went to ford dealer and check the car and told me that probably the misfire is due to bad quality fuel i didnt told him about the battery voltage because i havent realized the that goes that low

[Luke4efc]

41 minutes ago, GeorgeFocusmk3 said:

Do you have any ideas or other methods to determine if it is my altenator or the battery that is causing that

Have you checked the battery using a multimeter when the engine is running?

[Carl123]

20 hours ago, Luke4efc said:

Do you have any ideas or other methods to determine if it is my altenator or the battery that is causing that

Also, how old is the battery?

Is it the original one?