I'm experiencing flat batt / wont start issues about once or twice a fortnight. the car is 2010 FF

the alternator is clearly doing something because after a jump start and a little driving around, the car is ok starting maybe for a week or even 2 and then flat again at some random point.

having read lots of posts on here and elsewhere about alternator failures etc I decided to bench charge the battery for 24 hours. I put a digital voltmeter as a semi-permanent device fitted into the centre console fag lighter. started it and then watching the voltmeter, we see 12.6v, then down as low as 10.5 whilst cranking, recovery to 12.5v when it starts. however, even after 3 minutes idling (but no throttle peddle pressing) the system voltage is not increasing as might be expected due to alternator activity.

then i try loading the system by switching on front and rear screens, seats, tv etc and see the voltage as measured at the ciggy lighter dorp to around 11.7v but still no effect from the alternator. then rev the engine to 2000 rpm and the alternator bursts into life and up we go prgressively to 13.6v. then switch off the screens and tv and the system voltage rises to 14.3 and more or less stays for the duration of a 10 minute journey (no heavy loads like screens etc).

this seems to be a strange set of figures but a friend tells me this is typical of a smart alternator where the charging is managed by the engine management system. i tried starting it again this morning (started ok) and let it idle with the screens switched on and even after 8 minutes the alternator had not burst into life - it did as soon as the revs were increased to 1500 however.

is this correct functionality?