I had the alternator replaced a few weeks ago on my 2010 TDV8. Yesterday the car would not start (low battery voltage). I have checked the battery and done load test and the battery is fine. On starting the car the voltage across the battery terminals was around 12.3 volts and did not change when the engine speed was increased. I did a diagnostic check using my Foxwell 644Pro and no fault codes were evident.
I was about to take the car back to the dealership which did the work.

As a last resort I did a battery monitoring system reset (which my car does not have) and it went through the motions and reported a reset.

Again I started the car and the voltage across the battery terminals was 13.6 volts.

Can some one expalin what is going on. Why would the alternator stop charging the battery?