check the battery terminal voltage under the following conditions, after driving the car for at least 30 min:
1) At idle, min electrical load (ie, everything electrical off that can be turned off)
2) At fast idle (hold engine at 2500rpm for 30sec) min elec load
3) At idle, max electrical load (turn on everything you can, lights, heated screens/seats, fans etc etc)
4) at idle after 3min with max elect load (same as 3) but wait for 3 mins before taking reading
5) at fast idle, max electrical load
6) at fast idle, max elect load, after 3min
Do those tests and post up the results!