anyone got any ideas what could be up with my alternator / battery

car is a 2.5 tdi ranger (06)

its charging at 12.5v at tickover and about 12.8v at 3k RPM with nothing electrical on

with everything on it drops to 11.5 regardless of RPM

it has not had enough power to start a few times but has no pattern to it , i can be driving short journeys at night with everything on and not have a problem but 1 in 100 times it wont have enough power in battery

if i leave sidelights on for just 5 min without engine running it wont have enough to start

i have replaced the battery and just fitted another alternator from a known working truck but its still the same

is the voltage regulator separate or built in to the alternator like most ?

any other ideas ?????????????????