2099 XF x250 battery not charging
 

I have purchased a new alternator and had it fitted however still no charge can anyone advise what else could be the issue.
It started the warning light flashed on and went off, within seconds the warming light came on and stopped on at the same time the charging voltage dropped from 14.8 volts to 11.6 volts and never changed.
The alternator is brand new not a refurb