I'm glad that Tony has confirmed what I was thinking.
I know next to nothing about cars (or vans) but I have taught Physics and I know that an alternator must initially generate AC current but that what comes out of it needs to be DC in order to charge the vehicle's battery, so I was struggling to make sense of this question.
Wikipedia confirms Tony's statement (and what I was thinking must be the case):
". . . an automobile alternator contains six diodes, which function as a full-wave rectifier for battery charging"
https://en.wikipedia.org/wiki/Rectifier
. . . and this picture (from a vendor of alternator parts) seems to confirm it further:
http://www.jcrsupplies.co.uk/images/images/dscn43466.jpg.jpg