[T2] external battery charger

[Robert Mann]

       My neighbourhood is now graced by a camper only a couple y newer
than my '73 but with many upstaging features  -  2-litre Typ-4 engine,
autotrans, luxurious furniture from Adelaide, etc.   The owner lent me his
Haines workshop manual  -  especially created in England, different from
the Bentley.   This book asserts an external battery charger should not be
connected before disconnecting the battery.  No reason is given, nor can I
think of one.  I have charged my battery in situ many times, without
apparent damage.  Can anyone make sense of this prohibition?

Robt Mann
'73 1600dp Devon