I'm about to install an onboard battery charger.

Is there any reason why the charging leads can not go to the ends of
the battery cables at the selector switch instead of snaking into the
box and being put on the terminals? This would be easier and neater
and minimize the chance of things getting switched at battery change.

When we put the batteries in, the yard made a good point. Switching
one battery end for end so that either the two + or - terminals are
side by side greatly minimizes the chance of a massive short if
something gets across the terminals while the system is on.
Everything should be dead whenever the box cover is off but this is
real life and both boating and aviation are a good place for a belt
and suspenders approach.

--

Roger Long