>It is never a good idea to leave batteries connected in a parallel
>arrangement unless the following conditions are met
>1) The circuit is in ( or near ) constant use, where frequent charging is
>happening.
>2) The batteries are connected in parallel temporarily by a switch for
>purposes ( let's say ) of starting an engine with partially depleted
>batteries, and than, after the engine is started, the switch is restored
>to the non -  parallel mode. This is a common marine arrangement.
>Batteries connected in parallel in other than the above situations will
>tend to discharge each other, as the odds of their internal resistances
>being the same ( required condition for zero current flow ) are next to
>impossible.

The original post did not inquire about car batteries and an application
like many of us have in our vans. It was about connecting batteries
together to generate more current for different applicatino, which is
quite acceptable as long as the batteries are closely matched. This can be
done by purchasing them at the same time, from the same lot if at all
possible.

Car batteries have no chance of surviving such an application, as you say,
but decent deep cycle, not marine, batteries are used in this manner all
the time. I'm pretty sure that is what the original poster was asking
about.

If the batteries are not exactly matched, and they never will be,
equalization charge cycles will bring them close enough to where it
doesn't matter.

John