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.
Chuck
'83 Westy
'84 Jag XJ-6 VDP
-----Original Message-----
From: George Averill <averill@LDL.NET>
To: vanagon@GERRY.VANAGON.COM <vanagon@GERRY.VANAGON.COM>
Date: Sunday, July 28, 2002 4:30 PM
Subject: Batteries in parallel?

Does anyone have any information concerning problems which may result from
connecting two 12 v. batteries in parallel to obtain more current?  Seems
like I remember that there are problems to be considered, but can't remember
exactly what the problems were.  Maybe some solutions?
George