CCred68046 wrote:

Um....ok, but why "dissimilar"? I'm curious about two different
batteries.


One is an old 1000 CCA deep cycle for trolling and the other is a new 420 CCA
starting battery. Switching them to the both position will cause the good
battery to be discharged by the weaker battery. The way it was described to me
by a battery expert was "If you pour good milk into bad milk you have a bunch
of bad milk".

Get a regular 1/both/2 battery switch. There's no problem switching
those 2 batteries together when charging. Just use the 1 or 2 positions
when you want to troll or start and use both when the engine is running.

Except when brand new batteries are fully charged, there's almost never
a situation when a boat with 2 batteries will have both of them in the
same state of charge. People parallel them all the time to charge
without a problem.

Steve