Mark,

Except for trolling motors the primary purpose is redundancy. It's nice to
have a backup.

Both of my batteries are starting type. When starting and running I use both.
Swithching while running is not recommended. Damages the alternator.

While fishing or at anchor I switch to Batt #2 which powers the VHF,
sounder/GPS/Chart, and bilge pump.

Now and then I start and run on Batt #2 or Batt #1 just to be sure either have
the capacity to start the O/B.

The batteries will be five years old in April so I'll probably replace them. I
hate having dead batteries out in the middle of nowhere with no boat in sight.
Batteries are cheap tows are not.

Butch

Mark wrote: snip 1 - why is this system utilized (why not just 1 battery?)
and what is
the proper operation of this system?
2 - should I always have it switched to "both" so both batteries get
charged when running? what is the preferred switch scenario?
3- which battery is better to run the electronics to? (I imagine the
deep cycle.)snip