As others have written the correct answer FOR YOU depends on a few things like what type of batteries do you have, do you have regular access to shore poower, what kind of sailing do you do, etc.
On my Newport 28 (which for sale) I had two group 24 marine batteries, and the boat was on a mooring (no shore power). For a typical day sail, I would put the battery switch to ALL and leave it there. both batteries would be used to start the engine, both batteries would be charged when the engine was running, and both batteries would be used to run the GPS, autopilot, etc. For a few hour day sail I would not use enough power to really load either battery.
For a night sail, or an overnight at anchor I would set the battery switch to 1 or 2 and run off of that battery. The next day (or when it was time to start the engine) I would switch to the other battery to start the engine, and then back to the original one to charge it. When it was charged I would switch back to 'all'.
If one battery is discharged you don't want to tie both batteries together for starting, because all that does is have the good battery try to start the engine AND try to charge the weak battery.
Deep Blue C, 2002 C&C 110
Mt. Sinai, NY
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