I have a 20' wellcraft that I want to put a very simple dual batt setup in. any input on batt isolaters or should I just go with a batt switch , it would be nice not to have to switch anything

A 20' Wellcraft. I am assuming a single engine. The isolator you speak of would be a two bank, single alt isolator. Depending on your intended usage of the dual setup, you may or may not need an isolator.

If you plan to leave BOTH the batteries switched ON when running, and swap to ONE when at anchor (leaving a hot one to start the boat) then you can do as follows:

Anchor the boat. Swap to Bank A (the deep cycle marine battery). Run it all day long til it is low. Swap to Bank B (the marine starting battery - or an AutoZone special with a 3yr free replacement warranty that you bought for your truck, but it found its way into your boat). Start your motor. Pass the battery switch across the BOTH setting to the Bank A setting, whereas you are now charging your dead/low battery with your 40amp alternator.

NOTICE that you MUST get a battery switch that has the positions starting at OFF at one end of the rotation. It must pass through A, then BOTH, then B (or B, BOTH, A). This is important. I have seen switches that go OFF, A, B, BOTH, or some other thing. The importance here is that you must be able to go from A to B without unloading the alternator, else you risk popping the triacs, rectifier, or regulator (these days all these parts are inside the alternator).

If your deepcycle battery is nearly dead, you should not pause at BOTH for very long, as the dead battery will be receiving a charge rate from the hot battery of over 200 amps which is too fast. It is fine to pass through this zone, but it is like putting jumper cables on it every time it is dead and it will not last properly that way.

Remember to wire bilge pumps directly to the batteries, and not through the battery switch. You should actually have two pumps, each directly to another battery and choose one of them to use an automatic switch and run off your choice of battery. You may also want to wire your stereo MEMORY/CLOCK wire directly to the deepcycle battery. They draw next to nothing.

M

Some good advice up above. If you want simple then just go with the Switch and run it. The switch comes with a diagram for all the different ways you can set it up.

Jon