I run 2 cranking batteries and one deep cycle battery. The deep cycle is for stereo only 'NOT' connected to charging system, reason being is deep cycle has to be charged by a deep cycle charger. Charged at the same rate it was discharged. Fastest way to kill a deep cycle battery is short burst charging.

Randy

The "right" setup is a cranking battery on each and a deep cycle for your accessories. But that said, I've been running three dual purpose big guys no problem for about five years. I do however charge all three every time I get off the lake on my trickle charger. Your alternators are hard pressed to keep up with a deep cycle running a strong stereo for a period of time.

Hey Pete,

I am in the process of adding a third deep cycle battery. I will have one battery for each motor, then all accessories off the third battery. I'm also adding a third battery switch and battery isolator . Hope all is well!

My deep cycle is used for my stereo only also. It is hooked to the #2 on both my battery switches. I can charge it off either engine.
If I run it down some, I will switch one battery switch to BOTH and split the amps between the cranking and deep cycle.
Deep Cycle Battery is 6 years old.

Two deep cycles are fine if you run them in series and use an isolator.

Wait! In series? that would give you 24volts! What am I missing here?

So why 3 switches? Thats what I already have. What is the advantage of 3 switches vs 2. Cant figure that one out

...sorry I meant parallel with an isolator.