lsvbb,
This is how I view it whether right or wrong. In this application there is no justification for a triple bank charger. Unless you isolated the two stereo batteries with a separate switch the 3-bank charger would not be able to profile and condition the two batteries independently so you have gained nothing over the dual bank model. The only important issue in this case is the total charger capacity compared to the potential discharged battery consumption based on its amp/hour rating. The general rule is 10 to 13 percent so that a)you don't overcharge, and b)you don't undercharge which would manifest as not adequately chemically exciting a flooded battery and in turn this leads to permanent and irreversable sulfation (sulfer permanently bonded to plates...will not accept a charge anymore...battery is toast). With two stereo batteries having slightly over 200 collective amp/hours that exceed the ten times the 20 amp charger capacity (just over the recommended limit) this is why I suggested above that you have a means to selectively and temporarily take the starting battery off the charger. You might do this only when you have put the boat into storage and the batteries are known to be deeply discharged. This would allow Al to preserve his charger while ensuring that each battery gets enough current to properly desulphate and also satisfy his budget restraints. I'm pretty sure he doesn't want to replace his charger that was just purchased this season.

On another note, when you really understand the challenges of charging systems, especially in the context of big stereos and multiple batteries, you will find many conflicts. For example, extending the life of your batteries can be in direct conflict with preserving your alternator. There are no perfect charging systems...only the best overall compromise. As systems get bigger things definitely get more complex and the approach should vary with different systems. There is no perfect prescription that fits all systems.

David
Earmark Marine