My two cents' worth - between this thread and the previously-cited one, there is some confusion about what is meant by an on-board charger. Some are talking about a system that mounts in the boat, but plugs into the wall - just a carry-with-you battery charger with some transfer bells and whistles.
What the OP is asking about is a different critter - something to take power from the outboard motor and distribute it to multiple batteries. There are a few different brands and types, but they all share the same essential limiting factor - they only redistribute whatever's available as an output from your outboard's charging system.
In my case, for example, a Yamaha 50 outboard's output is more like a trickle charger than a real charger - only 6 amp output. That is enough to top off a cranking battery that hasn't been pulled down hard, provided you have a 40 minute boat ride between starts, but isn't enough to keep up with a deep cycle battery that's being asked to run lights all night long without the engine running. You draw juice from the battery much, much faster than you replace it.
So, first thing you need to determine is what the output on your motor is, and then figure out how long the motor has to run to replace the draw you've made on the battery.
oooh::big_smile: 
