Adding 2nd Battery...help
One thing to keep in mind when adding a battery switch is whether your outboard''s alternator works like a car alternator or works on a magneto. If it works like a car alternator, you''ll need a battery switch with a field disconnect. This keeps you from blowing the alternator diodes when switching between batteries, or if you accidentally turn off the batteries with the engine running. If your alternator is of the magneto type, I don''t think this is a factor.
In any case, common procedure is to start the engine with the battery switch in the "both" position, then switch to the "house" battery after shutting the engine down. This lets the alternator charge both batteries while the engine is running.
One concern I have is whether the alternator on your 15 hp outboard puts out enough juice to charge two batteries. I had an alternator added to my 5 hp outboard, and it proved to be a complete waste of money.
Hope this helps.
"One Step Closer"
Lake St. Clair, MI