Yes, your engine DOES charge your cranking battery while it's running...all the suggestions here will help keep it charged while not running...another consideration is an onboard battery charger...if you have a 2nd battery for bilge-pump, etc, you might want to consider an onboard charger as it is NOT being charged by your motor...I had 2 Guest chargers on my boat and LOVED 'em/11...plug 'em in and FORGET about them short term or long term...your batteries will be hot to go when you're ready!!...
