I''ll try to keep it clear. I don''t think a 9.9 O/B has an alternator unless it has a battery starter
. If it does then it can be used to charge both batteries. Wal-Mart and most auto parts stores have battery isolaters. They''re the same as tho one''s at marine suppliers but are usually a little less money. They''re pretty simple to hook in. If you have no alternator. Install the switch. You can set the switch to all and your charger will charge both batteries simultaniously. I suggest at least a 30 amp automatic charger as a smaller one will take all day and all night to top off most deep cycle batteries. I use a D8 for the house. It lasts more than a night or two for sure. I seperate my house from the Nav lighting
and Nav equipment too. Perko has an over-kill sized switch for a system your size that most boats use with multiple batteries. They too come with simple instructions for installation. I''ve never seen a failsafe system for the inadvertant. If all your using your batt''s for is lighting
and Nav equipment you don''t need any wire larger than 10 gauge to hook it all in with.
Hope I helped.