On our boat (which used to be equipped with an OMC 15), we went to one bank, period. The 9.9 can be pull started if the battery or starter fails for some reason. No need for an exotic setup with a start battery. Our deep cycle group 31 is plenty. The 9.9's amp draw (under 20 amps cranking) does not require a start-only battery. A deep cycle is fine.
Also, the 9.9 makes, at best, 5 amps (wide open), and just isn't going to recharge from any serious amount of house draw, period. It's basically set up to replenish what was used during cranking. Ours used to actually have a net LOSS if below full throttle with a bilge blower running.
While the solar unit (I would make sure it has a charge controller to keep from boiling the battery dry) will help while on the hook, to make up for anything much beyond an fm stereo, you will want to use a shore charger or other means. If you will spend time at the dock, and decent Guest or similar Marine, 3-stage charger will be fine. We used to use a 10-amp model (Guest 2610, I believe), and it was adequate, unless refrigeration was running while we were using all the cabin lights, etc.
Ditch the automobile-type charger. They are famous for leaking AC to the 12v side, and ours corroded the outboard LU so badly that I thought it was going to start leaking lube. Seriously.
We adding proper shore power, and refurbishing the 12v side of the wiring, we have since gone to a 20 amp charger (10 plus 10, tied together on our one battery), and just cannot overwhelm it in normal use at the dock.
We cruise a lot, and we tie to the dock a lot. Only time we have any concerns is on the hook. An LED anchor light helps a lot.
Current 12v setup:
One grp 31 deep cycle flooded battery
One 20a (10/10) 3-stage charger charger
6 amps from Nissan 9.8 OB when running 3/4 throttle.
No solar, no wind charging
No 1-2-both-off switch; just a cheap on-off batt sw.
Current 110v setup:
One 30a shore inlet
One twin 30a main breaker, feeding:
One 10a breaker for charger, and
One 20a breaker for outlets.
No refrigeration: Built-in icebox, and Coleman peltier-effect cooler, that can run on either 12v (or 110v with the adapter). Very electrically inefficient, BTW -- 4a of 12v all the time. But we usually run it on shore power, where it uses 0.4a of 120v.
Has worked great for 3 seasons now.
Paul Van Voorhees
Certified Tohatsu TLDI Technician
Mgr, Obersheimer Sails
Buffalo, NY USA