Hi Rod,
We run a single dedicated 36v 105 amp Lithium for our Ultera. A 31 agm for the electronics and another for cranking. We use allot of electronics and the agm 31 used to have trouble keeping up with the 9.9 kicker. We upgraded to a 15 hp kicker on the new boat for the stronger alternator.

We do run two 15 inch Solix, 3 optima down riggers usually constantly cycling, and occasionally radar along with misc. other small draws like the fish hawk and stereo and VHfs.

I had two go with the single 36v because of bow space/weight constraints. Also with a single 36 the charging is easier, we use a single 36v high capacity charger - it can fully recharge the battery in a couple hours.

I would never go back to a 3 - 12v system, but to each their own.