It is always wise to have the second battery, especially when using all the electronics you have. Also install a battery switch or isolater so that you can use/discharge one battery (deep cycle) only, when not using the big motor. This is the one I had all my electronics connected to, (radios, graphs, additional lights, Fish Hawke temp probe, etc. etc). This saves the main battery for start up and then switch to both to recharge when running. Of course that all depends on the size of the alternator on the big motor.